Today's Mystery Guest (RCA CTC-4)

[Electronic M]

Here is a 'cover all your bases' suggestion: if you have a 9-pin 'socket-saver/extender' plug it in to the chroma osc. tube socket and see if you have continuity between it and the solder terminals of the socket. Every so often the metal in a socket contact will break in a spot that is hard or impossible to see.

Also if there is a shield on that tube try removing the shield, and seeing if there is an effect on trying to adjust the osc. I've got a CTC-15 that don't won't chroma synch and don't want to oscillate if the osc. shield is on and grounded...So I just removed the shield as a quick fix.

[Phil Nelson]

Originally Posted by Electronic M

if you have a 9-pin 'socket-saver/extender' plug it in to the chroma osc. tube socket and see if you have continuity between it and the solder terminals of the socket.

Thanks, I did that very check yesterday afternoon. It occurred to me that I already had a socket issue on the video IF board, so why not check this one? This socket seems solid, knock on wood.

I will try to follow up on the other suggestions. It would be nice if I could just substitute a 3.58-mc signal, as you could do with a video or horizontal drive signal, but this situation does not seem that simple.

Phil Nelson

[dtvmcdonald]

Why not try a 3.58 MHZ signal from a signal generator? disconnect the
crystal, feed the signal from the generator to the no-longer
oscillator's grid and look on the scope at the two outputs of the
output transformer. See what happens if you vary the frequency around
3.58 MHZ (on the scope).

Another thing is to check the crystal ... connect the sign generator
to it with a 10K resistor and look on the scope. You should see the
sharp resonance as you tune. Remember that a previous thread had
what was thought to be a bad crystal; they tried two good crystals and
they didn't work. It was a bad part. After replacing the bad part
but leaving in a "good" crystal it didn't work ... but with the original crystal did.
So these circuits are picky about the crystal. If the crystal is bad, it may
be that only an exact replacement will work.

[Phil Nelson]

Hey, I just remembered that my Sencore VA62A video analyzer can supply a 3.58-mc drive signal. I hauled it off the shelf and fired it up. My scope has a hard time locking on the signal, but there seems to be a waveform of sorts.



I disconnected the crystal and connected the analyzer's 3.58-mc drive output to pin 6 (grid) of the oscillator tube. After adjusting the generator's output voltage, I began to see rolling colors moving upward on the screen, similar to what I've seen before when the color sync was messed up on my CT-100.



I tried adjusting the reactance coil (L127 in the RCA manual) through its entire range, but this had no effect on the rolling, which never changed speed. And I still don't see normal colors, only rolling patterns against the same green-blue bar background. Turning the TV's Color control had the same effect as before: no change in the colors, and the image will just extinguish if you turn Color down far enough.

The symptoms are the same whether I send the color-bar signal from my pattern generator through the TV's tuner and (lousy) video IF sections, or inject video directly at the 1st video amp.

I'm going to go back and repeat the procedure outlined on page 22 of the RCA manual, disconnecting the bandpass amplifier, trying to adjust T115, and so on.

Phil Nelson

[dtvmcdonald]

Its obviously best to use an accurate 3.58, but even the cheesiest signal generator will get close enough to test throughput ... just not phase.

[Phil Nelson]

Originally Posted by dtvmcdonald

Why not try a 3.58 MHZ signal from a signal generator? disconnect the crystal, feed the signal from the generator to the no-longer oscillator's grid and look on the scope at the two outputs of the
output transformer.

OK, I tried this (substituting the 3.58-mc signal from the VA62A analyzer at the grid of the chroma reference oscillator tube). It looks as if T115, the CW transformer, can pass the signal and be peaked.



In that photo, the scope was connected to terminal F on T115. By turning the bottom adjuster of T115, I could increase the amplitude up to a point, beyond which it fell off again. When I moved the probe to terminal D, the waveform looked about the same, although I didn't try the top adjuster.

None of which helps me figure out why the oscillator doesn't oscillate, but I'm taking it as evidence that T115 is operational, anyhow.

I had to move the adjuster quite a long way from its original position to find the peak, but perhaps that isn't surprising, given that many of the components attached to T115 have been replaced.

Phil Nelson

[old_coot88]

By sheer elimination, it begins to look like maybe the resonance point of the L42/C213 tank circuit is too far off from 3.58 to allow oscillation.

Do you have a signal genny that's reasonably well calibrated, and can be manually swept thru 3.58mc? If so, you might try sweeping the tank and watch for the resonant point on the scope.
(But first disconnect the tank from the tube cathode and C209. Connect the genny and scope to the top of the tank.)

[Phil Nelson]

My signal generator is a rotten old EICO 324. You can sweep it manually, but calibration is a laugh.

Those two components are small and rather buried.



Seems simpler just to replace L42 unless it is unobtanium.

Phil Nelson

[dtvmcdonald]

Try sweeping the combination of L42 and the 270 pF in parallel with it.
It should be close to 3.58 MHz. You could try disconnecting the 270 pF
capacitor and connecting in a standard 365 pF variable, and
varying that to see if you can get oscillation.