Does anyone know what to look for on a Zenith "Advanced System 3" 27-inch console (I assume on the 9-754 sweep board) that would cause a sudden pincushion problem?

This is the TV that I have written about before; it isn't used much and my son wants it more for video games than anything else. It had a "jumpy" picture previously that would eventually go away if used a long time or if NOT used for a long time. Tonight I happened to try it and it was working perfectly for about ten minutes, then got "jumpy" (as if the picture were moving up and down an inch or so), then suddenly developed the pincushion problem. It came on suddenly.

Interestingly, though it may just be coincidence, when the pincushion problem appeared, the picture stopped being jumpy.
(EDIT: It was coincidence, but I did discover that turning the TV off and on a few times stopped the jumping).

The other board is a 9-959 main module.

Thanks

Also edit ... if it's not worth repairing, does anyone in So Cal need a set with a picture tube in excellent condition?

Sounds a lot like bad solder joints...

Ooh... I wasn't even thinking that. I will check. Thanks!

Also, the wire connectors to the board might need the oxidation cleaned off the metal parts of the connectors.

Also, the wire connectors to the board might need the oxidation cleaned off the metal parts of the connectors.

I will check that too!

I looked for bad/cracked solder joints and saw none (though I did re-solder a couple just in case) and unplugged/plugged each connector a few times to clean. Still has the issue.

Looking at the schematic, I see no pincushion adjustment. Under "characteristics" for this board (9-754, one of a few covered by the schematic) it mentions "HORIZ. PIN/WIDTH ADJ - DIODE MOD."

Thinking that a transistor (or diode?) may be the problem. But I never actually learned circuit theory or how to really read a schematic so I'm kind of in the dark. I don't suppose anyone wants to take a look at the schematic, do you?

I'll probably not do too much else but it's an interesting challenge.

I looked for bad/cracked solder joints and saw none (though I did re-solder a couple just in case) and unplugged/plugged each connector a few times to clean. Still has the issue.

Cold solder joints are very rarely visible without a microscope. I would reflow absolutely everything is in the path of the issue, and everything on all PCBs that is either a heavy component like a transformer or large cap, or anything that receives stress like a connector or potentiometer.