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Old 11-13-2013, 03:24 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
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Ct-100

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...

Last edited by Tom Albrecht; 11-13-2013 at 11:00 AM.
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Old 11-13-2013, 11:07 AM
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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.
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Old 11-13-2013, 11:11 AM
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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.
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Old 11-13-2013, 11:13 AM
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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.
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Old 11-13-2013, 11:46 AM
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With the focus rectifier disconnected, isn't the vertical dynamic convergence transformer completely out of the flyback circuitry?
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Old 11-13-2013, 12:15 PM
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Quote:
Originally Posted by Tom Albrecht View Post
...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.

Last edited by Pete Deksnis; 11-13-2013 at 12:19 PM.
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Old 11-13-2013, 01:18 PM
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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.

Last edited by Tom Albrecht; 11-13-2013 at 02:05 PM.
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Old 11-13-2013, 03:56 PM
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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...
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Old 11-13-2013, 04:29 PM
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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.
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Old 11-13-2013, 04:55 PM
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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.
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Old 11-13-2013, 06:00 PM
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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.
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Old 11-13-2013, 07:11 PM
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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
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Old 11-13-2013, 07:32 PM
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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.
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Old 11-13-2013, 07:33 PM
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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.
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Old 11-13-2013, 07:36 PM
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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

Last edited by Tom Albrecht; 11-13-2013 at 07:43 PM.
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