Zenith 20Y1C50 Focus Troubles

[bgadow]

This set was given to me the first of May. At that time I could not quite get it to focus, the control would run out of travel. I found that R36, a 3w 47K resistor, was way off. The closest I had was 39K so I used that & had no trouble getting crystal clear focus.

I had only used the set once or twice since then. Saturday night I was watching another set & noticed that DX was pretty good so I decided to try several of the sets, including this one. When I turned it on I just had a blur; it got worse & then came the smell of burning plastic. No unusual noises. Later I dug into it. I have substituted the tubes in the ciruit, replaced the focus rectifier with a NOS ECG part & replaced the focus contol with a used one I had. (the old one seemed to sizzle a little, but then this one does, too, under the same circumstances) C91, 150pf 6KV disc cap, seemed suspect. It runs from rectifier + to ground. Its hard to see the cap since its mounted to the inside of the plastic board & the hv cage is not readily removable. I can unhook it easily enough though. Anyway, the plastic board in the area behind that cap appears to have experienced a meltdown! Using a mirror to view the cap it looks okay. I didn't have a substitute handy so I said, what the heck...I rigged up a series string of .0015 1kv caps about as long as my arm, figuring that would be close enough to either work or else put on a nice light show-and I saw absolutley no difference so just put it back the way it was.

I am measuring the voltage-at turn on, with a sharp raster, I am at about 5700 volts. The voltage will slowly but steadily drop & at the same time the focus will get steadily worse. But here is what stumps me: if I turn the focus control the voltage goes up but the focus gets worse. After 2-3 minutes the voltage will have dropped to about 5000-5200 volts. I can crank it up as high as maybe 5600 with the control but just make the pic much worse. Best pic is with the control all the way down. I have tried spraying everything on that board with freeze spray but with no real changes.

Any ideas? (I know this pic isn't very good; the things circled on there are from whoever used to own the Photofact)