I suspect in some applications NP0 ceramics might be OK, though the tolerances on ceramics tends to be not as tight as the silver micas. But the advice of wa2ise is best. Safer to replace a give type cap with the same type.

Well, I'm accumulating a to-do list:
Blue horizontal amplitude coil
Blue static pot
Silver mica caps

I took a look at the schematic for the blue static convergence pot - bah! It's a special - 100 ohms, with a tap at 25 ohms. I doubt I will find one. However, the red, green and blue DC pots are identical electrically and are tied in parallel - tops, bottoms, and taps -so. if the red or green happen to be sitting at a different rotation compared to the blue, I could just interchage the wiper connections and avoid the bad spot.

Waiting for some mica caps to show up in the mail - any day now, hopefully tomorrow, so I can start work on the weekend.

To do:
Blue horizontal amplitude coil
Blue static pot
Silver mica caps
Find out why the luma freq response is only 2 MHz - the manual shows 3 Mhz
See if focus control can be centered better

I think this hobby is proof of the law of conservation of difficulty :D

Here's the latest -
last weekend I pulled the chassis, but had nto time to work on it, I had it flat on the table, and tonight I turned it up to look at the bottom, and the missing core dropped out. It was somewhere all the way at the back of the chassis - I don't knoe how it could have arrived there.

Now the bad news - in trying to re-insert the core, I cracked it. The brass screw does have a slot internal to the core, so it could be driven from the rear. Apparently the threads were galled (maybe deliberately so that no one should screw it all the way in and lose it like happened here). Anyway, I'm guessing I got the screwdriver cocked and applied pressure to the ferrite, cracking it.

The pictures show
(048s) the original coil in place
(052s) Top - possible replacement? (would have to remove a lot of windings, I think)
middle - core from "possible replacement"
bottom - broken core from original

dagnab it! :sigh:

Wayne, if you will measure the length of the broken ferrite core, and the length of the protruding threaded rod, I may be able to help with a replacement core. I am assuming the material is the same as that used in horizontal width or linearity coils. I assume your coil is OK, other than the broken core? Send me an email.

Thanks jonh - email coming

And now the (slowly) continiung saga...

Thanks to John Folsom, I got a replacement coil (images attached). It doesn't look like the original due to having 2 windings for multiple uses. I decided to try using just the core and the mount with the old coil, which seems to have worked well (won't know for sure until I fire it up). First tried the core to make sure it would fit without binding. Then tried screwing the brass screw intot he old mount, but apparently the mount was damaged, so gave up and replaced the mount. As you can see, the replacement came with two different sized mounts. The smaller one fit the old coil.

I have had the mica caps for about 2 weeks, but no time to work on the set - will fix that tonight or in the coming week.

News on the DC convergence pots is not so good. The blue pot wiper is definitely got a big bad (open) spot. Also found out from looking at the wiring and verifying on the RCA schematic that it's not identical to green or red physically because it rotates the opposite way and the tap is towards the other end. Anyway, Cailube doesn't fix it, and in the process of checking all three I discovered the red DC pot has a bad spot too. Have to think some more aobut how to fix these

Wayne:

Here's something to try for the blue convergence pot. Get a 200 ohm pot (Nebraska Surplus sales has several types) and put a 50 or so ohm resistor in series with a 150 ohm resistor and connect the ends to the outside terminals on the 200 ohm pot. This will make the overall resistance 100 ohms with a centertap at the junction of the two resistors. You will have to mount it somewhere else on the chassis or perhaps you could find a pot that can replace 100 ohm section in the dual section in the set now.

Steve

Well, I've been working on the DC convergence some more. If you look at picture ...002 attached, you see the DC convergence in the earlier chassis, and in pic ...003 you see the later chassis I have where the blue lateral changed to a permanent magnet. In this case a 82 hm resistor has been added so that the circuit draws the same current. The reason is, this whole mess is the cathode resistance for the horizontal sweep output tube! The DC in the convergence coils is developed between the tap on the potentiometer and the wiper on the potentiomenter. Since I can't get tapped potentiomenters, I am going to use the circuit in pic ...004. The fixed resistors supply the tap point, and the three 250 ohm pots supply the variable voltage. Hopefully, the extra resistance won't reduce the dynamic current too much. We shall see.

The blue DC pot was open end-to-end as well as having a big bad area where the wiper is open. The resistance to ground at the cathode of the HO is supposed to be about 25 ohms, and measured 70 ohms. Maybe this is why the set has been sensitive to particular choice of HO tube.

I put the 250 ohm pots in a little open-backed plastic box, which I planned to mount just behind the cabinet back, so the shafts will protrude through the holes in the back. One oops - I forgot that the back insets into the cabinet frame, so there is no depth left to mount the circuit box. Will have to add a bracket of some kind.

Well, I got it put back together, and... it doesn't work right. Apparently too much resistance in series with the vertical convergence coils, which I was afraid of. Just for yucks, I tried reversing the convergence yoke (so the red coil is over the green gun and vice-versa). That resulted in better DC convergence (but still not enough range), plus the best DC setting put so much resistance in series that the vertical dynamic controsl have little effect.

At least the horizontal output seems to be OK, and no smoke was observed.

Well, back to the drawing board.

Oh yeah - still have to try the blue horizontal adjustment range with the new core.

Wayne,

I don't know if this will be of any help, but it is worth a try. I had a ctc5 Special with a bad crt, I sold it to a fellow by the name of Ed Milbourn in Indiana. He wanted my set as a source of parts to restore another CTC5 that he was working on. Ed may have the parts you are looking for and may be willing to sell you what you need.

Tell him Bob Galanter in Milwaukee who sold him the ctc5 gave you the referal. Ed is a real nice guy and I would think if he has what you need he would be happy to sell you the parts.

BTW, Ed is looking for a 21axp22 that he can use in the restoration he is working on. I think he may even be open to one that is weak but undervacuum and can be rebuilt. If anyone can help Ed on the crt issue, please contact him.

Ed's email address is:

EMilbourn@aol.com

Bob

Bob, thanks for the hint. I'll send an email and see what happens.

I won't be able to work on the set for the next 5 or 6 weeks, as I will be traveling for business (and some for pleasure). Fortunately, one of the pleasurable parts will be the Early TV convention in Hilliard, and I'm thinking I might be able to find some help there. If not, the one thing that will definitely work is to put in two pots in place of each existing one, with a movable connection to select either the top wiper or the bottom wiper.

I showed the problem to someone at work, and they said "I wonder how the engineers at RCA designed the circuit in the first place, assuming they didn't have the unusual tapped pots when they started?" Something to think about, but other than clip leads and decade boxes, we haven't thought of any way to breadboard it.

I made contact with Ed, and he can supply the pots. He's also going to Hilliard in May, so I look forward to meeting there. Thanks, Ed, and Thanks, Bob for the contact.

Time for an update!

Ed Milbourne indeed did supply the unobtainable pots. Progress then got interrupted by travel in May. The bad news is that two of three existing pots were bad, and of the three removed from a junker by Ed, tow were bad. Guess this was a weak point in this chassis.

Attached scan and previous attachments show original DC convergence circuit with 100 ohm pots tapped at 25 ohms (CTC-5 DC conv003), and an attempt I made to get results using a separate fixed resistor divider for the tap point CTC-5 DC conv004). this didn't work becuase the dynamic convergence drives are in series with this resistance and wete thrown out of whack. CTC-5 DC conv005 shows the final thing I cam up with, replacing each tapped pot with two pots and a SPDT switch to select either and upper or lower wiper position.

.

Here's what the result looks like currently.
DC convergence can be made for any combination of dynamic convergence controls. Unfortunately, it does not seem possible to get enough range on the vertical dynamic controls. Also, the horizontal dynamic controls still don't seem to do much, if anything. Now I will have to go snooping for waveforms to see what';s happening, I think. It's still hard to tell if the convergence coils are at the right position. The yoke prevents them from moving forward any further when it's in the right place for purity