I picked up a CT-100 earlier this year from a local collector, and after finishing off a bunch of other projects over the past few months, I've now turned to this wonderful RCA.

This set has seen a bit of work over the years. Many of the original paper caps have already been replaced, sometime between the 1960s and 1980s, based on the appearance of the replacement capacitors. The main power supply electrolytics were replaced with nice GE FP electrolytics that are still doing fine. The selenium rectifiers were replaced with diodes, and the ballast has been removed and replaced with fixed power resistors mounted in the area where the seleniums were.

Cosmetically, the set is not bad, with a cabinet that could use refinishing, but with the wood in pretty good shape except perhaps for a couple of the feet. Unfortunately the main front panel knobs are gone, so I'll be eager to find a possible source for replacements.

Everything else is there, and the good news is that the original 15GP22 tests good on all three guns.

I recapped a number of the already recapped paper caps (the replacement caps were a mixture of paper and polymer dielectric caps). I haven't done a full recap yet, but tried to cover the sweep circuits and video output stages.

Powering up briefly, I got sound immediately, as well as anemic high voltage (12 kV) and some semblance of a dim picture (bad sync, quite dark, but some video information making it to the screen).

I then recapped the HV cage, hoping that would bring the HV up to normal, but it didn't. I replaced some resistors in the focus section, putting original values in place of various jury-rigged components, and now notice that if the focus rectifier is connected, I get no HV. With the focus rectifier disconnected, I get 10 kV. There may be a (possibly brand new) short in the vertical dynamic convergence transformer, but we'll get to that later. For now, I'm leaving the focus rectifier disconnected.

Unfortunately, the flyback gets hot. I have a nice horizontal drive waverform on the horizontal output tube, and about -40V bias on the HO grid. Running the set briefly and then feeling the warmth on the flyback seems to indicate clearly that the source of the heat is the HV outer winding on the flyback, and most likely not the primary windings. It heats up pretty quickly, so I'm running much less than a minute. No nasty damage, but it is clearly getting too hot too fast. I gather this indicates a shorted turn in the HV winding; any other diagnosis possible? That would certainly be consistent with my anemic HV. The heating is the same whether the HV rectifier is connector or not, so that seems to rule out something loading down the HV as the cause of the heating.

Moyer does not have a replacement flyback available. Any junker chassis out there for a possible replacement?

Otherwise, I may try some surgery on the flyback. However, I don't want to do that prematurely. The original *looks* very nice and clean, and whatever surgery I do will irreversibly change that, and also has a fairly low chance of success. I've saved one flyback on a different set in the past by removing some outer turns on the HV winding, where the short happened to be. No guarantee by any means that would be the case here.

Open to advice on how to proceed.

P.S.: Moderator, I suppose this really belongs in the COLOR section! :) I'm so used to posting in the black and white section that I forgot about that...

Sounds like you need one of John's transformers, shoot him an email. RE low HV: disconnect the shunt regulator (6BD4), see if HV pops back up. The HV pot may be bad, do some futzing with the shunt circuitry, I wouldn't be surprised if you locate the fault. I've never seen a CT-100 flyback go bad, they seem to be better made than the later stuff. The outer winding getting hot tells me it's trying to put out the voltage, but it's getting lost to ground somewhere between there and the CRT.

I was probably editing while you posted -- I added one comment above mentioning that the flyback heating occurs whether the 3A3 is connected or not, so it seems to rule out something loading the HV as a cause. I had also tried pulling the anode cap off the 6BD4 earlier, and that didn't affect the low HV.

I'm encouraged to hear that CT-100 flybacks rarely burn up, so perhaps that means the likelihood of someone having a good one on a junker chassis is better than zero.

I most likely will need one of John's transformers, but I figure I don't really need it until I get the flyback situation sorted out.

It's probably just the convergence transformer, put one of John's in before you do any more testing. Otherwise you'll be chasing your tail, multiple faults and so forth.

With the focus rectifier disconnected, isn't the vertical dynamic convergence transformer completely out of the flyback circuitry?

...the flyback heating occurs whether the 3A3 is connected or not, so it seems to rule out something loading the HV as a cause. True Tom, but the 1X2B is still in the circuit, which drives the focus circuit, which is why Nick is so adamant about replacing the convergence transformer. One never knows for sure what is wrong until it's fixed of course...

Pete

Tom, I just read your latest post. Yes, I agree that removing the focus rectifier should remove a convergence transformer short from the flyback; I did not see a 1X2B reference earlier in the thread.

Hi Pete,

Thanks for chiming in -- I know both of you have a lot of good experience with these sets.

Anyone know of a spare CT-100 flyback floating around out there? According to Sams, a Thordarson FLY-100 is an option. The RCA part number is 79012.

Tom, sorry, the FLY100 is NOT a replacement, just another Saqms data error. I acquired a FLY100, and it is some B&W flyback. Grrrr.

I would agree that with the plate cap of the HV and focus rectifiers removed (and the tubes pulled) if the flyback HV winding still heats up rapidly, it is most likely a shorted turn.

I don't have a spare chassis, but I do have a universal coil winding machine and have wound several flybacks with varying degrees of success and failure. I unwound a bad CT100 flyback to get the turns count recipe and wound a replica on the old frame and core. Installed in my CT100, it lasted about 2 minutes, then the primary winding failed open somewhere.

I wound a replacement flyback for my 15" Motorola which worked and produced good HV and sweep, but the HV pulse and all the auxiliary pulses supplied by the flyback have a big negative going ring right in the middle of the pulse, which is upsetting the circuits which use the pulses (AGC, burst gate, color killer, color bandpass amp etc.) So the internal LC ratios are not quite right, even though I followed the recipe as closely as possible.

So winding replacements is an art I have not mastered. While I would not be eager to wind a complete flyback, I could be coaxed into wind a replacement HV winding. But first, do some searching for a junker chassis. I bet there are some out there...

John,

Thanks for the straight scoop on the FLY-100. When I called Moyer, the person I spoke with also indicated that his cross reference did not support the idea that a FLY-100 would be a suitable substitute for the flyback in a CT-100. In any case, he didn't have a FLY-100, so at least I didn't end up wasting money on that.

Great to hear that you've tried rewinding a CT-100 flyback. I also know of a coil winding machine I can borrow. The one here is a pretty simple affair, with just a variable speed motor (with foot pedal control), a turns counter, and a drill chuck. Is yours a little more sophisticated, with perhaps something to control the wire winding pattern?

I also have the perhaps misguided notion that it might be possible to remove most of the high voltage winding from another random flyback (keeping it almost entirely intact), and mechanically mount that on my CT-100 flyback after removing the original HV winding from it. Naturally I'd have to pick a donor flyback with an inside diameter on the HV winding that is slightly larger than the CT-100 primary, so it would fit. Could easily end up with tuning problems like you encountered, however.

I won't go after either of those just yet, since I agree it is worth trying to get the word out to see if someone may have a spare available.

In the mean time, however, I might look into some fine surgery on the existing flyback, on the outside chance that the short is near the outer diameter of the HV coil. Unwinding a relatively small number of turns from the outside might remove the short but leave enough for sufficient HV.

Tom,

I think attempting to unwind some of the HV coil is worth a try. If you can expose all of the HV winding, you might then be able to localize the shorted turn by seeing where it gets hot.

And the transplant idea might work, but it would be tricky to find one that fits mechanically ad has the right number of turns.

My coil winding machine was mad by the George Stevens Co, and is a very sophisticated device. It has interchangable cams and gears to set up the coil width and the ziz-zag winding pattern (also known as a universal type winding). Here is a photo of the machine winding a HV coil, like what you need.

That's a very nice winding machine. I always wanted something like that for rewinding power transformers, but I could never justify the cost or space requirement, for the few times I'd actually use it.

If you could remove the HV winding from the flyback ,could you then use the resulting flyback for all of the other windings except the HV winding and also use an independent external HV supply for the CRT anode? I suppose it may require some additional winding with L/R/C components to tune the flyback to match the original characteristics?

Dave

This George Stevens Co winding machine is designed for universal (flyback and peaking coil) type windings, and does not do layer type windings used in power and audio transformers.

On newer roundys (like CTC-16 era) I've heard of folks having successfully removed the fly's HV winding and getting their HV from a solid state HV trippler the input of which being connected to the HO Tube's top cap....This may also be a solution for your CT-100.

Two options to further confirm your flyback is at fault are to compare it's winding resistances to those listed in the Sam's folder, and to get a vintage flyback/yoke ringer (aka a flyback tester) and see how yours tests.

Good luck fixing it.

Dave,

I've seen it done. When I got my Philco 48-2500 projection TV, a prior owner (an engineer) had unhooked the original HV supply, and built his own little HV supply with an 807 tube and a nice big RF coil for the HV. I hastily removed all that and put the set back to original, only to discover there was a good reason why he had done that -- the original HV supplies in those sets had too high of source impedance, resulting in fluctuating HV with scene brightness, which in turn affected focus stability.

So yes, I think that could work. I'll probably try that as a last resort if necessary. Hopefully I can either get a replacement flyback, or modify/repair the existing one in some manner that works OK. So one way or another, we'll get this problem solved! Glad to see lots of ideas here from you guys.

Tom C.,

The tripler idea sounds very interesting! On the Philco set mentioned above, I eventually replaced the original poorly-designed vacuum tube voltage tripler with a solid state tripler to improve the HV supply, so I have a little experience working with a tripler module. That would be a very nice fix that would fit into a small space somewhere and not look too crazy. If the drive right off the primary is high enough, then the original regulator, etc., should still be usable. It seems quite possible that this could work. I notice that the focus supply runs off a tap *below* the 6CD6 drive connection, and generates 4.5 kV with a simple half-wave rectifier. All I need is for the top of the primary to generate 6.7 kV to get enough to triple to 20 kV.

This might be the next thing I try after seeing if some minor surgery can remove the shorted turn. Easier than trying to rewind the HV winding or retrofit an HV winding from another flyback. I suppose I could even "borrow" the tripler out of my Philco to do a feasibility test, and then order another if the result looks viable.

I did check all the resistances, and they are quite close to the values on the Sams schematic. If it's a shorted turn, it seems to be isolated to just one turn or a small number of turns, having no appreciable effect on the total DC resistance of the winding.

Tom

Comparing winding resistance will likely not reveal a shorted turn, as the short will only remove a small number of turns resistance from a large resistance total.

Do you have access to a "ringer" tester? They are pretty good at spotting shorted turns...

Have you checked the value of the cathode resistor on the 6CD6 yet, or read for cathode current? All this talk about serious mods so soon, and we're not even certain the flyback is the problem yet! Slow down, take a deep breath, do some trouble shooting. In the end if it ends up being the flyback, at least you did your homework first. All I'm saying is I definitely rule out every other possibility before jumping straight to unobtanium parts like flybacks, especially in a chassis like the CTC-2 where they rarely fail.

There are several controls on the chassis which impact HV production:

Drive varies the waveform going into the grid of the output tube, if it's low in amplitude the negative bias of the tube will not be right and tend to cause more current to be drawn heating things up. R175 (Sams) on my set had drifted causing incorrect operation of the drive control, I couldn't get rid of the drive line till I got the correct resistor in there.

Horizontal may be off frequency (wave coil), which will cause bad sync and low HV production- 2 of your original symptoms. The waveform in the oscillator itself may not be the right shape (equal peaks), the cure for both of these is to do the horizontal setup procedure in Sams.

HV pot may have drifted, if so the shunt may be dragging down HV even if everything else is working correctly.

You have a known bad convergence transformer- stop thinking about the fly till you cure that problem.

Is the CRT boot clean? If not, it will make a sizzling sound and even if it's not it could still have a path to ground somewhere that's draining the high potential away from the CRT. I went through 3 different ones before I found one my set 'liked'.

Or any one of a million other things on a very long list, but the point is you have to start somewhere before assuming the worst. My starting point is always to make sure the horizontal oscillator is in perfect health and on the right frequency FIRST. All other circuits in the chassis depend on horizontal to tell them when to do their thing: burst keying, color killer, convergence, ect. All other circuits are secondary to horizontal being correct.

My AGC pot was also bad, leading to a lengthy and unnecessary foray into CTC-2 IF alignment. Much was learned, including the important lesson of LOOKING FOR SIMPLE FIXES FIRST! :yes:

Slow down, maybe even walk away for a few days and think about it. Put John's convergence transformer in it, then retest. If it's still jacked up, go through the horizontal circuit with a fine toothed comb. Report to us with your results, the last thing we want is for your fly to be bad.

N2IXK,

I have a Ringer, but it works poorly. I've been on the lookout to buy a better one, but that's one acquisition I still need to make. The one I have has a sticky meter movement, and I have not been able to improve it. That ends up making the readings very unreliable. Would be nice to have a good Ringer right now, just to make doubly sure that the problem isn't somewhere else and the flyback really shows the expected lossy behavior in a Ringer.

Miniman,

I'm actually reasonably experienced at TV restoration, so quite a few of the important things you point out are indeed things I already paid attention to, or have reasons to believe are not relevant for the particular way I'm going after the problem. Nonetheless, I'm glad to see you ask all the right questions here, since I don't want to leave out something critical and make an unnecessary mistake.

The horizontal frequency is correct, because I can see just enough video stuff on the screen to tell that it actually locks weakly to a TV signal on the antenna input. Nonetheless, I got out the frequency counter to make sure, and yes, it's ~15.75 kHz. My ear had also confirmed from the start that the pitch was "about right."

HO grid bias is -46 volts, and the peak-to-peak drive waveform is 150 V on the grid (cf. 120 V P-P in Sams), adjustable with the drive control.

HO cathode current is 193 mA (cf. 167 mA in Sams), which seems reasonable given the improper power dissipation in the flyback itself, although if any of these numbers raise a red flag for any of you, don't hesitate to speak up.

The real smoking gun on suspecting the problem is a shorted turn in the HV winding of the flyback transformer is that it is getting quite hot in about 1 minute of operation, with neither the high voltage rectifier (3A3) or focus rectifier (1X2) connected. The only way there can be a load on the high voltage winding is if the load is within the coil itself - a shorted turn or something else very lossy. The heat really does seem to be generated in the HV secondary winding; it is much hotter to the touch than the primary windings. My finger may not be a 100% reliable temperature sensor, however, so I've checked it many times and I seem to be coming to a consistent conclusion on that.

I have the convergence transformer out of circuit for all of these measurements (by disconnecting the focus rectifier), so even though it looks quite likely that it is bad, it can wait until the HV problem is cured. It is definitely not having any effect on the flyback right now.

I have the CRT out of circuit most of the time, but have connected it a few times (as well as the 3A3 HV rectifier) and can see a very dim, out-of-focus raster on the screen, and it does not substantially load down the already weak HV (because the CRT anode current is almost zero under these circumstances). HV is quite low, around 10-13 kV -- a little lower in that range yesterday, a little higher today. No evidence of any current in the 6BD4 regulator tube (which is how it should be with the HV too low). Disconnecting or connecting the regulator tube has no effect (which indicates it is working correctly, or at least doing no harm).

I think I'm properly covering all the things that could account for improper flyback function, but if you see something I missed, I'll be more than happy to go after it. I, too, am not eager to prematurely modify the flyback in an irreversible manner.

One could always suspect the yoke, but if it's bad, it shouldn't be causing my HV secondary on the flyback to get hot. I have a test yoke I can connect, and perhaps I'll do that next just to make sure (in case my finger temp measurements are misleading me as to what's the source of the heat in the flyback).

On the other hand, there are lots of other circuits in the set that have nothing to do with the horizontal sweep, and my preference is to solve this problem before going after all those. I'm confident we'll get this set fixed in a step-by-step manner, and for me, horizontal sweep and HV are priority #1 pretty much in any set I work on. So indeed I agree with you that the horizontal oscillator needs to be functioning properly as step #1, and that was indeed done here.

When I get to the color circuitry, that's where I've got the most learning to do. I've restored dozens and dozens of black and white sets, so I've got a decent grasp of all of the circuitry and functions in black and white sets. But color is something new, so that part of this experience is going to be educational!

The B&K 1076 analyst is a good flyback checker. Another good way is with a WO-91B RCA scope with the mod from RCA bulletin. The mod is adding a capacitor to get a sweep output. The pattern during test is a good ringing waveform if the flyback is good.

I spent some time last night and this morning trying to check the flyback and yoke in additional ways.

I have a Sencore SS137 Sweep Circuit Analyzer (picture attached) that I had not used before. I spent a little time making repairs to it (new electrolytics, fixed a broken wire) and confirmed that everything on it appears to be working properly. It has many functions, of which two are useful here. One is that it has a substitute horizontal yoke coil. Connecting this substitute yoke in place of the yoke in the set is supposed to tell you whether there is a problem with the set's yoke. Connecting the sub yoke, I find the HV is slightly higher (14-15 kV), but no where near where it should be, so my conclusion is that the yoke in the set is probably OK. It also measures the current in the substitute yoke and displays whether it thinks the flyback (and overall horizontal sweep system) is OK. Although the measurement doesn't seem very sophisticated, it does claim the flyback is bad (provided the surrounding circuitry is functioning normally). The meter has lines for various deflection angles of black and white sets (above line is good, below bad), and a single line for "color" sets, requiring the highest amount of measured signal. My result was that the meter came up to the level for a 50 degree B&W tube, which is way below what is deemed correct for a "color" tube. Not sure I trust this one-size-fits-all test, given that the 15GP22 is probably a bit unique, and its sweep system may not have enough in common with what this tester thinks is appropriate for "color" to give a trustworthy result.

Fant's post above reminded me that people sometimes do a ringdown on a scope to check a flyback. Although I have never done that before, it occurred to me that although I don't trust the meter on my Ringer, it does put out pulses. I captured the ringdown on a scope (see attached photo). What do you guys think about the waveform?

The manual for the Ringer says 10 cycles is the boundary between good and bad, but I don't know how small a cycle needs to be before it is no longer counted. Perhaps one of you knows.

I have no idea where the threshold is set for the ring test, but I would judge that to be somewhat less than 10 rings.

The B&K 1076 analyst is a good flyback checker.
I used the 1076 with 100% success back in the day. Even with its simple neon indicator, it never once gave a false "bad" reading or showed a bad unit "good".

I would try the ring test on several known good (B/W is ok) flyback transformers to get an idea what the display should look like... I agree that that seems to fall short of the 10 ring threshold, but what does another flyback do in your set-up? Perhaps the scope input impedance is lower than that in the tester circuit, causing more rapid damping?

jr

Tom-

I am not too far away from you, and I have several devices that include flyback-testing ability if I remember right. They are a B&K 1077, a Sencore VA48, and a Sencore VA62. The catch is that I have not yet used any of them to do flyback tests, so there would be some learning to do in the process. I am happy to lend you any of these instruments and/or to help out if I can. I also have a CT-100 that I restored a couple of years ago, and I did a lot of work on its horizontal circuits as well. (In fact, it is still not 100% either, but I had put it on hold while other tasks came up.) You can probably find its discussion here with a search of my name.

Hi Chris,

It sounds like your situation is not so much different than mine -- various equipment that should do the job, but nothing that you've used often enough to really have confidence in its accuracy. I also have two B&K 1077 units that I could repair and try.

All,

I did a bunch of ringdown tests on various flybacks to see how they compare to what I'm seeing on the CT-100 flyback.

Here are the attached pictures in order:

1. CT-100 flyback, this time isolated from the circuitry (all connections removed, including yoke) - looks like 5 or 6 decent cycles only

2. Stancor A8134 NOS flyback, no connections

3. RCA 104309 NOS flyback with big coil, for 110 deg CRT (probably black and white), no connections

4. Traid D305 NOS flyback, for color set, no connections

5. Traid D311 NOS flyback, for color set, no connections

6. Traid D313 NOS flyback, for color set, no connections

Quite an obvious difference in terms of seeing a lot of decent ringing cycles on all the NOS yokes.

So that all looks nice and conclusive, but wait until you see the next post (I can't upload any more pictures in this one).

Now a little more data to make things confusing...

I happen to have an Emerson 637 in the shop. It's a 10" B/W set from 1949 and it works very nicely. Took the chassis out and tried the ringdown test on this set with a "known good" flyback, and look what I get!

1. Emerson 637 ringdown, with flyback in circuit (yoke, etc., connected)

2. Emerson 637, with yoke disconnected (removing other connections had little effect, also tried pulling 1B3 to remove filament load, but didn't matter)

3. Merit HVO-5 NOS, very similar to what's in the Emerson. No connections.

OK, so the Emerson flyback doesn't look very good in a ringdown test, but it works in the set! And it compares badly with an NOS flyback that looks very similar and might even be the right replacement (it's the fairly common early style flyback used in the late 1940s with a more metallic-looking core, not ferrite or laminated).

HV in the Emerson is 9 kV, which is just what it should be.

Not sure what to make of that!

And a couple more for a sanity check.

1. Crosley 9-407, with all circuits connected. Similar early style flyback as Emerson 637 above (the other one which rings down with very few cycles, even though the set works well).

2. GE 805 Locomotive, with all circuits connected. More cycles on this one. This set also works fine.

Still not sure how to interpret the Crosley and Emerson results. Maybe the early style metallic looking core is lossier than more modern ferrite cores? Then why does the Merit HVO-5 NOS flyback not show the same loss? It looks very similar in construction.

I have no idea where the threshold is set for the ring test, but I would judge that to be somewhat less than 10 rings.
Reading some of the details in the Ringer manual, it says that the usual threshold is 25% of the amplitude of the first cycle. So the bar is pretty high.

Tom, when testing with the flyback in circuit, if you leave the rectifier tube in the socket, the filament circuit looks like a shorted turn. Take one of your out of circuit flys and put a single shorted turn of wire around one leg of the core and compare the results.

John,

Here are some pictures to show exactly what you're referring to. The first is a NOS FLY-107 flyback with no circuit connections. The second is the same flyback, but with the filament winding for the HV rectifier shorted directly. The third is with an actual 1B3 filament across the filament winding.

These all show the expected behavior. A dead shorted turn has a huge effect, and a turn shorted with a small, but finite resistance, has a noticeable, but smaller effect.

The CT-100 flyback cases I've shown always had both the HV rectifier (3A3) and the focus rectifier (1X2) unplugged, since I was aware that this could be an issue. In the case of the post above with 6 pictures, the CT-100 flyback was tested with no connections whatsoever (no yoke, no filaments, no pulse windings connected to anything).

The one case I want to go back and double check is the Emerson 637 picture above, where I mention the yoke is disconnected and other connections didn't matter. I definitely tested it with the 1B3 removed, and also tested with most other connections removed (including yoke), but I want to make doubly sure I did it with all of the above disconnected simultaneously.

The Crosley 9-407 case above had everything connected, including HV rectifier tube filament.

So far, all tests are consistent with the CT-100 flyback having a shorted turn. The only confusing cases that are causing me to hesitate are the ringdowns on the Emerson 637 set, where I did not see the expected result for a working flyback.

The CT-100 ringdown isn't as fast as the direct shorted case posted here, but I think that makes sense. Shorting the filament winding has very low resistance, since the filament winding is thick wire. An unwanted short inside the HV winding (such as what I think I have on the CT-100 flyback) will have higher resistance both due to much thinner wire, and possibly because the short itself has some resistance (dirty/burned contact point).

This has turned into quite a sidetrack, but I'm actually glad to develop a good technique for testing flybacks, with enough experience on many different flybacks to have decent confidence in the results. These ringdown scope traces are probably a more thorough method of testing than simply relying on what some particular piece of equipment says in a "good / bad" test.

I think you are right, the resistance of the HV coil wire ends up being in series with the shorted turn and thus reducing the effect of the shorted turn on the ringing response.

One other thing you can try.. remove the solder from the terminal which connects to the plate cap of the HO and remove the flyback wire which goes to the HV winding and leaving the flyback wire going to the primary attached to the terminal. Run the ring test again, and you should see a normal response, proving the shorted turn. Of course, doing this does have some risk, one could break either of the two wires in such a way that they could not be repaired. But as the flyback is most likely NG, you don't have too much to loose.

Hi John,

I can do that this evening when I get home from work, but I don't understand why disconnecting this wire will change anything. If there is a shorted turn in the HV winding, it will still have the same effect, whether the bottom end of the HV winding happens to be connected to the primary or not. Top end, of course, is open circuit with the 3A3 removed. A completely isolated shorted turn will still overdamp the ringdown, just like a shorted filament winding, wouldn't you agree?

...One other thing you can try.. remove the solder from the terminal which connects to the plate cap of the HO and remove the flyback wire which goes to the HV winding and leaving the flyback wire going to the primary attached to the terminal. Run the ring test again, and you should see a normal response, proving the shorted turn.
Um.. not sure i follow correctly. But you're saying leave the HV winding ('tire') electrically floating but still physically in place(?).
Would not a shorted turn in the 'tire' still load the primary whether or not it's electrically connected? Seems like you'd have to physically remove the tire to do the test.
Or maybe i'm misreading whut U sed. :o

Yeah, my bad... what was I thinking? Bit I am pretty much convinced you have a shorted turn.

I retested the Emerson 637 set, which appeared to show too few cycles for a known-good flyback when I tested yesterday (and posted on page 2). This time I really had all connections removed, and it looks decent.

I think previously I had noticed that removing the 1B3 in that set had only a minor impact on the ringdown and then plugged it back in. However, after removing all other connections, it may have been having a bigger effect, being the only remaining load. So the previous picture with "no yoke" still had the 1B3 filament load. Got to be careful to make sure all connections are removed for a final test of a flyback.

So I think that removes all doubt about this particular method for testing flybacks. If you get something like 10 decent cycles of ringing or more, the flyback is OK. If you don't, it has high loss (low Q) from a shorted turn.

Looks like this will be a good technique for checking flybacks going forward.

Decided to unwind the HV winding of the flyback and see how many turns would need to be removed to find and remove the short. I was hoping it would be a short in the outer layer or two of the coil.

Got started by removing the black hard shell that coats the outer diameter of the HV coil. The material is quite different than most flybacks -- quite hard and durable. It was difficult to nibble at it with a diagonal wire cutter. First picture below shows the first nibbles out of the shell. Second picture shows the whole thing removed, which I also used a Dremel tool to assist. Once I get a few big nibbles out, then I was able to crack the whole thing off easily.

I left the Ringer and scope attached the entire time while I unwound the coil, so I could see immediately when the short was gone. After unwinding a few tens of layers, I could tell the unwinding was getting more difficult, and one spot in particular looked well cooked. Getting closer to the short!

Eventually got the short, and I also unwound one more layer of well cooked wire with the thought that a damaged winding layer would be likely to fail later. Still there are quite a few cooked layers present, but the short is gone. Alas, I had to remove quite a bit of the coil (perhaps something like 15%). Will it have enough HV back in the set? We'll find out.

The new ringdown is shown in the 3rd attached picture, and looks convincingly good. The last picture is the remaining HV coil, obviously smaller than when we started. I'll reattach the 3A3 anode lead and reinstall it in the set.

Dang, this is one of the more fascinating of troubleshooting mini sagas.

:)

Good Work! It will be interesting to see how much HV the coil can produce now.

jr

If it is not good enough you could always take the removed wire dip it in lacquer a few times to build up it's insulation and try to rewind that wire back on to the fly...

Looks like it will produce 20 kV. :thmbsp: The HV regulator is working, too. I can reduce the HV down to about 18.5 kV and up to 20 kV. When everything is working, I imagine I'll have to run it around 19 - 19.5 kV, just to have some headroom to maintain proper regulation.

I had also been thinking that if necessary I could try to add some turns back on the flyback. I won't be using the wire I removed, however, as you can see in the first attached photo. I didn't disassemble the flyback, but left everything enclosed in the core and frame while I unwound the coil. This meant that every few turns I had to cut the wire, to avoid having to pull a lot of wire through the opening. The insulation generally was destroyed as I unwound the coil anyway, so no use thinking about salvaging the old wire. Some new wire could be added with some kind of plastic mandrel walls to make winding reasonably straightforward. However, it looks like I may not need to do that.

The last picture shows a closeup of the flyback in the set. I coated the outside of the coil with some red insulating varnish. I may beef that up with something more at some point. I didn't want to glop too much on there yet in case I needed to add more windings.

As you can see, the focus rectifier is still not connected. Now I need to deal with the vertical dynamic convergence transformer issue and other things.

Looks like it will produce 20 kV. :thmbsp: The HV regulator is working, too. I can reduce the HV down to about 18.5 kV and up to 20 kV. When everything is working, I imagine I'll have to run it around 19 - 19.5 kV, just to have some headroom to maintain proper regulation. FYI: With the focus rectifier operating normally and with a low-contrast image, my CTC2 will crank out 22 kV; I set the HV 2 kV below that, and overall operation is satisfactory.

Pete

Tom that's a pretty good job, I would have done that too.... I think that if you keep the hv too high, it will draw extra current on the primary... What do you guys think....

Congratulations on a really delicate difficult operation on the flyback. Sounds like another historic CT100 is well on its way to being saved. Hope it all works out well.

Tom-

I am very happy to see your progress on the flyback problem, and I look forward to further news as you continue the project. Nice work!

I haven't had too much time to work on the set further this weekend, but I do have a vague semblance of a picture on the screen tonight.

http://www.videokarma.org/attachment.php?attachmentid=180742&stc=1&d=1384763635

As you can see, the red isn't very strong, but it is functioning. From the picture here, one might get the impression there is no red at all.

I'm not out of the woods yet by any means on the flyback. What I find is that with any significant brightness on the screen, the HV collapses down to 16 kV, and even lower at full brightness. So I'm definitely short a bunch of turns on the flyback. Focus is also very unstable, and I had to modify the range of the focus control to get even close to focusing properly.

I'll work on some other aspects of the set for a while, and then return to the HV problem. I may try adding more turns to the flyback, or try the voltage tripler module I have in another set (and run it off the primary of the flyback as was suggested back on page 1).

I have the vertical dynamic convergence transformer out of circuit now so I can apply the focus voltage. New transformer has been ordered from our fine colleague John Folsom.

I am not sure if anyone is aware of this but there is a Series of Tektronix tube type scopes from the 50's era that used an almost identical Focus control assy ( I can't exactly remember if it were focus but for sure a control in the HV area) it was a VERY large Scope, as a matter a fact I sold it to someone a couple of years ago for their CT-100 and it worked fine, the actual value was a small amount off but very close to the ct-100, the physical size and everything was identical, I had removed this myself from the older tube type scope for parts, the only part number on the control itself was 01080100., I remember the HV section of the scope had solder in small tube rectifiers, but its been a few years so I do not know the Model number.
I know the Focus and conv controls are impossible to find for the CT100
just passing this on from my experience on the CT-100

Fortunately my focus and convergence potentiometers are OK, but I imagine others who are not so lucky will appreciate knowing there is a possible source for replacement pots.

Gentlemen,

I have a pretty nice black and white picture on the set now (with sufficient red gun contribution as well). Color circuitry not working at all, but we'll get to that later.

What I'm having trouble with right now is focus stability.

I modified the HV regulator circuit to regulate well at a lower voltage for the time being, since my HV supply is a bit low. That allows the HV to be stable and well regulated at around 15 kV.

However, the focus is still not stable at all, and varies wildly (from nice and sharp to so badly out of focus you can't see anything). Probing the focus voltage, I see that it is moving around quite a bit (easily a factor of 2 with scene changes). Convergence transformer is out of circuit, so it is not the problem.

That got me wondering about whether the focus control was in fact correct. I had noticed when I got the set, that a previous tech had added extra resistors both above and below the focus control, which I thought peculiar. Measuring the resistance of the focus control I find it is only 1 M instead of the 5 M called for in both the Sams and RCA schematics.

Then I checked the DC convergence control and find it is only 10 M instead of the 15 M called for in the schematics.

Can you take a look at the attached pictures (first three) and let me know whether the controls look original? Or were they both replaced at some point by a tech that took the time to get shafts of the right length with no evidence of cutting, etc.? Maybe he couldn't get the right values.

Much to my surprise I found in my junk box a NOS 15 M "high voltage" control, IRC type HV-15 (last two pictures). Always wondered what that would ever be good for. I may swap that in for the convergence control (although the 10 M in there seems to work fairly well).

What I'm really curious about is why my focus is all over the place. With the 1 M control in there, the load on the focus rectifier is a bit lower resistance than it should be -- 11 M versus 13 M (I have added 2 M above the focus control to get a usable range, so only 2 M missing overall from the chain). That doesn't really seem like enough to really make such a big difference. Obviously the focus range adjustment is much smaller than it is supposed to be, but that wouldn't appear to explain the instability.

Would having a low second anode voltage somehow cause the CRT itself to draw more current on the the focus electrodes and overload my focus supply, in a manner that appears to be scene dependent?

It also seems to like to focus with a fairly low focus voltage -- more like 1-2 kV instead of the 3-4 kV in the schematic.

The 1X2 tests good, and I've also swapped it for an NOS tube, so I don't think that's the problem.

Not quite sure what's happening here.

Those look (superficially) the same as the pots in my CT-100. I found a couple of old photos that may be helpful.

http://antiqueradio.org/art/temp/RCACT-100HVCagePots1.jpg
http://antiqueradio.org/art/temp/RCACT-100HVCagePots2.jpg

Whether mine are original, who's to say, although I have no reason to think that they are replacements.

Nice progress on this set, by the way. You work about 100 times faster than I do.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Tom, have you replaced the focus filter capacitor? The one on the wiper of the focus pot.

Yes, I replaced that with .01 uF 6000 VDC capacitor from ASC.

Re. the erratic focus, is there any chance there could be an open condition in the CRT socket, either on the lead going to the focus anode or the conv. anode?

The convergence pin showed a lot of white corrosion, which looked like evidence of poor contact in the past. So I shined that one up. I also tightened all the socket pins, since I have intermittent contact on the filament. Much better, but still fails to have proper contact on the filament once in a while when I first put the socket back on the CRT after working on the chassis. The focus pin doesn't look bad, but that in itself doesn't really mean anything. The convergence pin seems to be working, since the convergence control has a big effect on convergence, and the convergence is stable once set.

I do wonder a bit whether the focus pin is making poor contact. The way the socket is designed, it's a little tricky to get a wire in there to wrap around the CRT pin as an independent method to see if the voltage is really getting to the pin. Sounds like I should probably put some effort into that. Or, if you have any particular suggestion how to go after that possible problem, let me know.

I would use an ohmeter on RX1 to verify solid continuity up thru the focus lead to the socket lug. If good, then maybe re-flow the CRT pin (is it pin 6?).

I've learned quite a bit more about what's going on with the unstable focus, and I suspect all of this is related to the flyback being different than the set was designed around (fewer turns on HV winding).

I checked out the connections from the focus supply through the potentiometer and all the way to the CRT pin 6. All looks OK.

Observing the behavior, it was clear that the focus voltage was varying a lot with picture brightness. A little more observation revealed that the focus rectifier filament was rather dim since I had modified the HV regulator to bring the voltage down to 15 kV (which I had done hoping to improve HV stability with too little voltage coming from the flyback).

Right away I was able to observe that by increasing the HV, the focus rectifier would glow much more brightly. In other words, loading down the flyback by regulating the HV down also knocked down the focus filament current a lot. With too little current, the focus supply had too little current to remain stable with changing current draw by the CRT focus electrode.

On the other hand, if the HV regulator was set too high, so that it was basically allowing the HV to be unregulated, there was an equally interesting effect. Now, the filament of the focus rectifier would increase and decrease in brightness along with changes in the scene brightness. In other words, a varying load on the HV also caused focus instability.

It looks like the regulation has to be just right -- low enough to keep a constant load on the flyback with changing scene brightness, but not so low that the flyback is overloaded and the focus rectifier filament runs with too low current.

At the moment, I can't really find a sweet spot where the focus is totally stable, but it is far better than before. It's also a question of how much brightness and contrast this set is really designed to deliver. With relatively low contrast, I could probably get things stable.

Or quite possibly, with a proper flyback, everything would work much better. I'll give some thought to either adding more turns to the HV winding (which will be a bit of a project, requiring much more disassembly and modification of the flyback than I did before), or seeing if a voltage tripler in place of using the HV winding can work.

So far, this is all very educational as far as the HV system of the CT-100 goes!

Tom:

Very interesting analysis. One thing I discovered when I restored my set is the HV/Focus is a marginal design even with an original flyback. High brightness/contrast scenes can regularly cause the regulation to fail and focus to go out. At first I thought it was just my set but this was then confirmed by other owners to be normal. In order to provide a stable picture, the contrast and brightness need to be set lower than we are used to in modern sets. I do find it hard to believe that this is the way these sets were designed but it seems so.

It would be interesting to hear what everyone who has a working CT-100 is seeing for focus stability, brightness, and contrast. Mine is almost acceptable now that I've backed off the contrast, and set the HV regulation to around 18 kV.

What it cannot handle well at all right now is a white screen. Quite a few commercials these days include a white screen with some text and a small picture. When a scene like that comes along, I get total focus collapse, with the screen practically going blank in a cloud of fog. Almost all other "normal" scenes are OK.

I'll probably leave it like this for a while and take some time to work on the color circuitry, and come back to the flyback / HV issues later.

Tim is correct. "blooming" is common on CT-100's when large bright/white images appear on the screen. This due to the design of the HV regulation circuits. Keeping the brightness/contrast at their minimums while maintaining a viewable picture helps. The focus on my CT-100 does tend to jump in & out but is unrelated to the HV design. I believe I have a bad focus pot. Current back problems prevent me from pulling the chassis to confirm.:nono:

-Steve D.

Maybe the marginal 'stiffness' of the HV/focus supply is just part of the charm and challenge of using the set. Maybe sorta akin to using an early radio.

I haven't noticed any of the symptoms being discussed, though I'm lucky to have a tube with perfect emissions on all three guns so it's pretty easy to get a bright image without taxing the HV supply.

http://videokarma.org/attachment.php?attachmentid=173976&d=1334520695


Again, it's important to set up the horizontal and HV regulation circuits correctly. When doing a set, I always test the HV unloaded with the regulator disconnected to see where it tops out. My CTC-2 makes about 22.5kv, so I know it will easily make a stable 20kv with the regulator tube doing its thing. Other sets are not as good, my Wingate struggles to make 22.5 with no image on the screen and the regulator cap off. My CTC-4 on the other hand is absolutely leathal, I'm not sure where it topped out at since it overloaded my probe but WAG is around 38-40kv? They can be all over the map.

Tom: if the filaments are being affected too much, you might consider taking the filament windings off the flyback and moving them to a step down transformer. Take a 120v primary transformer from something small like a clock radio, and remove the secondary windings. Replace those with a few turns of HV wire and run that to the rectifiers, the current you save won't have to come from the flyback anymore and may just give it the oomph it needs to make respectable pictures again. You could test the theory by temping in some 1.5v batteries to power the rectifiers, if it doesn't help at least you didn't spend any money. Just a thought.

Good to hear that some CT-100s can produce a bright stable picture. Do you think the CRT is the difference? Normally I might expect a high emission tube to have greater danger of loading down the HV than a lower emission tube. Or maybe a tube with more usage somehow "wastes" electrons on something other than lighting up the screen?

Since the set is reasonably functional as a black and white TV at the moment, I'm looking into why there is no color. It looks like this will be a fairly labor-intensive bring-up.

The first problem I noticed is that the chroma reference oscillator does not run properly. It fires up for a few milliseconds, and then goes quiet for about 1/2 second. This cycle repeats continuously. I found that this behavior is occurring because as soon as the reference oscillator produces a signal, it causes the chroma sync phase detector to output a strongly negative signal, which has the effect of shutting down the oscillator (I'm not sure a negative signal to the reactance tube should cause the oscillator to shut down, but it does). Adjusting the tuning of the phase detector transformer has no effect. If I short the output of the phase detector to ground, the reference oscillator runs continuously. I haven't found any bad components under the chassis in the surrounding circuitry (except the 100 meg resistor in the color killer circuit, which I replaced), so I will check the coils and transformers next. Everything is recapped, including most micas at this point.

[new info] - When I short the phase detector output to ground so the reference oscillator runs freely, it turns out that its frequency is 2.47 MHz, not 3.58 MHz. Furthermore, it keeps right on running at 2.47 MHz if you pull out the crystal, and when the crystal is replaced with another 3.58 MHz crystal. So perhaps the tuned circuits do indeed need some attention. Will check that this evening. Maybe the phase detector is fine, simply doing what it does when the reference oscillator frequency is way below target.

Another problem I notice is that if I look at the output of the first video amp on the scope, the chroma sync burst on the back porch of the horizontal sync pulse is not visible. I gather this is probably a video IF alignment issue, causing insufficient bandwidth to pass 3.58 MHz? I suspect that's a big problem, so will do an IF alignment soon. Adjusting the fine tuning doesn't seem to provide any setting where the chroma sync burst comes through, at least not without a general massive increase in noise as I tune beyond the point where the black and white video looks reasonable. I imagine if I can't see the chroma sync burst, there won't be any chroma signal getting through either, so color is not going to happen even if the reference oscillator is brought to life.

Got the chroma reference oscillator fixed. It's an oscillator design that is new to me, and is quite sensitive to the value of a "choke" in the control signal from the phase detector back to the oscillator to adjust its frequency. Although Sams calls it a "choke," that is quite a misnomer -- it's actually really an inductor with a specific value affecting some RF response on the control line. Put in a coil much more closely matched to the original and got it running OK.

Still no color, however, so I ran some checks on the video alignment, and found things are not even close. This explains the lack of chroma sync burst on the detected video and so-so sharpness on the black and white picture (apart from focus issues). It appears that virtually every slug on this set has been fiddled with in the past. It's got traps in the middle of the passband, the wrong center frequency (by more than 2 MHz!), a generally peaked (rather than flat) and narrow passband shape, and worst of all, one of the little IF coils in the tuner pushed inside the tuner and damaged (core cracked and stuck). I'm repairing that coil -- nice that that top of the tuner comes off so easily for service.

Alignment looks like it will be very time consuming... and may uncover some additional problems.

Convergence transformer arrived today, so that is also ready for attention. Lots to do.

Are you aware of the chronic problem these sets have with video peaking coil failures?

I restored a 21CT55 CTC2B chassis and that inductor in mine had also failed. Like Electronic M says, the peaking coils are common trouble areas. Check all of your inductors and suspect all that are coated in a white cement like substance. It seems to corrode everything it touches.

The CTC2 and CTC2B are similar in a lot of areas. When I aligned mine I found it helpful to inject a color bar pattern at the 1st video amp (pulling the last video IF tube) and get the video and color problems fixed before getting a final good RF/IF alignment.

Dave

I came across lots of good info online about the failure of coils in these sets. I tested all those first thing when I started and replaced five that were open. I thought this particular one in the chroma reference oscillator circuit wouldn't be particularly critical in value, since it appeared to be an RF choke intended to pass only low frequencies. That turned out to be an incorrect understanding of how that oscillator is designed, so I guess I created my own problem in the process of replacing that coil. Once it was replaced with one close to the original value, the chroma reference oscillator started working.

The circuit is basically a phase locked loop, and usually the control voltage output by the phase detector would be filtered to remove all trace of the RF. Such is not the case here -- apparently a significant RF component appears on the control line and needs to be there for the oscillator to function properly. An unusual design to me.

I'll keep an eye on the remaining white coated coils in this set, since they are obviously subject to failure sooner or later as well.

Dave, thanks for the tip on feeding a composite video signal into the video amp; that sounds like a good "divide and conquer" approach. Since I can't get a nice video signal through the IF chain just yet, that might be a morale booster to get some color on the screen earlier in the process!

After a full evening's work on IF alignment, the video IF response looks better. Not an exact match to what Sams thinks would be ideal (rolloff slope on high end of passband is too steep), but hopefully OK?

The first photo is the bench setup. Second is the connection to the tuner I used for injecting the IF sweep for quite a few of the alignment steps. This has very short leads (less than 1" beyond the shield). The third picture is the IF response from injection point at the diode in the tuner through to the video detector. Although I don't show my markers here (they are not very photogenic on the scope), I have the scope carefully calibrated, with the center being exactly 44 MHz, and each division exactly 1 MHz. The null point on the low end is 41.25 MHz, and the top end goes to zero just under 47 MHz.

Alignment of this set is quite an undertaking compared to a typical black and white set, which I can usually align in a pretty short time. And there's still plenty more to do in the color circuitry.

Wow! I've never seen a response that excellent. No wonder those
sets give great pictures. Is a 21CT55 as good?

Yes, they have the same IF and are both a royal bi!tch to align if you aren't careful.

Well, today there were several steps forward, and one big step backwards.

Was still having trouble getting the chroma reference oscillator to lock properly, and then figured out that I had a grid emission problem on the 6AN8 in the oscillator. A voltage check found the grid of the reactance tube much too positive given what the circuitry around it was doing. After swapping that tube out (it looks fine on a tester), I was able to lock the chroma oscillator. At that point, I had lots of wild colors on the screen, and got busy with alignment of the chroma bandpass filter and various other tuned circuits in the color demodulators. Then worked on purity, which was only so-so around the edges (maybe I need a degausser?). Finally installed the new convergence transformer. While setting up to do convergence adjustments, I adjusted the width, height, linearity, to get a nice looking crosshatch on the screen. So far so good, and then...

I heard a gentle pop, and the picture went dark. Very quickly took a look at the flyback, and low and behold, the tape holding the 3A3 anode lead in place had actually caught fire! Obviously an arc occurred (maybe triggered by my changing the width setting?). Quickly blew out the flame, and then pulled out the flyback to see what condition it was in.

Quite surprisingly, all windings good, and no shorts. Good ringdown. But it would obviously arc again without repairs. I unwound a few layers from the outside diameter, and am coating with corona dope and varnish while I contemplate what to do next.

There were significant problems running things with reduced HV because of the turns I previously removed from the flyback, so simply getting back to where I was earlier today probably isn't a real solution.

If I add more turns to this flyback, I wonder if it will arc again? Doesn't look promising.

Maybe tomorrow I'll try a voltage tripler running of the primary of the flyback. If I get enough voltage that way, I can remove the HV winding entirely. If I don't, maybe I'll unwind most of the HV winding, leaving just enough to run the tripler?

I wish there were someone out there with a spare flyback or a junker chassis or set. That would be a much more straightforward solution. But there might not be any spares out there that people are willing to part with.

Another solution, one I would use, would be to disable the HV system
entirely and find somewhere an appropriate Spellman HV-in-a-box supply and
use that until the correct working flyback turned up. And the HV regulation
would be spot on as these thing are really well regulated. I used to have several
... not on inventory! ... but they got thrown out by the disposal Nazis around here.
You can't hide a Spellman supply, just set in down on the floor of the cabinet.

This might cause a focus voltage headache however, and would of course
reduce the load on the whole horizontal system a lot and that would have to
be taken care of.

Maybe something like a HV board from an arcade XY monitor would be an idea for sets with unobtainium flybacks and parts.

They've been building replacement boards and flybacks for these on and off for a while, and they have the focus divider built in. 19KV is 19KV no?

Had to take a few days off from this project. Too many radios/TVs/hi-fis came in and the shop was filling up. Got those done, so back to this set.

With the current condition of my flyback, I decided to follow up on this great idea that was suggested way back on page 1:

On newer roundys (like CTC-16 era) I've heard of folks having successfully removed the fly's HV winding and getting their HV from a solid state HV trippler the input of which being connected to the HO Tube's top cap....This may also be a solution for your CT-100.

...

Good luck fixing it.

Gave it a try this evening. Pulled the TCG523 tripler that has been serving very nicely in my Philco 48-2500 projection TV, and wired it up to the CT-100 chassis. I've got the HV winding of the flyback coated with a relatively thick coat of insulating red varnish, with the end of the HV winding buried in the varnish. That seems to be working OK to keep the HV winding from arcing right now. If this ends up being a usable solution, I'll coat it even better, or possibly remove it altogether.

The first picture shows the tripler on the bench temporarily wired up to the chassis. The input of the tripler is connected to the top of the primary -- the same place the 6BG6 plate is connected. The ground connection of the tripler is connected to the chassis, and the HV lead is resting against my HV probe.

Fired it up, and got 26-27 kV at the tripler output (see second picture). Putting the focus rectifier in place to provide at least a little load for the flyback lowers it to around 24 kV, which seems like a really lucky break. Just about right. The focus rectifier is wired as it originally was, off its own tap on the flyback, and powered by its own filament winding. The 3A3 HV rectifier is removed, and its filament winding not connected to anything.

Connected the tripler output to the HV regulator circuit in the set by touching the HV lead from the tripler to the metal housing of the 3A3 socket. The HV regulation works fine. If I set it to 20 kV, I get a mild red glow from the inside of the 6BD4 regulator (similar to what I had before at lower voltages when I was using the reduced-output flyback). Third picture shows the setup and the HV probe reading. I can adjust the HV from 16 kV to 22 kV with the CT-100's HV adjustment pot.

This looks like it may be a viable solution for dealing with my bad flyback. Will try it with the CRT next.

By the way, don't pay too much attention to the way I've got the replacement convergence transformer hanging off the back of the chassis. I will pot it in the original transformer housing once a few other issues are out of the way here.

Well, here's what I see on the CRT when using the tripler. Looks like there is plenty more work to be done on the color demodulator and video driver circuitry. Convergence is also pretty bad, so some serious work needed there as well. Anyway, this seems to be a step forward. My education in color TV continues...

Hi Tom, This looks like it's going to work!

So do we have any ideal of the circuitry involved in one of those HV tripplers? I'm assuming there are a couple switching transistors? Can't be too much going on in such a small package.

It's a classic half-wave voltage multiplier composed of a ladder of capacitors and diodes. A schematic can be found here: http://www.datasheetarchive.com/dlmain/Datasheets-X2/DSA879000-27.pdf . Look for the ECG523 diagram near the top of the page.

Now I remember seeing those schematics in my old ECG book. Amazing to think how something so common today, as a few silicon diodes, were keeping this circuit from being used back in the day.

6BG6 is a typo . . . isn't it? :worried:

Does 6CD6 sound better? 6BG6 was indeed a typo. Just finished a black and white set a couple of days ago and the two tubes look similar.

Spent some time this evening on alignment of the color demodulator. Followed the RCA manual for the Chroma AFC, Chroma bandpass, and Color Matrix adjustments. Fortunately I've got an old Sencore color bar generator that generates the I and Q type pattern the RCA manual instructions are based on. Had to spend some time with the Sencore and a modern color set first to get the color bar generator working well enough. After alignment, it looks like this CT-100 can produce some decent looking color.

Purity is not good (see first pic), regardless of adjustment of the cross purity adjustment and field cancellation. Best is with no current in the purity coil (unplugged, or adjustment fully counterclockwise). Would you guys say this needs degaussing, or something else?

If it needs degaussing, can the field cancellation coil be drafted into service as a degausser with a variac, or is that coil capable of too little current and field to do the job? (I also don't want to burn it out, of course.)

The remaining pics are simply showing that we now have reasonable color.

Convergence still has big problems, so that will be a project for another day.

Overall, we're getting closer... :)

Great work Tom :thmbsp:

looking good. Not a expert on the CT-100 but I always start with a good degaussing. On sets that have them I start by fully retracting any perm magnets that may be used for edge purity (my CTC-5 had those). then use on of the large old school degaussing rings.

Thanks to a tip in a PM from one of our friendly VK members, I was able to get the purity much better (see first picture). What I had not yet tried was changing the position of the yoke along the neck of the CRT, and that helped a lot in terms of cleaning up the top edges of the raster for red purity. Now the color rendition is acceptable over the entire screen. The small amount of pink in the upper right corner is not easily noticeable in a normal picture.

I tried using the field cancellation coil as a degausser to see if any further improvement could be made. After all, the cancellation coil does bear some resemblance to the automatic degaussing coils built into newer sets. The wire size looked like it could handle an amp with no problem, so I applied AC current up to 1 amp (about 40 volts AC) and then backed off slowly. This had no effect on purity afterwards. It probably takes a lot more current than what I applied (automatic convergence coils give a nice buzz for the moment they work, and this coil never got into that regime). In any case, I'm not interested in testing the limits of the cancellation coil, since I might damage it, and the purity is actually pretty decent now anyway.

Worked on the convergence a bit. Horizontal convergence is working properly, and I was able to get good convergence across the horizontal axis (see second pic). The vertical convergence is not working properly -- adjusting both the vertical convergence amplitude and the vertical convergence shape controls has very little effect on the screen. Also the "smile" when probing at the plate of the convergence amplifier looks possibly inverted. I wonder if I have the phase of the new convergence transformer reversed? There may be some other problem in the vertical convergence circuitry, so I'll look into that further next time I get a chance to work on the set.

I also mounted the new convergence transformer inside the housing in which the old transformer was potted, and have positioned the solid state voltage tripler module inside the HV cage (not permanently mounted yet, but it soon will be). Also adjusted the range of the DC convergence control by adding 5 megohms on the top side (for some reason my control is only 10 megohms as opposed to the 15 megohms on the schematic; adding this resistor get the sweet spot properly within the range of the control).

I also made a slight modification to the focus control circuit. I'm still having loss of focus on very light or white screens. There isn't any evidence that this is caused by loss of regulation of the HV, so I think it is the focus voltage that is sagging or otherwise changing in high brightness scenes. I adjusted the range of the control (it is only 1 megohm as opposed to the 5 megohms on the schematic, so its range is really limited) and also increased the resistive load on the focus rectifier to both improve regulation and pull the voltage down into a range where good focus in properly centered in the range of the control. This may have improved focus stability, but I am not sure yet. It still won't stay in focus on the very brightest scenes. Nonetheless, it is watchable at this point.

"Oh the tenacity!" Seriously, I am envious (not jealous) and very happy to be able to follow this thread. So very few of these sets out there. My only "roundie" is a CTC-9 that is in the queue. I only hope that I can be as persistent and have good results.

Thanks for taking the time to do the work and share it with us.

Here is a tip on how to obtain a cheap degaussing coil if you ever feel the need to have one. Step one: Find a 'modern' CRT set on the curb or grab a 1$ thrift store special. Step two: Remove the internal degauss coil from it (sets with screens over 20' tend to have good degauss coils that can handle line voltage for a few minutes). Step three loop the coil into a ~16" ring, wrap that ring with electrical tape, and connect a line cord to the leads of the coil (make sure the cord is over 7' long if you do not want to use extension cords). Step four: test it out. If it does not get too hot to hold in ~4-9 minutes it should be good enough to get the job done.

Great idea, Tom. I would suggest adding a series on/off thumb operated switch in series with the AC line, as it is useful to be able to turn the coil on and off with the flick of a finger.

Here is a tip on how to obtain a cheap degaussing coil if you ever feel the need to have one. Step one: Find a 'modern' CRT set on the curb or grab a 1$ thrift store special. Step two: Remove the internal degauss coil from it (sets with screens over 20' tend to have good degauss coils that can handle line voltage for a few minutes). Step three loop the coil into a ~16" ring, wrap that ring with electrical tape, and connect a line cord to the leads of the coil (make sure the cord is over 7' long if you do not want to use extension cords). Step four: test it out. If it does not get too hot to hold in ~4-9 minutes it should be good enough to get the job done.

Or, there are several coils available on the bay: http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1313.TR0.TRC0.XColor +TV+degaussing+coil&_nkw=Color+TV+degaussing+coil&_sacat=0&_from=R40. Prices vary. The larger coils work best.

-Steve D.

Thanks for the tips on the degaussing coil. I checked eBay a few days ago, and only saw one for $39, which I thought was perhaps a little steep. Buyers for degaussing coils these days are probably few and far between! Anyway, there is certainly a nice selection of degaussing coils today, so I decided to order one.

I've parted out a few color sets over the years, and have always pulled the degaussing coils as a very useful source for a lot of nice magnet and coil wire in a very accessible form. I may have one of those fully intact somewhere, but I guess it will be nice to get one of the commercial degaussing coils instead of trying to make use of an old coil from a junked color set. Certainly sounds workable enough, though.

I've been making some measurements of what's happening when the focus goes out on bright scenes. I'm becoming convinced that the problem is focus voltage instability, not a HV regulation problem. The HV hardly budges, while the focus voltage changes by several hundred volts out of about 2.5 kV. Does anyone know of a solid state replacement for the 1X2B? I may also try a string of a bunch of 1N4007s just to see if a lower impedance rectifier would make much difference. I suppose I should check whether the voltage at the output of the 1X2B (as opposed to at the focus output to the CRT) is stable before worrying about the rectifier itself. It's possible that the voltage dropping potentiometer and resistors are the problem, driven by a change in current on the focus grid. The circuit looks like it was designed with the assumption that the focus grid would have little or no current flow, but I certainly seem to have enough to cause a problem.

I've been hearing from some of you that defocusing on bright scenes is par for the course on these sets, so this experimentation with focus stability is perhaps really more about understanding the design limitations of the set, and not true restoration, I suppose.

I'm also wondering if there is just a touch of gas in my CRT, which may be having some subtle effects.

Still working on the vertical dynamic convergence -- that remains one trouble spot with the set. It's watchable, but would be nicer at the bottom of the screen with better dynamic convergence.

Well, we've gotten far enough along on this that it looks like electrical restoration will be a success, so I guess it's a keeper. :)

Next I'll work on the cabinet. I won't post a blow-by-blow on the cabinet work, but I thought it would be nice to document the original as-acquired cabinet condition. Pictures attached. Will post some more when the cabinet is done.

A few more pics of original condition. Note some damage to the feet on the right side.

Last few pics of original condition.

Does anyone know if a decal set is available for this set? Would be nice to restore the front panel control decals.

I also don't have the original main knobs for the front. I imagine those will be hard to find. Any tips on a possible source would be greatly appreciated. I believe I saw an NOS tuning knob on eBay a couple of years ago. Didn't know I would ever need one then...

I've been hearing from some of you that defocusing on bright scenes is par for the course on these sets, so this experimentation with focus stability is perhaps really more about understanding the design limitations of the set, and not true restoration, I suppose. I've had the same question/supposition and have observed that the blooming/focus problem occurs on modern video, whether cable or DVD, especially commercials and DVD menus. There is no CT-100 focus anomaly ever with vintage video DVDs that were originally generated with the TK-41 (early Laugh In, for example) or restored 3-strip Technicolor movies. It is my contention that the TK-40/41 and the CT-100 were designed in concert, one with, and for, the other. And that well may be why they can't handle today's dynamics.

If that's correct, it's similar to the copy protection blocks that appear on the CT-100 screen but not modern TVs. Designers blanked out retrace lines on the CT-100; how could they have imagined a need to blank brute copy-protection!

Pete

Does anyone know if a decal set is available for this set?I had CT-100 decals made by Scott Wright at Solidesign:

http://www.solidesign.bizland.com/decals.html

I looked through my box of CT-100 stuff and I guess I donated my last spare set. If you contact Scott, he can print you a new sheet from the old art file. He prints as many sets as will fit on one sheet, so then you will have some spares to share with future CT-100 owners.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

I believe it's less this above, and more the fact that anything taken off the camera and then sent through any kind of image processing, digital, or analog, can take the image and expand it's dynamics to the limits of the image window for transmission..... For example, while the camera may have photographed a dark scene, meant to be black, it may not show up as black unless it's tweeked a little by a video processor.... Photoshop also allows this for stills, it's called adjusting Levels.... Almost like adjusting the contrast so some of the grays are lost....

In a video, if the blacks are extended below black, they can effect sync, if white is extended beyond white, you get some noise in the sound...

Most likely anything sent over a live camera feed keeps everything out of the too black, or too white areas, Most likely the norm back in the day.... Today the black, and especially white in DVD menus are not photographed, but generated, and un naturally sent whiter than white, causing noise, blooming in that older set...

I guess I donated my last spare set.

I still have them right here next to me, I don't think I'm going to use them after I saw what Bob's Halicrafter's resto looked like. You could really see the outline of the decal against the wood, it just looked out of place to me. I guess I can send them along to Tom, same price I got them for. :thmbsp:

Hi miniman82,

I was just about to send off an email to Solidesign, but if you're willing to send me a set, I'll do my best to put them to good use. Great to see how folks here share resources and tips.

Quick question here for anyone who has worked on these or faced a similar situation on a different set -- Is there a preferred way to get the safety glass off the plastic CRT mask? Looks like the original rubber or expanded foam seal liquefied at some point and ran downward a little, and then very helpfully turned to stone with quite good adhesive properties (see picture). I'm hoping not to damage the paint on the front of the mask, which might happen if I simply try to pry it apart. Maybe a little (but not too much) heat?

Tom

Hi miniman82,

I was just about to send off an email to Solidesign, but if you're willing to send me a set, I'll do my best to put them to good use. Great to see how folks here share resources and tips.

Quick question here for anyone who has worked on these or faced a similar situation on a different set -- Is there a preferred way to get the safety glass off the plastic CRT mask? Looks like the original rubber or expanded foam seal liquefied at some point and ran downward a little, and then very helpfully turned to stone with quite good adhesive properties (see picture). I'm hoping not to damage the paint on the front of the mask, which might happen if I simply try to pry it apart. Maybe a little (but not too much) heat?

Tom

I have had to remove several safety glass plates from RCA plastic crt frames which were stuck together from the foam rubber gasget that was used. I found that if you saturate the rubber with WD40 or mineral spirits the rubber will start to get gooey and then it will be much easier to seperate the glass from the crt frame. AFter seperating the 2 pieces, I use mineral spirits and fine steel wool to clean the rubber off the inside face of the glass. The plastic frame is a bit more difficult to clean. I use a plastic "Bondo" spatula as a scraper to remove the bulk of the gooey rubber that was softenend by the mineral spirits. Any type of soft plastic will probably work so as not to damage the crt frame. Then when you have the bulk of the rubber removed from the crt frame, use a rag saturated with mineral spirits and rub the remaining rubber residue from the crt frame. I even had good results doing this on a CT100 crt frame and did not do any damage to the painted areas.

I did use the heat method on a CTC4 safety glass once, but the solvent method above works well if you are patient and there is less risk of breaking the glass.

RE: the decals issue. It is my understanding that if you are doing a complete refinish and you place the decals on wet varnish, and then apply several more coats to the cabinet, that the decals will be less visable. I have yet to try this method so I cant vouch for it. But Nick is correct, if you apply the decals on the surface, the carrier of the decal will be VERY visable.

There are various setting solutions that model builders use to brush over the decal after it's been first set. The solution softens the film, allowing the decal film to pull down tight against any surface irregularities when it dries, which eliminates voids under the film that show up as white spots when the film dries. Hiding the film edges becomes a process of applying many coats of clear finish over the area - enough so that the edges can be feather sanded in. There are various
thicknesses of decal films available too. However the thinner ones will be much more fragile to apply. I use to use this stuff back when I was into model railroading when I was younger. It was really amazing how many irregularities you cold conform a decal over.

I think during manufacturing that they used the silk screening process. There's no way they could have taken the time to apply individual decals, unless it was for a prototype.

On both my CTC-4 and CT-100, I heated the frame/glass with a pair of hair dryers. Slowly apply pressure to pry them away from each other, they should come apart with little effort. Bob's right, it's a PITA to get the crap off the frame. I used a razor on the glass and some goo gone, that worked fine. But I ended up damaging a little of the paint on the mask scraping it off; I guess I always meant to repaint the damaged areas, but apparently it hasn't bothered me enough to take the thing back apart to fix it. Most of it is out of sight anyway, but some of them do drip down a ways and are more bothersome.

Thanks for all the advice on getting the black crud off. I looked over all of the above, and ended up using the following procedure which worked well:

1. Heat the edge of the glass with a heat gun, gently prying the glass up with a screwdriver and inserting little spacers as I proceeded along. It didn't take much heat -- perhaps about 100-120 degrees F was enough to get things apart. Whole operation took less than 5 minutes to get the glass off.

2. Used a razor blade to get most of the crud off the glass. No problem. A little residue left behind was easily removed with paint thinner and a rag.

3. I didn't have any mineral spirits on hand, but tried paint thinner on the crud on the mask side. I found it is a very marginal solvent for the crud, and using paint thinner alone would have taken a very long time and a lot of rubbing (mineral spirits might have done better). What worked very well was to wet the surface with paint thinner, and then use a razor blade to carefully scrape the crud off, rewetting things as material was removed. This is probably much safer than trying it dry. The paint thinner softens the crud slightly, and more importantly, lubricates the surfaces and the razor blade, so you have a nicely lubricated sliding interface for scraping the crud off. This step took maybe 15 minutes, finished up with rubbing with a cloth wet with paint thinner.

All clean, and no scratches.

------------------

On the decal subject: On previous radios and TVs I have restored, I apply the decals between coats of clear finish. The result looks perfect, with not a hint of visibility for the decal film. I have not found it necessary to apply the decal with the clear finish wet. I simply let it dry, and apply the decal using the usual water float method with a tweezer. After the decal dries in place, the next coat of clear finish can be applied. A few coats over the decal with light sanding in between gets rid of any trace of a hump from the thickness of the decal film.

Back to cabinet stripping...

Wow Tom, at this rate you'll likely be done with the set by Christmas! way to go!

Thank you for the decal note, Phil. I will need a set when I eventually have my CT-100 (re-)refinished. It has no decals now.

Been very busy with the cabinet refinishing for past couple weeks. Lots of dents to steam out and/or fill, plywood and veneer repair on the feet, etc.

Anyway, I've got the entire cabinet grain filled and stained, and wanted to confirm that the very bottom of the sides should be substantially darker than the rest of the sides -- everything from the horizontal veneer cut downward (basically the last 3-4 inches at the bottom).

How about the bottom skirt on the front? Should that also be a shade darker than the rest of the front? That was not easy to tell on mine, since the original finish was in quite poor shape at the bottom.

I'll darken the bottom areas further once I know whether that is correct.

I'll be happy to defer to the experts, but my cabinet is a uniform color from top to bottom. Of course, now it has been refinished, but I just looked at cabinet photos in its as-found condition and there's no indication that there was darker toning anywhere near the bottom (or anywhere, period).

As you've probably noticed from examining interior areas protected from sun fade, the original color was rather reddish.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Well, a sample of one, but mine is not noticeably darker at the bottom... In fact, in front, the feet and area between them under the speaker area are slightly redder than the rest of the cabinet.

jr

the feet and area between them under the speaker area are slightly redder than the rest of the cabinet.That's consistent with that area being somewhat shaded from UV exposure. If you disassembled the cabinet and looked at finished edges never exposed to sun, you might be surprised how red they look.

Phil Nelson

That is indeed true, but there may also be some color differences in the base material as well, as the color changes are quite abrupt and on the "seam" lines.

jr

In that case, maybe they were originally somewhat darker and my cabinet was so faded that it all looked the same. I'm working with sample of one, too :)

Phil Nelson

Thanks for the feedback -- sounds like both of yours are uniform, and were thought to be that way originally as well.

Anyone else? Take a look at the first picture in my post at the top of page 7. That's what's making me think the bottom should be darker. In any case, I haven't done anything yet to make the bottom darker, so if the consensus is that it should all be the same, it will save me a step.

Here is an example of what I mean... The piece coming down on the right side is quite brown (almost like walnut to my eye) Below the speaker area Reddish brown, like *most* of the cabinet. Below that more reddish, except the foot is a little more brownish. I hope that it was a little more uniform when new! This set is un-restored (obviously) pretty much "as found" in 1971.

If I may say so, my set has about the nicest original finish of any I've ever seen. The entire surface looks uniform to me. http://tvontheporch.com/images/picture_1879_11_290x359.jpg (http://tvontheporch.com/images/picture_1879_11.jpg)

Well, that all looks pretty convincing. I suspect mine may have been modified at some point -- maybe a partial refinish of just the side feet areas or something like that. I'll go with uniform color on the new finish.

Some pretty nice looking CT-100s out there. Thanks for posting!

the foot is a little more brownish.You may be seeing normal assembly-line variations. The foot is likely a different type of wood than the veneered front. Manufacturers used toner rather than stain because it did a better job of giving an even color on both the expensive veneer and the cheaper structural pieces. (Plus, toning lacquer dried very quickly, unlike stain.) Wood is a natural product, and not every piece of the same type of wood is identical.

Cabinets were made on an assembly line by humans, not robots. On ARF someone posted a video of RCA TV production in the 1950s. In the finishing segment, the cabinets are on a moving conveyor and the lady sprays on lacquer with a hose that looks like it could cover a school bus in a few seconds if she opened the nozzle a little more. It's remarkable what a good job the workers (generally) did under such production pressure. As you probably know from finishing, even a moment's inattention can produce unevenness, and different kinds of wood may still look a little different under a uniform coat, owing to differences in the tightness/depth of the grain and the way it takes the light at different angles. Then you need to factor in the unpredictable effects of UV fading over a period of decades. I have seen a photo of a CT-100 that sat in a sunny store window for years, and its color looked nothing like an original.

Phil Nelson

Indeed! The two sets at the Early Television foundation illustrate this:
http://www.earlytelevision.org/images/2_cabinets.jpg
jr
http://www.earlytelevision.org/rca_ct-100.html

I'm more of an electronic guts type of guy than a cabinet refinisher, but here are cell shots of what I like to think is pretty much a pristine CT-100.

shots 13-12-22 13.15.10 and 13-12-22 13.15.42 taken on the right side of the cabinet clearly show a dark area below the seam. But the dark clearly begins below the seam. The area was illuminated with a 6500 degree K lamp and looks the same by eye as in the photo. Phil is probably on the mark.

shot 13-12-22 13.17.38 is of my operational CT-100; it has an early history of poor storage environments. It too shows a darker lower member although it is harder to tell compared to the other set.

A new observation for me: In both cases it is evident that the grain pattern flows from the veneer side panel to the lower member. Perhaps you can detect it in the marginal cell photos. Did they use the same sheet of veneer to cover the side panel and hardwood frame members? Seems a bit extravagant, but perhaps a quality damn-the-cost practice was invoked because of the status of RCA's first color set?

Pete

Actually, it's a continuous sheet of plywood down the sides all the way to the bottom of the feet. The horizontal line is nothing but a shallow cut through what is otherwise continuous veneer.

OK, so I guess some sets had a darker finish at the bottom, and some didn't. Mine is like Pete's, except that the border between lighter and darker coincides more with the horizontal cut on mine.

Any correlation to build date, mine is fairly late (B80036743)... perhaps they simplified the toning process on the later sets? :scratch2:

jr

Any correlation to build date, mine is fairly late (B80036743)... perhaps they simplified the toning process on the later sets? :scratch2:

jr

Jr. Doubt the build date had anything to do w/the final finish on the CT-100 cabinets. Probably more to do w/the skill of the person applying the toner/lacquer or even the lateness of a shift and a tired employee. Much the same as any mass produced product during the pre automation era & tireless robots of today. Many RCA cabinets & chassis were pulled from the line during final inspection for repair.

Photo courtesy: Dave Arland

-Steve D.

For what it's worth, mine is B8000952, cabinet #797. So that may be a relatively early one, and it has the darker area at the bottom. I decided to go ahead and darken that area again, now that I know it's not likely some kind of modification after manufacturing.

The first two pictures in my post above are of one of the last CT-100's built; the third picture is of one of the first built. So we're all over the place with build dates.

Pete

I just checked my 100 here in Wisconsin (Cabinet number 20) and it too is darker below the seam. My other two 100s are in California so I cannot check them for a while but if I recall they too are darker below the seam.

Steve

I never noticed it before but mine is also slightly darker below and beginning at the seam. I have Cabinet #951.

It sounds like your situation is not so much different than mine -- various equipment that should do the job, but nothing that you've used often enough to really have confidence in its accuracy.

:) Makes me feel better reading that!

This is a great thread. I continue to learn a lot by reading the travails of other collectors. Thanks to everyone involved for making me less ignorant. Some day I may even tackle one of my color sets...

Happy New Year!

-- Dave Sica

:) Makes me feel better reading that!

This is a great thread. I continue to learn a lot by reading the travails of other collectors. Thanks to everyone involved for making me less ignorant. Some day I may even tackle one of my color sets...

Happy New Year!

-- Dave Sica

Ditto.... I think everyone can say that....

I've been busy refinishing the cabinet for several weeks. In the future, I'll remind myself not to refinish things in the middle of the winter, since the drying time for every step is much longer -- even in our relatively mild California winters. Often had to set the cabinet outside in the sun to move things along!

http://www.videokarma.org/attachment.php?attachmentid=181363&stc=1&d=1389904925

http://www.videokarma.org/attachment.php?attachmentid=181364&stc=1&d=1389905245

Overall it came out pretty decent. I'm not sure the color is exactly right, although it's hard to know exactly what the original color was on these.

This was a labor-intensive cabinet restoration, with plywood and veneer repairs in addition to the usual refinishing. I'll give some more details about that below in another post.

Here are some shots of the work in progress:

1. Main cabinet ready to strip (front moulding and speaker grill slats removed):
http://www.videokarma.org/attachment.php?attachmentid=181365&stc=1&d=1389906434

2. Front moulding and speaker grill slats ready to strip:
http://www.videokarma.org/attachment.php?attachmentid=181366&stc=1&d=1389906434

Removing the moulding and speaker slats made stripping and sanding much more straightforward. Luckily, they came off easily without any damage.

3. This pesky gap in both of the top front corners caused me a fair amount of grief:
http://www.videokarma.org/attachment.php?attachmentid=181367&stc=1&d=1389906434

First I tried clamping and gluing. It required so much force to close it up that it simply came apart again after unclamping. I settled on lighter clamping and gluing, which didn't really close up the gap, but would hopefully stabilize it. Then I filled it with wood filler and sanded back. Unfortunately, it still cracked open a little during subsequent finishing steps and required some rework. Came out reasonably OK, but not perfect.

4. Plywood repair for rear foot:
http://www.videokarma.org/attachment.php?attachmentid=181368&stc=1&d=1389906434

This rear foot was badly damaged, so the original plywood was removed and replaced with new plywood. Although this picture shows some veneer remaining above the new plywood up to the groove, I later removed that as well, and put a single new piece of veneer in place from the bottom of the foot up to the groove, with only a vertical seam against the existing veneer.

5. Rear foot after plywood and veneer repair:
http://www.videokarma.org/attachment.php?attachmentid=181369&stc=1&d=1389906434

6. Veneer repair for front foot (the plywood was still usable on this foot, with a little wood filler to fill in some missing gaps):
http://www.videokarma.org/attachment.php?attachmentid=181370&stc=1&d=1389906434

You can also see above that there is substantial wood fill on the front of the front foot. They were badly rounded and beat up from this set being moved (or shall we say "dragged across the pavement by idiots") way too many times over the years.

7. All stripped and ready to stain:
http://www.videokarma.org/attachment.php?attachmentid=181371&stc=1&d=1389907085

8. Stained and ready to reassemble:
http://www.videokarma.org/attachment.php?attachmentid=181372&stc=1&d=1389907085

The stain I used was Zar brand "Dark Mahogany" followed by "Cabernet." The Dark Mahogany alone was not sufficiently red, and adding a coat of Cabernet got it to where I wanted it. I like Zar stains because they do not overemphasize the grain, giving a final appearance closer to the original stain + toned lacker used by RCA.

9. Masking in preparation for painting the gold stripe on the front edge of the front moulding:
http://www.videokarma.org/attachment.php?attachmentid=181373&stc=1&d=1389907085

10. Gold strip painted using Rust-o-leum "Aged Brass" spray paint:
http://www.videokarma.org/attachment.php?attachmentid=181374&stc=1&d=1389907085

In retrospect, I think "Aged Brass" isn't really dark enough, and I would suggest using "Antique Brass" (a little darker color also available) to get a closer match to the original.

11. Gold stripe done:
http://www.videokarma.org/attachment.php?attachmentid=181375&stc=1&d=1389907085

Tom,

Looking really good. That one troublesome corner was probably warped and no amount of clamping & gluing would bring it back to true. I wish I had had the knowledge I have now when I refinished my CT-100 years ago. Much warmer here in So. Ca. this winter.

Best,
-Steve D.

Multiple coats of clear finish were applied. After a couple of coats, the following two steps were done.

12. Wall of hole on side of cabinet (for Focus and Convergence controls) painted with very dark brown paint to cover up the grain and plywood structure:
http://www.videokarma.org/attachment.php?attachmentid=181376&stc=1&d=1389907522

13. Decals (Thanks, Nick!!) were applied:
http://www.videokarma.org/attachment.php?attachmentid=181377&stc=1&d=1389907522

After more coats of clear finish, with sanding in between, the edges of the decal are almost invisible on the finished cabinet.

So there you have it... That's what's been keeping me busy for a few weeks (in addition to Christmas travel, job, and various radio and TV restoration projects for other folks on the side).

Now I can reassemble things and get back to work finishing up a few remaining electronics issues with the chassis.

Beautiful work Tom! It probably looks pretty close to what it did when it left the RCA factory.

Steve

Looking good!

Today's nit: there should be a small gold dot between OFF and VOLUME. The guy who made my decals accidentally omitted the dot; my refinisher added it back with a fine artist brush and gold paint.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Hi Phil,

I probably won't bother with the gold dot, since I don't even have knobs for my front panel controls! I suspect my CT-100 will be known as "the knobless one." :)

On behalf of a number of people who have benefited from the decals you originally had made, thanks very much for making them available.

This looks really cool.

Just looking at the photos transport me to a time before I was born.

Life was so much simpler when this set was new.

Cabinet is done now, except for the brass frame around the safety glass, which I'm still contemplating what to do with. If I polish it, it's hard to get it back to the right "antique brass" color.

The set is reassembled and working reasonably well. After fixing a problem with the vertical dynamic convergence, I can get the convergence pretty close:

http://www.videokarma.org/attachment.php?attachmentid=181404&stc=1&d=1390199103

Here's a closeup of the lower left area:

http://www.videokarma.org/attachment.php?attachmentid=181406&stc=1&d=1390200291

As you can see, the bottom left area is not quite fully converged. The interaction between the vertical and horizontal dynamic convergence controls makes this all rather tricky. The convergence directions in both Sams and the RCA Service Manual offer only limited advice. One thing I notice is that although there are three horizontal convergence adjustments, the directions only mention two. The third, in the high voltage cage (at the top) has a big effect, but I didn't see any instructions on a procedure for adjusting it.

Does anyone have a clear understanding of what each of the three horizontal dynamic convergence adjustments really does? Any tips on further improvement of convergence?

The picture is pretty watchable now, although color tracking is marginal. Although all three guns test very good on my CR-70, the red gun does test a little weaker than the other two, and this becomes very apparent when adjusting the background, drive, and screen controls. The red screen is set at max (about 440 volts on the red screen), the other screens near minimum, the green drive is set at minimum, and no amount of fiddling with everything else can really get the tracking to be excellent.

Here's a screen shot where things look reasonable normal as far as color goes:

http://www.videokarma.org/attachment.php?attachmentid=181405&stc=1&d=1390199103

You can age polished brass with gun bluing liquid from a sporting goods store. You can also buy proprietary stuff for the same purpose, but I don't know whether that has any real advantage.

I was shown how to do this by a guy with an antique lamp restoration business. The brass should be clean -- as in alcohol-wash clean -- and handled only with forceps until the aging is done. It's easier to do with small pieces that you can dip. For long pieces, I guess you would brush or spray it on. As soon as the brass reaches the desired shade, you wash off the solution.

That convergence looks pretty good. If there are secret tricks for making it perfect, I'd love to learn them. I don't think any of my old color sets has ideal convergence.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

that sets looks as good as it did when brand new!i saw several of these back in the 50s and they never had convergence that close.even new right out of the box!you did a beautiful restoration on this.post more pics

Agreed amazing job on this set! Clearly a labor of love

Trying to obtain "perfect" convergence on these vintage sets will drive you nuts.
You have to know when to walk away.

-Steve D.

Three signs that tell me when the convergence is good enough:

1. After crouching for hours, my back feels like someone stabbed a sword into it.

2. The air is thick with foul language.

3. I hear voices from the other room: "Are you still fiddling with that? We're sitting down to dinner!"

:)

Phil Nelson

Will those little magnetic paper strips help with the corner convergence?

Tom, your work on that CT-100 is fantastic, a real gift to the early television community.

I would not have known that you could disassemble the cabinet and especially the speaker grille that way. Your results give me high hope for improving my own CT-100 in the future.

I've been enjoying watching this set over the past few days, but there was one major shortcoming -- the focus stability. I could adjust the focus for a poor compromise: slightly out of focus for a dark scene, nicely in focus for a medium bright scene, and completely blown-out of focus for a bright scene.

Measurements showed that the problem was not insufficient HV anode voltage regulation, but rather unstable focus voltage. The focus supply is completely unregulated, and unfortunately it seems that the current drawn by the focus grid of the CRT varies quite a bit with scene brightness.

Looking at the circuit, I got the idea of using a high voltage zener diode to stabilize the focus voltage:

http://www.videokarma.org/attachment.php?attachmentid=181432&stc=1&d=1390445303

I replaced the resistor below the focus pot with a 3000 volt zener. Or more precisely, a string of 15 Z4KE200A 200 volt 1.5 watt zener diodes. Here are a few pictures:

http://www.videokarma.org/attachment.php?attachmentid=181430&stc=1&d=1390445303

http://www.videokarma.org/attachment.php?attachmentid=181431&stc=1&d=1390445303

Somewhat to my surprise, this is a fantastic fix. The focus goes from being completely problematic to rock solid. I can turn the brightness and contrast up all the way, to the point the picture looks very bad in other respects, but the focus stays sharp under all circumstances!

Now I realize this isn't really "preservation," but given that I have had the experience of seeing how the set works in its original state, I would say I have experienced that enough, and now I'm ready to watch a set that really works.

Given that my high voltage cage doesn't look particularly original any more because of the big voltage tripler module, this little insertion of a string of small zener diodes is hardly noticeable, and of course is easily reversible.

Great idea!

Genius!

How creative. May I ask what gave you this idea?

Phil Nelson

Well, a zener diode is basically the simplest kind of voltage regulation circuit, quite useful when a fairly limited range of current supply is needed. I was curious how high a voltage one could find in a single zener, and generally found that common ones top out at 200 volts. Since they are quite inexpensive, putting a bunch of them in series is still quite cheap, and they don't take up much space.

I've used zener regulation in battery eliminator power supplies I've built for 1920s radios and later tube-type portable radios, so I'm generally familiar with their usage for this purpose.

The current in the focus circuit is very low, so not much power dissipation is needed. Using 1.5 watt rated diodes is complete overkill, but there was no real cost difference between these are lower power ones, so I went with these.

I also bought some 4 kV rectifier diodes with the idea of putting several in series to replace the 1X2 focus rectifier tube in case that was also needed. But so far, it seems to be fine with the original vacuum tube rectifier, so I'll leave that in there in the interest of keeping as much of the vintage circuitry going as possible.

I've done a little more circuitry modification this evening to improve the color tracking from dark to light. Basically, with the condition my CRT is in (which doesn't seem all that bad on the tester, but does show a weaker red gun compare to the other two), I found that it was not possible to adjust the green and blue drive low enough or the green background high enough, if I had the screen settings about right for good grey level color balance. So I modified the range of all those controls, and now the color tracking from black to white is reasonably decent.

These changes have slightly hurt the dynamic convergence, so I'll go back and see if that can be improved by readjustment now that the other settings seem to be in the right range. In particular, adjustment of the screen voltages seems to impact convergence (and focus) so it's not surprising that convergence will need some touch up.

Adjustable high voltage shunt regulators based on a string of zener diodes combined with a totem pole arrangement of bipolar transistors were fairly common in high end display monitor high voltage power supplies throughout the 80's. They were used for the Focus, G2 and G1 outputs, and could be very tightly regulated if used in conjunction with a feedback loop and error amplifier. If I recall correctly, we used 1N5281 zeners and 2N6517 transistors for the Focus and G2 circuits, and 2N6520 transistors for the G1. I used to design them for a living. Great idea for your CT100!

Does anyone know what the original back cover screws looked like on this set? I've got the original back cover, but it was tacked on with just a few dissimilar screws. I'll get a set of something that looks similar to the originals if I can find out what they were.

Tom,


If you're interested in CT-100 'hot rodding', I dabbled with a few circuit mods that RCA themselves included in later chassis.

For starters the vertical oscillator circuit in the CTC-2 gets its plate voltage directly from the LV power supply, where on later chassis it would come from B+boost. This change was necessary in later sets having larger screen sizes, because with the vertical circuit powered from the LV supply changes in horizontal width would not have any influence on vertical height. The result was that on an overly bright scene which might tend to load the horizontal section (really any situation where the potential of the horizontal circuit changed drastically, such as line fluctuations), horizontal width would tend to shrink while vertical remained largely unchanged. This causes the picture to appear somewhat 'squeezed', much like drawing an image on a rubber band or silly putty and stretching it out. Thus with vertical potential directly tied to horizontal the entire picture size would change in a more or less equal way, rather than losing only width and ending up with funny raster geometry. It's really noticeable with a crosshatch on screen, less objectionable with live images.

I moved the vertical plate supply to B+boost on my chassis and it works quite well, only other thing I had to change was the value of R144 (Sams) to account for the different supply voltage. Picture size now tracks well regardless of picture content, no more stretchy raster. I don't remember what value I ended up with, I just futzed around with different value resistors till I had something that worked well with the range of the height control.

I believe later sets also had the shunt regulator cathode returned to B+boost, rather than B+ or ground. This leads to less voltage across the tube, potentially making it more stable as a circuit but also lessening the stress on it in operation. It's not uncommon for shunt regs to have red plates in operation, but some of them really get hot. Not that 6BD4's are getting scarce or anything, just for the sake of what I will term 'generational improvements' in the chassis. No reason for the CTC-2 to fall behind in other areas, when it was blessed with such great picture quality right?


Lastly, I seem to recall the resistors in the shunt reg control circuit had drifted a lot, leading to no real ability to bring HV into it's proper range. After I replaced the out of spec ones, I could now vary HV into the correct range.

Great looking set, there's room to be much better! :thmbsp:

Thanks for the tips on possible modification of the vertical deflection power supply. I'll keep an eye out for the type of image squeeze you describe, and fix it if it looks like it's a problem. So far I don't see it, so perhaps that happens more with the larger screen sizes of later sets as you describe.

My shunt regulator is working fine, so I won't need to change resistors there. On the other hand, man did this set need a lot of work to get this far. If I was looking for a trial-by-fire set to learn about color TV, this set ended up giving it to me and more. Particularly in the area of alignment it needed many hours of work. And of course all the stuff with the flyback and voltage tripler ended up giving me more education than I necessarily had intended to sign up for!

After making some minor modifications to the adjustment ranges of the various color drive, screen, and background control ranges, I gave the convergence some more attention. I was able to get it much better after figuring out that the position and orientation of the yoke has a pretty big impact on convergence (as well as purity, raster scan linearity, and probably a few more things!). After some careful adjustment of yoke position and observing the interaction with the convergence adjustments, I got it looking reasonably decent:

http://www.videokarma.org/attachment.php?attachmentid=181436&stc=1&d=1390545223

No problematic areas left in any part of the screen, although there is just a touch of misconvergence on the left. Mainly I wanted to fix the poor convergence I previously had on the bottom, because that is where text banners often appear, and poor convergence really messes up the fine print.

I also found I was able to get the vertical linearity better at the bottom of the screen by replacing the vertical osc/output tube.

I think I *might* actually be "mostly done" with electronic restoration. Color fidelity is by no means outstanding, although I'm not sure how good these sets can really be. I know my CRT red gun is a little weak, and that is certainly making it difficult to get good color tracking from black to white. Also, certain colors like orange are never quite right. Nonetheless, it's quite a lot of fun to watch now. I'm probably overdoing it here, but here are a bunch of screen shots off live over-the-air TV this evening:

http://www.videokarma.org/attachment.php?attachmentid=181437&stc=1&d=1390545223

http://www.videokarma.org/attachment.php?attachmentid=181438&stc=1&d=1390545223

http://www.videokarma.org/attachment.php?attachmentid=181439&stc=1&d=1390545223

http://www.videokarma.org/attachment.php?attachmentid=181440&stc=1&d=1390545223

http://www.videokarma.org/attachment.php?attachmentid=181441&stc=1&d=1390545223

With all the work that has gone into aligning, adjusting, etc., I simply can't imagine that the typical TV sales shop was capable of really doing justice to one of these sets when installing in a customer's home. In New York City or Philadelphia, maybe some RCA factory hot shot set up the sets for customers, but in other places I imagine the skill level of the installation techs was mixed at best.

I remember as a kid in the late 60s and early 70s seeing color sets at neighbors' homes that were wildly out of adjustment, and those were sets a lot newer that this that weren't nearly so difficult to adjust. I can only imagine how some of these CT-100s looked in customers' homes in the mid 1950s! Maybe some of you remember those times and can tell me I'm wrong...

that convergence is as good as i ve ever seen.amazing its perfect!set looks great and i applaud your work.if i remember correctly,these sets had a different look as far as color fidelity.afterall,these were among the first color sets and there were many improvements to be made.overall,they had a nice look to them and i actually own one.mine is in climate controlled storage.restored by stan cash back in the 90s,the picture is beautiful.i got it due to the death of original owner.cabinet was restored and flawless.convergence is very good,but not quite as good as yours.that part of the job took stan many hours.he stated it was difficult to get it correct but its looks good.my crt is excellent.these are a wonderful piece of history and glad that another one is in operation.

...I remember as a kid in the late 60s and early 70s seeing color sets at neighbors' homes that were wildly out of adjustment, and those were sets a lot newer that this that weren't nearly so difficult to adjust. I can only imagine how some of these CT-100s looked in customers' homes in the mid 1950s! Maybe some of you remember those times and can tell me I'm wrong...
I've seen customers back in the day who, since it was a color set, would crank the color up all the way.
And BTW, the degree of convergence you've achieved on this set would pass muster on any roundie.

I remember as a kid in the late 60s and early 70s seeing color sets at neighbors' homes that were wildly out of adjustment, and those were sets a lot newer that this that weren't nearly so difficult to adjust. I can only imagine how some of these CT-100s looked in customers' homes in the mid 1950s! Maybe some of you remember those times and can tell me I'm wrong...


An uncle of mine had one for a few weeks in 1954, this in Ft. Worth TX.
This was a loaner while his mono giant console was being fixed. It looked
just fine in color. In B&W the purity was less than perfect but watchable.
Convergence I remember as no problem.

He liked color so much that when the 21CT55 came out he bought one.
In Ft. Worth we had the big advantage that all local programs
were in color, right from day 1. And there were lots of studio
productions. On the 21CT55 they looked fine too. I've told this story before.
Oh but did I mention he was 1/2 mile from the transmitter?


Doug McDonald

An uncle of mine had one for a few weeks in 1954, this in Ft. Worth TX.
This was a loaner while his mono giant console was being fixed. It looked
just fine in color. In B&W the purity was less than perfect but watchable.
Convergence I remember as no problem.

He liked color so much that when the 21CT55 came out he bought one.
In Ft. Worth we had the big advantage that all local programs
were in color, right from day 1. And there were lots of studio
productions. On the 21CT55 they looked fine too. I've told this story before.
Oh but did I mention he was 1/2 mile from the transmitter?


Doug McDonald

Hey Doug,

I bet your Uncle viewed this station break slide on his new 21CT55 back in late '54.

-Steve D.

Wow, what a nice set. the linearity of that test pattern is superb.. this set is older than my father!
I know the feeling of hearing, 'we're sitting down to dinner!' and all you want to do is just finish the damn thing already. BTW, I've seen CRT sets from the 1990s which had awfully misadjusted convergence. I have battled crt set-up on mostly rectangular sets and it really is a pain.
From one Tom to another, excellent job:thmbsp:

Wow what an accomplishment! Boy I'd love to come see your set in person someday. I believe I work in the same town as you. We might even be working for the same employer. I don't have the ability to collect and restore televisions since all of my time and space is taken up with 35mm movie and projector collecting. I "collect and restore" sets vicariously on this board through reading everyones posts and stories here. Congratulations on your remarkable achievement! -Sean

Hey Doug,

I bet your Uncle viewed this station break slide on his new 21CT55 back in late '54.

-Steve D.


Well, maybe on the loaner. It was months later that he got the 21 incher.

Doug Mcdonald

Wow looks like you really got her humming!

I'd like to improve the color tracking a little more if I can. What I've noticed is that the color is quite good if the overall scene is dark, but on brighter scenes, the red becomes weak. What is peculiar to me is that the red becomes weaker in a bright scene than it is in a dark scene. For example, if there is some red lettering or a red object on the screen, and the rest of the scene shifts from darker to lighter, the red stuff actually gets dimmer. Likewise, a face that looks normal if the background is dark loses its red content if the background gets lighter, even though the brightness of the face is unchanged.

I'm wondering if there is a problem in my DC restoration circuitry (particularly for red), or if the DC restoration circuitry is having trouble dealing with the difference between the guns in my CRT?

I've got the red screen at max, blue and green screens much lower, and both the blue and green drive below their original minimum settings (resistors modified). The red background with low brightness is too high, even though green and blue background are set at max. In general, the brightness control does not have the correct range -- it basically always needs to be fully clockwise, and still the image is a little dark, although not bad.

Here is the data on the CRT (see pictures below: #1 red, #2 green, #3 blue). All three guns test with very high emission. Key difference is the cutoff bias:

Red cutoff bias: -51.5 V
Green cutoff bias: -66 V
Blue cutoff bias: -67 V

I also checked the screen voltage during the emission test on my CR-70:

Red screen: 259 V
Green screen: 188 V
Blue screen: 225 V

So although all three guns are capable of high emission, there are significant differences between them in terms of bias and screen current/voltage.

Should the set be easily able to handle these differences within normal control ranges?

Any indication of something not quite right in the video amplifiers or DC restoration circuits?

If it's merely a matter of the set not being able to handle the gun differences within normal design range, I'd like to modify things to work better with the CRT in its current state, if I can figure out what would work.

Since many of you have far more experience with color sets than I do, I thought one of you might have some ideas.

i have a cr70 also and with that much emission your crt is NOT the problem. the sencore testers are spot on as far as im concerned. steve

I also checked the screen voltage during the emission test on my CR-70:

Red screen: 259 V
Green screen: 188 V
Blue screen: 225 V



Tom,

Checked the screens of my 'benched' CTC2 and came up with:

B8000194
G2
R 389
G 344
B 345

As you can see the levels are quite a bit higher. Although there are six feet of extra wire on the 15G, the 'bench' operation is normal and did not require a basic set up. Also, I dug up an unfinished CTC2 setup procedure from 2009 that never got put on my CT-100 site before it went down. Added some stuff and posted it here on AK: http://www.videokarma.org/showthread.php?p=3093798#post3093798

Pete

Also agree that the CRT is not the problem. I don't recall, have you replaced all the original white inductors in the chassis? Every single one of those was bad in mine, even one that tested good at the beginning of the resto went bad after the electrons started whizzing around. Don't trust them, they are always bad!

Have you tested the 6BC7 resto tube, or done any subbing yet? The schematic doesn't have 'drive' controls listed, so I assume you mean the video gain pots on the little phenolic boards. What are your actual G1 voltages doing with the chassis driving the CRT? If the voltages are wildly different, I'd expect to find trouble in the output circuits of the individual matrix channels. Not saying they are going to be exactly the same voltage, but the fact that you're having to mod things points to circuit troubles. Sams has the inductors on each matrix output at around 5 ohms (L54/55/56 respectively), but they are typically wound on high value resistors because they make a quick easy former. If a coil has opened up in the matrix output, there could still be some voltage getting through on just the resistor. Food for thought, ohm them out to check.

This is very likely a DC voltage issue at the G1 terminals and not a video issue, the fact that the brightness isn't working correctly is a clue. All it does is set the DC offset on the G1's of the CRT, thereby varying their relative intensity. You could look at it as an adjustable cutoff control, if you like. Find your missing DC voltage, you'll likely find the problem.

Thanks to both of you for the tips and measurements. I'm not sure why, but my CRT seems to need some extra help to produce a good picture. I wonder if it has just a touch of gas in it?

I was able to make quite a bit of progress after reading your posts above, and taking some time to understand the DC restoration and brightness circuitry in this set. Bottom line is that I had to make a modification to get the brightness into a usable range. I changed the value of R282 on the Sams schematic from 1.8 K to 900 ohms (put another 1.8 K in parallel) and this brought the brightness control into a normal range (or perhaps it even goes a little too bright at the top now).

http://www.videokarma.org/attachment.php?attachmentid=181511&stc=1&d=1390724208

As you can see, the brightness control is at the bottom of a big voltage divider that starts with the screen controls connected to B+. The CRT cathodes are connected to the junction of the 1.8 K and 4K resistors, and were previously running about +135 volts. The brightness control couldn't bring the grids high enough. Changing the 1.8 K to 900 ohms brings the top of the brightness control closer to the cathode voltage (and lowers the cathodes to +124 V, more or less what the schematic says it should be).

This allows me to run with normal brightness and not need excessive contrast just to make the picture bright. More on that in the next post on the function of the DC restorer.

Other mods I previously made are shown here:

http://www.videokarma.org/attachment.php?attachmentid=181512&stc=1&d=1390724208

To coax enough red out of this CRT and to tone down the green and blue, the blue and green gain control ranges have been modified (to allow further reduction) and the green background range has been modified. This provides good grayscale from black to white, as long as the contrast is not set too high. Not sure if the green background mod is really still needed at this point (I should try removing it).

The following scope pictures illustrate the behavior of the DC restorer in the CT-100, and also show why my picture turns green if the contrast is too high.

These are all shots of the red CRT grid signal. The brightness control and DC restorer (a separate diode for each CRT grid) set the level of the bottoms of the blanking pulses on the video.

With the brightness all the way down, the bottoms of the blanking pulses are at zero (bottom of scope grid):

http://www.videokarma.org/attachment.php?attachmentid=181513&stc=1&d=1390724644

The scope vertical scale is at 20 V / div. Increasing the brightness a little just moves the whole waveform up. In this case, so the bottoms are at 40 V:

http://www.videokarma.org/attachment.php?attachmentid=181514&stc=1&d=1390724644

Now the brightness is set to a good point (bottoms of blanking pulses at 60 V), but the contrast is still fairly low. Note that the red cathode is at 124 V, near the top of the screen, so things are still pretty dark overall:

http://www.videokarma.org/attachment.php?attachmentid=181515&stc=1&d=1390724644

Now I increase the contrast control, and you can see that the bottoms of the blanking pulses are still at 60 V (held there by the DC restorer) and the contrast grows the waveform upward for a brighter picture:

http://www.videokarma.org/attachment.php?attachmentid=181516&stc=1&d=1390724644

Here is the point of maximum usable contrast, and the picture is quite good, with good greyscale color tracking. The tops of the waveform are now at or slightly above the cathode voltage, so this is the threshold for CRT grid current starting to flow:

http://www.videokarma.org/attachment.php?attachmentid=181517&stc=1&d=1390724644

If I turn up the contrast more, grid current in the CRT pushes the whole waveform down. Since there is a 100 K ohm resistor going to the DC restorer cathode, the action of the DC restorer trying to maintain the bottoms of the blanking pulses at 60 V is now overcome by the lower impedance "grid leak" biasing of the waveform down by the CRT grid current:

http://www.videokarma.org/attachment.php?attachmentid=181518&stc=1&d=1390724644

What this means is that a lot of the red content in the picture is now being pushed toward black, even though a few of the brightest red areas are at maximum brightness. The effect of this is to cause the whole picture to shift toward green and goof up the color balance. Since the green phosphor is capable of higher brightness at lower current, the green is no where near this kind of grid current situation, so the green keeps getting brighter while we start losing red. The contrast control can be advanced further still, driving the bottoms of the blanking pulses all the way down to zero on the red (and still topping out at around 130 -140 V), giving an extremely green picture.

I suppose I really should have captured some screen shots along with the last two scope shots, but I trust you can imagine the problem.

So with these modifications, I now get a well behaved picture if I turn up to the contrast just to the point that things start turning green, and back off a little. Good greyscale tracking, and colors stay pretty true from dark to bright. The picture is not all that bright, and is best viewed in a reasonably dark room. Not very viewable in a sun-lit room by any means.

So I'm not sure why my CRT seems to need these circuit mods to work. There may yet be something wrong in the circuitry, but it looks from various voltage measurements and scope traces that the circuitry is doing what it should.

By the way, the tip on the white coils was probably a good one. I do still have a few of those white coils in place, but I checked them again and they're still OK for the moment. In the video output stages, there are no white coils left. L51 and its brethren were bad when I got the set, so those have been replaced. L54 and its brethren are not white coils, but red ceramic coated ones that seem to be fine (maybe were previously replaced?). A few white coils still left in some other circuits.

I also swapped the 6BC7 DC restoration tube for a brand new one, but that didn't have any positive effect.

For completeness, here are some voltage measurements with the set running well at its optimal contrast point:

Screen voltages:

R: 448 V
G: 275 V
B: 306 V

Cathodes: all at 124 V

Grid voltages (varying with scene, of course, so don't pay too much attention other than the general range of these):

R: 87 V
G: 88 V
B: 95 V

One question that naturally arises when it seems that modifications are needed to generate what should have been possible with the original circuitry is "What's different now?"

One thing that has been apparent all through the restoration process is that the B+ voltage has been quite high on this set. The original selenium rectifiers had been replaced with silicon rectifiers at some point in the past, and that actually caused at least a 10% overshoot in the voltage. The RCA service manual shows the main B+ as 400 V, and Sams shows it as 380 V. I was getting 440 V.

This played some role in why the brightness control range wasn't right, since it resulted in increased bias by virtue of having the CRT cathode about 10 volts higher than it should have been. This, combined with the fact that my red gun cuts off at about 50 volts instead of 68 volts like the other guns (and no doubt what the red gun did originally) was causing some problems.

The previous power supply work had a pretty sloppy appearance, with the diodes and a bunch of power resistors added in the space where the seleniums had been. The original ballast is gone. I had left the previous work in place up until now since it was functional. But now that the high B+ is causing some problems, it's time to get the power supply producing the correct voltage.

The main change, of course, was simply to add an appropriate power resistor in series with the AC feed to the silicon rectifier voltage doubler, as shown here:

http://www.videokarma.org/attachment.php?attachmentid=181526&stc=1&d=1390796811

Using a value of 8 ohms brought the voltage down from 440 to 399 volts. I also put the fuse back into the circuit, since the previous modifications had removed it. If RCA felt there was a good reason to have a fuse in this circuit, I thought it would be best to put it back in there.

The previously installed power resistors weren't quite the right values. In place of the 315 ohm ballast resistor, they had a 280 ohm resistor (two 560 ohm in parallel). In place of the 800 ohm ballast, they had 750 ohms (two 1500 ohm in parallel). These lower resistances further increased various power supply voltages.

I put in the correct value resistors using combinations of power resistors to hit the values exactly. The 315 ohm needs to handle substantial dissipation. I have three 10 watters in series, and they get rather hot -- dissipating something like 25 watts total.

The 8 ohm resistor also generates quite some heat. I have a 30 watt chassis-mount resistor in there, and the whole vertical sheet metal structure gets pretty hot. Those seleniums must have made pretty good spacer heaters as well as the big ballast tube used originally.

"Before" and "after" shots of the power supply wiring are shown in the attached images.

Lowering the B+ back to the correct target value should keep things running cooler in this set. With my solid state tripler, etc., the flyback was running pretty hot, so I'm glad to cool things down a bit.

I was able to put the brightness modification back closer to the original, with the 900 ohm modified resistor I showed above now at 1100 ohms (originally 1800 ohms) and plenty of upside brightness range.

An unfortunate side effect of lower the voltage this much, however, is that many adjustments were thrown out of whack. Convergence had to be redone among other things, but the second time around was certainly much easier than the first. I also had to lower my focus regulation zener string from 3000 V to 2800 V to get the focus range adjusted to the new reality.

Picture quality is now very good, with good greyscale color tracking. The only real complaint one might have is that the picture is not very bright, but I understand that is par for the course with the CT-100. It serves best as a night-time TV. :)

Tom, thank you for your continuing detailed notes.

Well, this set is basically done. To cap things off, here are some shots of the set in its current state. Thanks to the kindness of some VK members, I have some of the front panel controls now. If anyone has the rest of a channel selector knob they can spare, I can put it to good use!

I've been enjoying watching it in the shop for the past week or so, and this evening I'll be moving it into the house so it can have a better climate controlled environment and stay cleaner (as well as being easier to enjoy watching).

And a few more pictures.

Thanks for all the help, everyone!

Tom

Beautiful! I should take some daylight pics of mine. Second thought, my back would protest so upstairs it will stay!

And a few more pictures.

Thanks for all the help, everyone!

Tom

And a few more pictures.

Thanks for all the help, everyone!

Tom

Tom, Fantastic job. Looks like your just missing the plastic channel # insert on the channel selector knob. Hopefully someone out there can supply you w/one.

-Steve D.

Absolutely gorgeous! Congrats, Tom!

-Clark

Tom, Fantastic job. Looks like your just missing the plastic channel # insert on the channel selector knob. Hopefully someone out there can supply you w/one.

-Steve D.

Someday I hope 3D printer and some sort of 3D scanner will be able to replicate these parts. Cause I need one of those for my 21CT55.

Dave

Tom,

Here's a 1-1 scale scan of the channel #'s paper insert. Not sure it's of any use to you, but good for reference.

-Steve D.

Some further followup on this set -- The CRT red gun has always been fairly weak on this set. Seems it died a natural death with a worn-out CRT after years of usage way back when. No problem with loss of vacuum on this CRT; just not much brightness from the red gun. You could get an OK picture on it in a dark room, but it's no fun to watch in such a state.

Adding a brightener helped a lot, but I still noticed another problem: The DC level of the CRT grids is set by the DC restoration circuitry and brightness control. This works fine, unless you try to push the signal to the point that the grid goes positive with respect to cathode. Then grid current flows, and the DC level of the grid gets pushed down as in any grid-leak scheme.

For example, on bright scenes, the DC level gets pushed down, which lowers the red level not just on the bright parts of the picture, but everywhere. So bright pictures were very weak on red.

I've done some experimenting and found a way to boost the red grid signal, allowing it to go positive and draw grid current without any effect on the bias. I added a cathode follower as shown below. It is capable of delivering enough current to the grid to hold the DC level and appropriate brightness.

http://www.videokarma.org/attachment.php?attachmentid=196468&stc=1&d=1517469546

The CRT cathodes sit at around 125 VDC in this set, and with this cathode follower in circuit, I can drive the red grid well above that while holding the black level at around 85 volts where things look good.

Now the set looks good in a moderately lighted room -- something I've never really had before. Much more fun to watch than before.

Anyone else tried this method to squeeze a bit more life out of a CRT that has seen better days? If this were not a 15GP22, I'd give it a good whack with a rejuvinator, but that seems far too risky with such a valuable CRT.

Thank you for doing this interesting research and for sharing your success, Tom.

Grid current is not recommended according to the data sheet, though I suppose you have nothing to lose.

http://scottbecker.net/tube/sheets/093/1/15GP22.pdf

Thanks for posting the data sheet. That's one of the reasons why I posted this -- to find out to what extent operating with positive peak voltages on the red grid may cause some damage. I see on the data sheet that the max rating for grid voltage is +2 volts. Is there an additional reference on there specifically about grid current? I may have missed it.

One thing I noticed in the circuitry is that while the green and blue cathodes are fed by a fairly stiff voltage divider, the red cathode has an extra 2.7K cathode resistor. This reduces the gain of the red and reduces the positive grid voltage with respect to cathode. Shorting this resistor results in a pretty nonlinear response, where the red beam gets too bright on positive peaks and gets completely out of balance with the other colors. I tried reducing the value of that resistor and came away with the impression that it's best left alone. Intriguing, however, to think about how the red signal could be further boosted in some useful manner that gets rid of the nonlinearity. Maybe some kind of active feedback on the beam current? With a little help from a modern op amp that would be possible to do.

If any of you knows anything further about the risks of boosting the red in this way, let me know.

As miniman82 points out, I don't have so much to lose here, but I also don't want to completely kill the red very quickly by trying something completely crazy.

I didn’t look, but from experience reading lots of data sheets I can tell you that going beyond 0 on the grid will typically result in grid current being drawn. For most output power pentodes this isn’t problem occasionally, my concern here would be overheating the grid cup and warping it. That could cause a permanent short and damage the gun.

My instinct tells me that shouldn't be a problem. The grid goes at most 10-20 volts positive with respect to the cathode (I think it is in fact only a few volts because of the cathode resistor), so the power dissipated in the grid will be tiny. Probably gets way more heat from the nearby cathode than from the actual power dissipated in the grid. I was more worried about whether this will cause some kind of cathode stripping or something like that.

My instinct tells me that shouldn't be a problem. The grid goes at most 10-20 volts positive with respect to the cathode (I think it is in fact only a few volts because of the cathode resistor), so the power dissipated in the grid will be tiny. Probably gets way more heat from the nearby cathode than from the actual power dissipated in the grid. I was more worried about whether this will cause some kind of cathode stripping or something like that.

Running it into the positive grid region is probably along the lines of a mild clean and balance on a rejuvenator. Odds are it won't hold it's emission for long...It is probably either going get brighter as the cathode cleans or darker as the remaining cathode gets used up. If brightness improves with use you may want to revert it to stock.

Absolutely stunning cabinet and electronic restoration on this set. The screenshots are the best I’ve seen to date on a 15GP22. I was thinking a 3D printer for the knob as well.

Late to the game on this thread, don’t know how I missed it.

Are sure you weren't looking at Nick's screen caps?

Are sure you weren't looking at Nick's screen caps?

The screen caps in post #150 look pretty decent to me...

Those screen caps were not very bright when I took those a couple of years ago. Always have needed to run this set quite dark to get enough red.

Today, I took a quick shot during the Super Bowl. This is the west coast, so there was still significant light coming into the room from the window at 5 PM or when I took the shot. As you can see, red is decent. I'll take some nicer screen shots after I've had a chance to fine tune things better.

http://www.videokarma.org/attachment.php?attachmentid=196501&stc=1&d=1517805073

Noting the concerns about positive grid voltage and grid current mentioned above, I made an update to the circuit by adding two 1N4148 diodes in series between the red grid and cathode. I used 1N4148 to have fairly high speed diodes. Two in series since they are only good for 75 V reverse voltage each. This limits the grid to only +1.2 V on positive peaks of the video.

Here's the circuit:
http://www.videokarma.org/attachment.php?attachmentid=196502&stc=1&d=1517805073

With this circuit, the DC restorer sets the black level ahead of the cathode follower, and the diodes clip the peaks off the video drive to the CRT grid. I'm pleasantly surprised that clipping the positive peaks still lets plenty of red drive through (so I don't really need the strongly positive peaks on the grid signals).

I compared this to the original to reconfirm that this was all doing some good. The difference is very compelling. Without the cathode follower, the DC restorer isn't able to keep the black level fixed on the red gun any time the red signal becomes large. As reported above and on the last page, grid current competes with the action of the DC restorer, and the DC restorer loses. Now with the cathode follower and diode clipper, the DC restorer keeps the black level fixed as it should, and I can get decent red.

Not like a new tube, but pretty watchable. With the clipper in there, hopefully I am not damaging the red gun.

Are you still using the brightener?

I hadn't seen those caps before (maybe three years ago), they do look good. Please keep us posted on how well the tube holds up. My set can be viewed in moderate light (even blinder-inhibited daylight) but could be much improved by your scheme. I'm just going to leave the burn-in period to you. :)

Tom, As it happens, I re read your entire post from 2013 yesterday. The set still looks spectacular. Looks like you're still looking for the plastic channel selector insert. Thought about powering up my CT-100 for a Super Bowl photo but got caught up in the game. Also on the west coast and a lot of light streaming in from my west facing windows.

-Steve D.

Are you still using the brightener?

Yes, the brightener is still on there. I'll try it again soon without just to compare, but I have a hunch it needs to be there.

Tom, As it happens, I re read your entire post from 2013 yesterday.

That's quite a reading assignment you gave yourself! Thanks for the nice comments. Still a work in progress, trying to get this tired old CRT to give us a nicer picture than it wants to.

Yes, the brightener is still on there. I'll try it again soon without just to compare, but I have a hunch it needs to be there.



That's quite a reading assignment you gave yourself! Thanks for the nice comments. Still a work in progress, trying to get this tired old CRT to give us a nicer picture than it wants to.
I had a set that had a very weak red gun and I bought these boosters they look like brighteners but have no transformer in it I have one left for the green gun but can be used for any gun you just have to reverse the green wires to the red wires and this unit has an adjustable pot to adjust the red, and it does work. I had posted about this and no one ever seen or heard of this. This unit is for the base of the 21fbp22 or any color tube with the same base. It is a plug and play has both male and female ends. I had taken one other I had apart and all that's in it is a pot and a resistor and wires tied together.

I had a set that had a very weak red gun and I bought these boosters they look like brighteners but have no transformer in it I have one left for the green gun but can be used for any gun you just have to reverse the green wires to the red wires and this unit has an adjustable pot to adjust the red, and it does work. I had posted about this and no one ever seen or heard of this. This unit is for the base of the 21fbp22 or any color tube with the same base. It is a plug and play has both male and female ends. I had taken one other I had apart and all that's in it is a pot and a resistor and wires tied together.I remember those...
http://www.videokarma.org/archive/index.php/t-258583.html
Look near the end of the page for how they work.

.

I remember those...
http://www.videokarma.org/archive/index.php/t-258583.html
Look near the end of the page for how they work.

.
Yes I seen it and maybe it does just increase the bias for the given gun but when you have a tube that has a weak gun this is the answer, plug and play rather then make changes to the existing circuit when the tube is going down hill to begin with. As I was told by lots of VK members awhile ago is that these black and white or color sets will never be perfect. But we all seem to try to get then perfect and find mistakes from the original designers, but that's ok.

Made some further progress on the CT-100 after some time away from it. Two key findings:

1. I actually get more red with the red screen turned DOWN, not up. I modified the circuitry to lower the voltage range of the screen controls, and this actually helped. I don't understand why this is the case. Anyone have an understanding of how the screen control is behaving this way? I've confirmed that the voltage at the CRT socket goes up and down with the screen control setting as expected.

2. While building the cathode follower into the chassis more permanently, I happened to spot check some components under the chassis, and found the choke on the plate of the Q synchronous detector was open. I'm sure I checked this coil when I first restored the chassis a few years ago, but undoubtedly this coil opened in the mean time. This was confusing my interpretation of what was going on with the red. I'm surprised at how well the color worked with this choke open, but it's certainly much better now. Also replaced some drifted resistors while I had it on the bench. Not a bad idea to pull the chassis on a set like this every few years to catch the newly deteriorated components.

I've now got the cathode follower built in to the chassis, hiding inside the cage where the selenium rectifiers once resided. This leaves the chassis looking relatively original. The cathode follower is worth its weight in gold for enhancing the red on this set. It's officially "daytime watchable" now, which makes it a lot more fun to have around. Here are some daytime viewing pictures. Blinds are closed (with sun directly on the windows, which still lets a lot of light into the room).

Here are some quick screen shots. Didn't have nice reds like these in the daytime until now.

http://www.videokarma.org/attachment.php?attachmentid=197157&stc=1&d=1527379373

http://www.videokarma.org/attachment.php?attachmentid=197158&stc=1&d=1527379373

http://www.videokarma.org/attachment.php?attachmentid=197159&stc=1&d=1527379373

Made some further progress on the CT-100 after some time away from it. Two key findings:

1. I actually get more red with the red screen turned DOWN, not up. I modified the circuitry to lower the voltage range of the screen controls, and this actually helped. I don't understand why this is the case. Anyone have an understanding of how the screen control is behaving this way? I've confirmed that the voltage at the CRT socket goes up and down with the screen control setting as expected.

2. While building the cathode follower into the chassis more permanently, I happened to spot check some components under the chassis, and found the choke on the plate of the Q synchronous detector was open. I'm sure I checked this coil when I first restored the chassis a few years ago, but undoubtedly this coil opened in the mean time. This was confusing my interpretation of what was going on with the red. I'm surprised at how well the color worked with this choke open, but it's certainly much better now. Also replaced some drifted resistors while I had it on the bench. Not a bad idea to pull the chassis on a set like this every few years to catch the newly deteriorated components.

I've now got the cathode follower built in to the chassis, hiding inside the cage where the selenium rectifiers once resided. This leaves the chassis looking relatively original. The cathode follower is worth its weight in gold for enhancing the red on this set. It's officially "daytime watchable" now, which makes it a lot more fun to have around. Here are some daytime viewing pictures. Blinds are closed (with sun directly on the windows, which still lets a lot of light into the room).

Here are some quick screen shots. Didn't have nice reds like these in the daytime until now.

http://www.videokarma.org/attachment.php?attachmentid=197157&stc=1&d=1527379373

http://www.videokarma.org/attachment.php?attachmentid=197158&stc=1&d=1527379373

http://www.videokarma.org/attachment.php?attachmentid=197159&stc=1&d=1527379373

That looks extremely nice Tom! I think your screenshots are the first I've found that look remarkably similar to how a working 15GP22 looks in person. Do you do anything special when snapping screenshots?

This also provides a bit of hope to anyone with a rather anemic looking 15GP22, provided of course it's still under vacuum.

Really looking nice. I think if you took some closeup shots in the dark the images would look spectacular.

Do you do anything special when snapping screenshots?
These are simply quick shots with my iPhone. I guess I got a little lucky with the lighting level. I didn't have to adjust the brightness up or down from normal viewing for those shots.

This also provides a bit of hope to anyone with a rather anemic looking 15GP22, provided of course it's still under vacuum. If anyone has a CT-100 with a weak picture tube, I'd recommend this method of squeezing more life out of it. Not sure how long this will hold up, but I don't see any change after a few hours of viewing. I don't watch it all that much, so it will be a while before I'll be able to tell how well it's holding up. Much nicer to have it watchable, and something that can be demo'd without having to have a completely sympathetic viewer.

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

Nice work!

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

I believe you're onto something here. A weak B/W tube will often give blackened highlights when driven too hard, and show poor, non-uniform focus. Dial the brightness and contrast back and some tubes will behave more normally.

I've seen this effect blamed on a dead cathode center region in period papers. It's possible the same effect that causes this behavior in B/W tubes is causing the strange behavior of Tom's weak red gun.

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

Is the spot size from the red gun larger under these conditions? I'm guessing that with lower g2, much of the beam current is being generated from a larger area of the cathode surrounding the very center, which is likely fairly well depleted by now.

just a WAG,
jr

I think you're on the right track. Now that I've modified the screen control circuitry so that the adjustment range of the screen controls is at a little lower voltage, as I go from the lowest screen voltage to about 1/3 the way up, red brightness increases. Beyond about 1/3 up, the red brigthness decreases, and near the top of the range, I think the entire picture (all guns) is defocusing a little. This might be consistent with additional current dragging down the voltage on either the first or second anode, without additional beam current actually making it to the screen (it's hitting elements within the gun).

I don't think I'm seeing a bigger spot size on the red. Picture is pretty sharp overall. Don't know if that's consistent with the idea that the center of the cathode may be worn out, but I'll simply report what I see.

Tom, I know that I am late to the party. I did see the pictures that you posted a few days ago. Just wanted to say how wonderful it looks. I must admit that I only recently became aware of the 15GP22 CRT thanks to this forum and the Antique Radio Forum. I will admit to having never seen a real 15" color set (vintage that is).

Very Nice!

I just had an idea. I improved my 7JP4 with the grid-cathode short remover
of my Sencore. Maybe my 15GP22 arcs are not caused by gas ... I never saw
a purple glow. I'm going to try that short remover on every single pair of
electrodes in the gun, including ones between different guns (excluding
heater-cathode). It can't hurt. I'll first check it with a 2kV supply set to
current limit at say 50uA. That should catch gas.

Sure sounds like it's worth trying. If we can stave off the death of one more 15GP22, that would be very nice. Let us know how it goes!