Today's Mystery Guest (RCA CTC-4)

[old_coot88]

I'd say the tube socket is fine, provided there's no intermittency when you wiggle the tube.
-
If you end up using the new CW xfmr, it'd be a good idea to clip out those two resistors before installing it. That way, if the resistors are actually needed, they can be placed externally.
-
If it's the primary bad in the old xfmr, it may be re-windable using roughly the same gauge wire and same number of turns.
The primary is untuned, serving only as coupler or 'tickler' to the secondaries, so its exact inductance is not critical.

[Phil Nelson]

When I reconnect the old quadrature transformer (still in place and connected to everything else), the 3.58-mc waveform is visible and I hear a tone WHEN there is no signal to the receiver.



When I turn on the color bar generator to inject a signal at the 1st video amp, the waveform is flatlined.



I guess the oscillator is capable of running, but something else is killing (or swamping out?) the oscillation when a video signal is present.

For what it's worth, I reviewed my notes of voltage readings in the color circuits. I found wrong voltages on the grids (pins 2 and 7) of V125, the 12BH7, R-Y/G-Y demodulator tube, measuring near zero when those grids should have around -20V (or around -15V according to Sams). At the time I attributed this to the oscillator not running, since the outputs of T115 connect to those grids, but who knows?

Another anomaly at V121B, the 6AZ8 bandpass amp, where the grid should have about -12V and I measured only about -3V or less.

I haven't forgotten the toasted appearance of the old transformer.

Phil Nelson

[old_coot88]

So the osc runs now with the old T115 in place but wouldn't before. What happens if you disconnect C202 (thus severing the osc from everything 'upstream')? Does it still run? If so, is there any sign of color barberpoling when you inject signal?

[Phil Nelson]

I disconnected C202. The waveform and tone are still present when there is no signal at the receiver. When I turn on the color bar generator, I see strong barberpoling on the screen.

Phil Nelson

[DaveWM]

burst keyer not working right?

[Phil Nelson]

Related to the burst keyer, I never got as far as trying the AFC adjustment procedure listed on page 23 of the RCA service clinic manual (specifically, adjusting T112, the phase detector transformer, or R228, the AFC balance control, which affect the keyer output). Pages 46-48 have more info.

Is it OK to use my digital multimeter for voltage measurements in those procedures, or will it load down the circuits too much? I don't have much faith in my old RCA Voltohmyst, but I could try using the scope for voltage measurements instead.

Phil Nelson

[old_coot88]

Wonder if you coulda cleared a tin whisker or something in the old T115 whilst removing and reinstalling it.
Now that you got barberpoling, try reconnecting C202, and go back further and disconnect R242. If still barberpoling, you should be able to zero beat the osc using the reactance coil, though it won't have any sync.

(Digital meters supposedly are high impedance like a VTVM, so shouldn't cause any loading problem.)