Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Phil,
Since you've already done the alignment procedure per the manual, presumably the tint ('hue') control shows some evidence of working when you turn it.
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The display looks like possibly one of the two demods is really wonky or maybe not working at all.
Assuming the hue control is working, I would try this.. using a cap of .01 - .047 or thereabouts, short out each of the demods in turn, watching to see what the tint control does while one demod is shorted.
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At the 12BH7 demod tube V125A, short the plate (pin 1) to ground*. This kills the R-Y signal. On a normally working set, the display will show some colors missing, but with the tint control still functional.
Now short the plate of the other (G-Y) channel (pin 6). You should get a different set of colors, and the tint control should still work.
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If either channel shows little or no color, and/or the tint control has little or no effect, I would look further into that channel for the problem.
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Of course if both channels pass the test, then the wonky-color fault lies somewhere else.
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*Or to B+, doesn't matter since it's signal ground.

[Phil Nelson]

Thanks, that gives me some things to try. I haven't redone the alignment procedure since the latest replacements (which included a couple of resistors on the demodulators).

The tint (hue) control does have an effect. Colors shift around in the bars, but they are never correct.

Edit: I still haven't done the alignment, but here are quick photos showing the results of shorting the demodulators.

1. Demodulators normal (nothing shorted). Wrong colors, of course:



2. Demodulator pin 1 shorted:



3. Demodulator pin 6 shorted:



The tint control affects colors in both cases.

Gotta run some errands, but I can return to it this evening.

Phil Nelson

[old_coot88]

On second thought, before doing anything else, check for proper voltages on the 6AG7 demod driver tube V123. If good, check xfmr T114 for continuity on the secondary.
If good, scope the signal coming out of terminals A and D of T114. That's the 3.58 bandpass signal before it's mixed with the 3.58 osc. signal.