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

[old_coot88]

Did you check the reactance coil itself for continuity, and +280V on the plate of the reactance tube?

(open reactance coils were pretty common on CTC-15 forward, dunno about CTC-4s.)

[Phil Nelson]

I measured about +270V on the plate (pin 6) of the reactance tube (V122B). The reactance coil L127 has continuity, measuring about 10 ohms resistance.

Phil Nelson

[old_coot88]

How about plate and screen grid voltages on the osc. tube? And continuity of the coil going to the cathode? If all good, maybe the xtal.

[Phil Nelson]

I measure +249V on pin 1 and +118V on pin 2 of V124A. Resistance from pin 3 to ground is zero, suggesting that coil L42 has continuity. Maybe I will dig out one of those spare crystals and try substituting it. They are all the plug-in type, so I'll need to solder on some little leads.

Phil Nelson

[jr_tech]

I suspect that the xtal substitution will answer the question, but then I was wrong before about your CT-100 xtal

Be careful soldering to the pins, heat sink them if you can.

jr

[Phil Nelson]

Well, I subbed two different 3.58-mc crystals and took care not to overheat their pins.

No change in symptoms. I'm still lacking the negative voltage on pin 6. Still seeing the greenish color bars that disappear when you turn the Color control down.

Seems unlikely that all three crystals would be bad, although I guess anything is possible.

Phil Nelson

[miniman82]

Ignore the reactance tube for now, or at least keep the input grounded for the moment per the adjustment procedure so you can focus on getting the oscillator running. It's obvious you have no output from it, that's why you have zero grid volts on V124A and also sagging plate volts- that tube is probably turned on 100%. Make sure the .01uf caps to ground coming off pin 2 are not leaky (C2110/C211), then see if you can lift one of the primary winding wires of T115 (pin B or E) and measure resistance. It's possible your CW transformer has shorted or something, but tuned circuits can be deceptively hard to troubleshoot despite their apparent simplicity.

One last note: although the inductors in the CTC-4 chassis are not the dreaded white ones that always go bad like the CTC-2(B) chassis had, it's always possible you have a bad inductor still. Make sure L42 checks in at the proper resistance, and also ensure C209 isn't leaky.

[old_coot88]

The schematic indicates there should be three ferrite slugs inside the CW xfmr. Are all 3 present and intact? No doubt they've been diddled with if somebody's been in there before.

Also, you stated R246 is 22K, where the schematic shows it as 2.2K.
Is the replacement red-red-orange (22K) or red-red-red (2.2K)?
In any case, that would not stop the oscillator from running.