CTC-4 Director 21

[old_tv_nut]

Do you know if the capacitors are OK
C201 2p
C202 120p
C209 6.8p
C203 .01
C213 270

How about the reactance tube itself (6AN8)?

The reactance tube works by amplifying the effect of C201 by the variable gain of the tube.

You can check if the oscillator can be pulled by putting, I don't know for sure, but maybe 100pF, from the plate of the reactance tube to ground. If this can pull the oscillator to a lower frequency, and the reactance tube cannot, the problem is in the reactance tube circuit.

[SwizzyMan]

Originally Posted by old_tv_nut

Do you know if the capacitors are OK
C201 2p
C202 120p
C209 6.8p
C203 .01
C213 270

How about the reactance tube itself (6AN8)?

The reactance tube works by amplifying the effect of C201 by the variable gain of the tube.

You can check if the oscillator can be pulled by putting, I don't know for sure, but maybe 100pF, from the plate of the reactance tube to ground. If this can pull the oscillator to a lower frequency, and the reactance tube cannot, the problem is in the reactance tube circuit.

I will check the tube first. This is somewhat confusing though because the previous crystal was able to oscillate at the right frequency and maintain color lock until the crystal broke. And after doing some reading around it seems Phil Nelson had this same problem with his 4 and adding a cap in parallel with the tank circuit allowed the correct frequency to be reached.

[old_tv_nut]

Originally Posted by SwizzyMan

I will check the tube first. This is somewhat confusing though because the previous crystal was able to oscillate at the right frequency and maintain color lock until the crystal broke. And after doing some reading around it seems Phil Nelson had this same problem with his 4 and adding a cap in parallel with the tank circuit allowed the correct frequency to be reached.

I think you are right to try the tube first. If that's not the problem:

A crystal for this circuit is cut to parallel resonate with a certain net amount of capacitance. That is, the crystal is operated above its series resonant frequency, and behaves like a large inductor, and is tuned with a small parallel capacitance. If the tank coil is tuned right on frequency, the net capacitance is supplied by the reactance circuit through the 120 pf C202.
The tank being on the far side of the 6.8 pF, it takes a large change in the tank tuning to change the total parallel capacitance across the crystal. I don't doubt that it worked, but a more direct way might be to put a small cap (5-10 pF) directly across the crystal. This way, the tank doesn't have to be detuned so far to get the required net capacitance. Adding capacitance across the tank means you are detuning it from exact parallel resonance to be capacitive when the oscillator is locked, while I think it is really intended to be more or less exactly parallel resonant for oscillator operation.

Different TV manufacturers may have specified different parallel capacitances, for nominal parallel resonance of the crystal. (probably between 18 pF for Zenith [if i recall correctly] and possibly higher values [maybe up to 32 pf but probably in the 20s] for other manufacturers). So, it's possible the crystal you got is not cut for the net capacitance in this chassis, and some more capacitance is needed. This should come from the reactance circuit, not the tank circuit, which is why you tune the reactance coil to zero beat the oscillator.

[SwizzyMan]

I tacked on a 5 pf cap across the crystal and I can almost get the oscillator to run at the right frequency. It still wont lock even after 20 turns on the reactance coil. Here is what I could get (note the bars rapidly change color every second but I no longer have the barber pole rainbow.)