Today's Mystery Guest (RCA CTC-4)

[DaveWM]

Phil, are you back to the orig coil?

so in the final analysis did it turn out to be a bad detector diode?

[Phil Nelson]

I haven't reinstalled the original L42 coil yet. I did reinstall the original T115 quadrature transformer.

It seems possible the detector diode was always bad, but when first looking at that output way back when, I only scoped the grid of the 1st video amp, which gives a skeletal luma-like signal, even with a new diode. I should also have looked at the cathode of the video amp, where color information (including burst signal) appears. Perhaps the signal coming out of the video detector network wasn't as bad as I first assumed.

Since that time, I have also replaced dozens of resistors in the chrominance circuits, the video board, and the video IF board. Those replacements corrected a number of wrong voltages. It's tempting to look for a single "silver bullet" solution, but I suspect that the TV started out with a bunch of problems (maybe including a funky diode), which I'm still in the process of correcting.

Phil Nelson

[Zenith6S321]

If you have a Sencore VA62, it also puts out the color bar pattern.

Dave

[old_coot88]

Phil,
Have you checked/replaced any resistors in the IF strip yet? If not, I would start with the screen grid and cathode resistors of the three IF stages. And then check the screengrid & plate voltages of all three stages.
If all's good, then scope the screen grid of each stage. They should all be flatline. Any hash or signal of any kind on them would indicate a bypass cap problem.

[Phil Nelson]

That reminds me, I need to fix the color bars on my VA62. It makes beautiful B/W patterns, but the colors roll. Just call this the house of half-baked equipment .

In the IF section, I have replaced R115, R116, R118, R119, R120, R122, R123. That was in the context of working on AGC, and I see that it did not include the IF cathode resistors. It can't hurt to check out that section some more.

Phil Nelson

[old_coot88]

If, in checking voltages, you find any seriously out of whack on plate & screengrid of 1st and 2nd IFs, there's two honkin' 10K resistors in parallel (R320, R321). Since this pair is a B+ dropper, they could sure mess with those voltages.

[Zenith6S321]

Originally Posted by Phil Nelson

That reminds me, I need to fix the color bars on my VA62. It makes beautiful B/W patterns, but the colors roll.
Phil Nelson

You might try turning the VA62 color bar frequency adjusting capacitor screw circled in this picture. I had to adjust it on mine to get the right colors in the bars. Remove the four case bottom screws, the four rear case screws and remove the bottom cover and case cover. Remove the two screws on the top of the control panel inside the case. The top will swing backward on a hinge like shown in the picture. Its worth a try...

Dave

[Phil Nelson]

Ah, I remember that convertible-top chassis. When I first got the VA62A it had a bad ripple, which I fixed by replacing a couple of p-s electrolytics.

That is one heckuva twitchy adjuster, or else there's something else going on that makes this oscillator circuit unstable. After much careful tweaking with my longest diddle stick, I was able to get the rolling to snap into regular color bars. When I put the case back on, colors went back to rolling.

The ripple has also begun to resurface. I have gotten a good in-circuit cap checker since I bought this thing. While it's on the workbench, I may as well check the other electrolytics and see what else has gone bad in the last few years.

Phil Nelson

[DaveWM]

any updates Phil, last time looked like you were getting close.