I recently decided to proceed with the restoration of this CTC-4. It needed a bunch of tubes, the usual complete replacement of wax/paper and electrolytic capacitors, and a few resistors (oddly, all low-value ones that had drifted higher; none of the high-value ones tested bad). I connected it to my Sylvania test jig with a Variac and ammeter, and it had a raster on first power-up, woo hoo!
Initially, I got no signal through it, but with a few tests using a Sencore VA62, I found it had a bad video detector diode. That diode was already a replacement; since Phil Nelson also had a bad one in his CTC-4, I wonder if these sets are hard on those diodes. With the diode replaced, I get a pretty good signal through the RF input:
I started testing the UHF tuner using the VA62's output, which can be set to any channel up to 83, and found variations in performance among several 6AF4 UHF oscillator tubes. With a new RCA tube in place, UHF works well all the way to the top of the band, amazing to me for a consumer product sold in the mid-1950s.
Now for the bad news: I decided to go through the horizontal alignment procedure from the Sams Photofact folder, which includes adjusting the horizontal waveshaping and horizontal hold/frequency coils, followed by the HV and linearity adjustments. Well, I did the waveform adjustments and everything seemed fine, then as I got ready to connect my high-voltage meter, I saw a big, bright arc inside the HV box, almost like a fire had started in there! I pulled the plug quickly, and after a minute or two to let my heart calm down, I looked at the flyback and felt it; it was pretty warm, and most of the rubber "tire" coating had sagged down, exposing the top where the arc went to the top of the HV box:
(The sagging is to the left in the first picture, which was taken with the chassis flat; normally, it is vertical.) You can see where the wax has drooped low enough to completely expose part of the coil on the lower-left part of the "tire" in that picture.
Before I saw the wax damage, I powered up the chassis after everything cooled down, and as soon as the HV came up, the arcing started again, so I pulled the plug quickly of course.
Apparently, the "waveform adjustment" procedure, which has steps such as "turn the coil until the slug is all the way out, then bring it back" (putting the horizontal frequency way off for a few moments), caused the flyback to get warm enough to start melting the wax, and I did not think to watch it or otherwise be concerned, until the arcing happened. Aside from hating myself for messing up a part that was fine for 50+ years, my next step will be to remove the wax and inspect the arc spot for damage, to see if I can just apply a bit of corona dope there, then cover the whole tire with sensor-safe RTV silicone and hope for the best. If I remember right, the picture was still on the screen even when the arcing first happened, so I hope that means the flyback may still be OK once re-insulated. I tested its resistance from the H output tube cap to the HV rectifier tube cap, and got 490+ ohms (versus 510 ohms on the Sams schematic), so it is not open but could that be a sign of some shorted turns?
The remainder of the horizontal adjustment procedure is to set the high voltage, and adjust the horizontal linearity for minimum current to the yoke. So, there is no step to check the H output cathode current, but should I measure it anyway, and possibly re-adjust the waveshaping coil while monitoring that current (as well as my oscilloscope), if in fact the chassis seems to work again OK once finish the flyback repair?