A few days ago I decided to fire up my very first restoration - an RCA 630, done about 15 years ago. Everything seemed OK, then, all of a sudden the picture was wrapped around so that part of the image that should be the top was now showing up at the bottom. It looked something like vertical fold-over, but I knew it wasn't. The vertical sync was stable, but it seemed like the sync bar was happening in the middle of the picture -- it's not easy to describe.

I tested, and substituted all the tubes in the video, sync and vertical circuits. They were OK.

I really hated to pull that set off its shelf (80+ lbs), but there was no other choice. You can't just pull the chassis with the crt on it, so it's a pain.

Once on the bench, I did voltage checks, and looked for some capacitor that went bad or a burned resistor. Nothing.

Perhaps by luck (or skill), I found the problem -- the vertical centering pot was it. Not burned out, but not making good contact. I opened the wirewound pot, sprayed it with cleaner and presto, the vertical was fine.

Thought someone else might find this interesting.
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Ron

I've had similar experiences with restored TVs & radios that sat unused for a long time. Previously-cleaned pots (or switches, anything with moving contacts) can relapse, causing "unexpected results."

I tell myself to go around the place and play everything regularly, to exercise the controls and let the heat of normal operation dry out coil forms, etc., but somehow, that doesn't get done as often as it should.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

Pots, switches, fuses and connectors - anything with moving parts - are the most unreliable part of electronics.

As a design engineer (my day job) I go to some trouble to eliminate them from my designs as far as possible. They are the items that have the most influence on the right-hand side of the bathtub curve (google it)