Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

I suspect the oscillator is simply not running. Trying old_coot88's test, when I turn down the color control, the color in the color bar pattern is totally extinguished.

When I check voltages on that tube (V124A, 6AZ8), the grid at pin 6 shows 0V instead of the expected -10V. I see no waveform at all when I put the scope probe at (or near) the plate, pin 1, as miniman82 suggested.



The other voltages at that tube are in the right ballpark and the resistance at pin 6 is correct (47K).

I guess the obvious question is, why no negative voltage at pin 6?

The other check, watching for drifting color bars while you move the oscillator slug, is part of the color AFC alignment procedure that I mentioned earlier.



When I made this check, no drifting bars were visible, leading to the conclusion that the free running frequency of the oscillator is not correct.

Phil Nelson