RCA CTC4B "Gainsborough" picked up in San Francisco

[ChrisW6ATV]

Thank you all for the helpful information and comments.

Nick, I may put a piece of phenolic on the top surface of the HV box if/when I get this set going OK again; does "fish paper" accomplish the same thing, if I find some of that instead?

Username1, Tom, and Old Coot, I will go ahead and put a current meter on the H output cathode when I am ready to fire this set up again; also, I will run the Variac low to start, and observe the results of adjustments I make. The adjustment procedure for a CTC-4 does not specifically address the cathode current, but I will do so anyway.

Eric, my thought was to use some corona dope on any suspicious windings in the arc area once I have the wax carefully removed (and before I coat it with the silicone). Do you think it is a viable alternative to epoxy paint?

Bob, this set has a CTC-4B chassis, and seems to match the "late production" schematic diagram in the RCA 1955-1966 Field Service Guide. I thought I saw an RCA note that said all Gainsboroughs were "late production" models and were built with the changes in place; there is a list of them in the Guide that I will look at and compare with my set. There is no hang tag or any other service information in the cabinet or chassis. This is a maple-cabinet set, I think; it is a much lighter color than any walnut wood I have seen. Indeed, I would love to put a 21AXP22 back into it one day. The broken CRT was a 21AXP22A with the date code 6048, and from various tube date codes, this set was likely in regular use to perhaps 1966 or so.

[miniman82]

We're talking 24kv+, I don't think paper is gonna cut it. You need something thick with high dielectric strength like phenolic, nylon, plastic, ect. FWIW, once you fix the obviously high horizontal current situation I doubt you'll have the issue crop up again. It happened because the insulating wax melted away, allowing the HV to find the path of least resistance.

[old_coot88]

I was gonna say the same thing re. fishpaper. It's useful in lower-voltage applications, but I sure wouldn't use it here.

[Phil Nelson]

I used some of this 90KV-rated cambric tape when transplanting a glass CRT into my CTC-4. I still have much of the 20-yard roll left. Send me a PM and I'll mail you some to insulate that spot where the arcing occurred.



I would reserve judgment on the flyback until you can remove the wax and carefully inspect the winding.

Apart from this little incident, if you're getting a good picture from RF input, that sounds like great progress.

Regards,

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

[ohohyodafarted]

Chris,

Just goes to show you learn something new every day. I had always wondered about the 3 different CTC4 chassis. I did some investigating and in one of my RCA service bulletins I found a chart defining the 3 different chassis. The 21ct660u and 661u in the two low end Seville and Haviland models, used the early production CTC4 chassis. The 21ct662u Director, used the CTC4A. The 21ct663u Cheltenham and 21ct664u Gainsborough used the CTC4B.

There are a fair number of circuit differences, mostly in the color circuits, between the CTC4 and CTC4a. I don't have a CTC4B RCA schematic so I am not sure where the circuit differences lie in the CTC4B version.

Unfortunately I got my Cheltenham without a chassis. It was just a gutted cabinet. I have a CTC4 and CTC4a chassis to work with but I do not have a "B" version. And what is worse, the CTC4A I have has been canabilized so I am not certain which direction I will proceed. I am leaning toward rebuilding the "A" chassis but that will entail finding a pile of spare parts that are missing. Clearly the CTC4 early version is not correct for a Cheltenham.

[ChrisW6ATV]

Nick and Old Coot, thank youfor the comments regarding fish paper. I just had no idea of its properties.

Phil, thank you for the tape offer. I will get in contact with you once I evaluate the flyback. As Nick said, reasonably it should not come up again; it would be more of a "just in case" plan but still maybe worthwhile. And, I will likely want to use some of that kind of tape for the CRT HV connector as well, once I am ready to install the chassis into the cabinet.

Bob, I wish you luck with that Cheltenham restoration (maybe, reconstruction is more like it), I know you are a person who can definitely succeed with the project. If I can help with identifying details of the CTC-4B chassis, I will be happy to do so. At the least, I could take and forward lots of high-resolution pictures from various places and angles. My chassis had very little work done other than tubes over its likely ten years in use; I only identified a couple of obviously-replaced capacitors plus the video detector.

Today, I had both a funeral and a banquet to attend, so I only have had time to get online a bit, but tomorrow I hope to evaluate the flyback and post an update.

[ChrisW6ATV]

Here is the location of the arcing, after cleaning most of the wax from the flyback HV coil.



The view in this picture is about 5/8 inch wide, with the darkened area being just over 1/4 inch long. My main concern would be whether the dark parts are now carbon or something else that I should attempt to remove, before applying some corona dope (or other substance such as HV-rated epoxy as Eric mentioned?), followed by the silicone.

Also, I doubt that I could remove "all" of the wax on the coil, especially after seeing discussions of vacuum-impregnating flyback coils. I would be concerned that the materials or methods to do so could be risky themselves. Is some remaining wax likely to doom the silicone coating integrity as a whole? I appreciate any suggestions here.

[Username1]

Great Picture ! If it were me, I would use a tooth brush, try and clean all that burnt
looking stuff off. Try and determine if there is a lot of damage to the insulation of
each wire.... You may need to treat that, varnish ?

I read the piece on the hot wax and the vacuum pump, really good ! It was mostly to
promote boiling off of moisture. Lower the atmospheric pressure and water boils
off at a lower temperature.... Just as they use a 12 Lb cap on your car radiator, it
increases the boiling point of the coolant.

You have to be sure after you coat it with whatever your going to do, it has no air
pockets, it can over time promote oxidation and corrode and open up a wire....

But it looks to me the wire is in good shape ! clean it coat it,
patch that tire, and back on the road !



.

[ChrisW6ATV]

Thanks. I am carefully using a pin point to remove what I can. Even with a magnifying-lens lamp, I cannot get as good a view as the camera can, but I will try more pictures after some cleaning work.

[Electronic M]

I'd probably try to clean that spot a bit better. There is no need to Vacuum impregnate the fly with wax, and I think putting silicon on the fly is overkill. What I'd do is heat the portion of the wax tire that fell away (probably with a hair dryer) till it became soft enough to smoosh back on to the fly, do that and mold it back into the shape it should be in. If that proved impossible I'd try making a mold or form around the fly to pour molten wax in to cast a new wax tire.

The wax can always be remade if it fails, but silicone is going to be hard to remove if bad.

[old_coot88]

That char contains a lot of carbon which of course is conducive to shorted turns.
After cleaning as much as humanly possible, I would try this before applying any sealant:
With the 3A3 removed, power up the chassis (preferably with a variac) and with the room darkened, look for any sign of incandescence in the cleaned area. Any specks of incandescence would indicate leakage paths between turns.
----
(Edit) Why do I think shorted turns would incandesce in this instance? Well, since a couple of turns of wire generate enuff juice to light the 3A3 filament, a single turn ought to light up a speck of carbon white hot.)

And if there's no incandescing, see if any heating can be detected in the cleaned area. An IR camera might come in handy. If there's no heating detectable it's probably good to go.

[ChrisW6ATV]

Here is another picture. Any cleaning better that this would probably require some kind of solvent, which would likely damage the good insulation nearby as well. I am planning to coat this area with corona dope before I apply power in any case.



Tom, the original wax is not feasible to re-use, as far as I could imagine doing. My concern with trying wax again is that if anything happens in the future such as a short at the same spot, another wax meltdown would likely destroy the part and maybe much worse. You have a good point about wax being more easily re-done/reversed, though.

Old Coot, without the HV rectifier, am I still looking for/expecting the full high voltage to develop in the flyback coil, at least if I get the variac anywhere near 110+ volts? If I am, then I would need to plan for and insulate against new arcing, I would expect, since the entire coil is now uncoated.

[ChrisW6ATV]

Another alternative I will consider is to remove part of the coil winding to a point below/before the damaged area. Tom Albrecht did this successfully on a flyback that had a shorted turn near its outside layer.

[miniman82]

Great picture, you can clearly see the portion that got zapped. What you're looking at here is the outer turns of the step up ring, which consists of very fine copper wire wrapped with a single layer of even smaller fibers. The burned portion has had its covering exposed, which is why it looks different from the adjacent turns. I would very carefully attempt to remove any remaining burned stuff with a microscope and dental tools, then apply melted wax with a small brush till they are again covered. You could then install it with the cage removed to see if it will work, then take it back out and finish building the tire back up. This is going to take time to get right, so take it one step at a time and don't rush. Those wires are close to the size of a human hair, doesn't take much to break them.

[ChrisW6ATV]

Thanks for the detailed notes, Nick. I will see if I can get more of the burned material off the wires before any coating or tests.