Today's Mystery Guest (RCA CTC-4)

[Electronic M]

Resistors fail randomly sometimes, and a tiny percentage of new caps are bad shortly after failure....Don't discount all possibilities.

[Phil Nelson]

Yay, the oscillator is oscillating again.



I replaced three mica caps and a couple more resistors in the horizontal AFC/osc circuit and my favorite waveform is back. Also HV (25KV) and a screen image. Two of those micas (C106, C110) were rated for 1KV, so I tend to point the finger at them, although I have no way to test a 1KV cap.

Which puts me back where I was a week ago, wanting to see some color in those dratted color bars. Before messing with color, though I'm going to behave responsibly for a change, and work through the procedures for HV adjustment and HV performance check. The manual says that involves measuring current at the 6BK4 regulator cathode as well as at the HV fuse.

Interestingly, when I put on a color bar pattern, I saw some rippling waves of color, rather than monochrome bars, and the Color and Hue controls actually had some effect on the image -- the first time ever.

That gives me hope of solving the color issue eventually. But first I need to do the HV adjustments and some other voltage checks, so I can play the set for more than 60 seconds without fearing a meltdown.

Thx again for the advice.

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

[Phil Nelson]

After a detour to restore yet another CTC-11 (which needed very little work), I'm preparing to tackle the color issue again. In the meantime I got a new pattern generator with variable chroma output, which should be ideal for following up on Nick's suggestion to vary the chroma while looking at a tube such as the chroma bandpass amp.

While I have the CTC-11 on the workbench, it seemed a good time to try this method and see what the results look like on a similar TV that displays color very well. Here's an animated GIF showing pin 1 of the CTC-11's chroma bandpass amp (6AU6A) at 0%, 50%, and 100% of chroma output from the generator:



Pin 1 of the chroma phase detector (6AL5) also has a waveform that changes obviously in response to varying chroma input:



My idea is to get the CTC-4 back on the bench and scope the bandpass amp and phase detector and watch for change when I vary the generator's chroma output. I hope this will answer the question whether a usable color signal is making its way through the TV's video IF section.

We're leaving tonight for a mini-vacation, so I may not be able to try this until this weekend.

Phil Nelson