CTC-7 chroma issues - Videokarma.org TV - Video

miniman82 · #1 10-09-2010, 03:17 PM

I got the CTC-7 up and running, but I'm having an issue.

Using the NTSC gen, I can get color bars but they disappear after a few minutes. After they go away, nothing will get them back and I'm left with luma only. As the color fades away, I get issues with waves of colors vertically though the picture, as if it won't lock or something. The reference osc is running normally, I verified the waveform via o-scope. It's almost as if there's a cap that's slowly charging, which kills the chroma signal when it reaches a certain point.

Any ideas?

marty59 · #2 10-09-2010, 05:53 PM

Something around the Burst Amplifier or Color Oscillator is changing value, hit it with some freeze spray first to see if anything changes.

ctc17 · #3 10-09-2010, 08:21 PM

I would sub out the color crystal first thing. It doesnt have to drift very far to cause exactly what your describing and seems to be the no1 issues I run into in the old rca sets.
Spraying the crystal wont tell you either. When they get flaky its to a few degrease of change, not freezing a warm crystal.

You can pop any colorburst crystal in there for testing. One or the small ones out of a solid state set will work for testing. If thats the problem you want to locate an NTE358.

There should also be instructions in the sams for fine tuning the osc frequency. Usually you inject color bars and ground out a few points. The color bars will float and barber pole, you will adjust the osc slug to slow them down as much as possible. This beat matches the osc in the set to the osc in your generator. You just need to be sure you have a fairly good generator that the chroma crystal hasnt drifted in.

First sub out the crystal before tweaking any slugs though.

miniman82 · #4 10-09-2010, 09:37 PM

Well after talking it through with my brother (he's an electrical engineer), we ended up discovering that the burst amp tube's plate voltage was high by at least 100 volts. Turns out that either the factory, or more likely a misguided tech, had mistakenly wired the plate to the +385v supply rail, vice the 270 one it was supposed to be fed by. After flipping to the next page of the RCA service manual, it instantly became obvious why this was done. On one page, there is the schematic for the 7 A-F versions of chassis. On the very next, there is the 7N and P versions. The N and P's do in fact have the burst amp plate wired to +385, but that would not be correct for my chassis. My best guess is someone in the past flipped to the wrong page of the same manual as I have, and thought they saw an 'error'.

I corrected the wiring, but it made no difference. I still have the bars going through the picture, and nothing will make real color snap in.

I can try the crystal from my other chassis, but the last 3 times I saw this issue pop up on different chassis the crytal didn't fix anything- anything is possible I guess. Here's what I'm seeing at this point.

old_tv_nut · #5 10-09-2010, 10:26 PM

If it's not the crystal, then the suspects would be the small-value caps in the reactance control and oscillator (C737 4 pF, C729 15 pF, C732 82 pF, C731 220 pF).

Also, V701 and 707 are both 6U8A's, so you could try swapping them in case it's a bad tube.

old_tv_nut · #6 10-09-2010, 10:51 PM

By the way, I don't have the oscillator adjustment procedure, but I think it should involve disabling the phase detector (V706A,B), either by pulling the tube or by shorting the output (junction R727A,B [two 2.2 meg] and C726 [0.0047]) to ground. If the oscillator frequency drifts badly as it warms up with the phase detector disabled, then the problem is definitely not in the phase detector. Otherwise, there's a possiblility it could be there.

Phil Nelson · #7 10-10-2010, 12:13 AM

Ugh, your screen shot brings back painful memories.

I practically lost my feeble mind over my color sync problem. And I still don't consider it really-really fixed, just in remission. One of these days I should quit moping and fix it for good.

No solutions, just wishing you luck.

Phil

miniman82 · #8 10-10-2010, 10:03 AM

Phil, I know what you mean. Ive checked everything I know how to check, the only thing that remains is possibly a slightly off frequency local oscilator. That would cause the synchronous demod in the 12AZ7 to go wonky, along with a loss of color.

Wayne, thanks for the tips. My brother did mention that even small value ceramic caps can 'age' over time, and cause frequency dependent circuits to go bad. We'll see what I come up with later today. Gotta go praise the Lord now.

Don Lindsly · #9 10-10-2010, 12:11 PM

Replace the burst amp and chroma oscillator tubes. Align the color AFC per the instructions. If it lines up as expected, the crystal is OK. When setting the oscillator, with the phase detector grounded, the colors should just float by, but not lock. If not, the crystal is probably off frequency. Tubes and proper alignment fix most color sync problems.

Make sure the fine tuner is on edge near the sound bars.

miniman82 · #10 10-10-2010, 04:04 PM

Took the crystal from my CTC-9 and tried it in the 7, so far so good. The colors locked solid, and I couldn't shake them no matter what I did. Ran it for 10 minutes straight and they didn't so much as waver, so I guess the crystal was to blame after all. Ended up having to replce the oscillator socket while I was at it, since a pin broke while I was trying to remove the old crystal.

Love it when an easy fix comes my way!