what do you think? strange zenith symptom on 20Y1C38 chassis

[drh4683]

Im a bit stumped on this one. Perhaps is nothing to be concerned about at all. I try to have my TV's 100% working correct, even small problems annoy me.

This 20Y1C38 zenith has a rather unique symptom which I've never seen before. Im getting these retrace lines on the screen, but only if I advance the AGC control too far (once the screen begins to overload). Once I back off, they just go away all together. If I advance the AGC all the way to black, I still get these retrace lines, but they are not consistently spaced and tend to jump around.

At first, I thought it was some of this closed caption info comming across the screen, but I get this consistent pattern on all channels.

I checked out both germanium blanking diodes and damper tube of course. Your obvious tests have been performed and voltages are very close to normal on the AGC and Y circuits.

I can only make scope measurements with no signal to compare with the schematics, yet with no signal everything is fine. If the AGC is backed off to appropriate level, not a trace (pun) of retrace is visable, which I guess is a good thing.

In the photos you can see what this looks like. For the sake of getting good photos, the contrast is cranked to near max, brightness lowered a bit and AGC into overload. Color level turned all the way down.

The last picture is how the set works when the AGC is backed off of overload which is how it should be adjusted and has a great picture.

What do you think?