Zenith 20Y1C50 Focus Troubles
 

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)

How is the HV...I know if the HV is too low or too high, even if you can adjust the focus voltage to the correct value, you will have poor focus...maybe problem in HV regulation circuit?
Also how is the crt...is it strong...no arcing?

I checked the hv last night. Previously I had set it with a VTVM but I have now rigged up a spare hv probe to work with my digital multimeter. I am averaging 25,600v. I cannot adjust it any lower. When I tried it last night it took longer than usual for the focus problem to show itself, long enough that I thought maybe it was just an intermittent. But it did finally go slowly to a blur-and while this was happening the hv did not change. I also measured hv while varying the focus control-no difference. No visible problems in the crt neck; I pulled the socket but the focus pin looks okay. I haven' t decided what to try next. At some point I guess I should find out why the hv is on the high side; I'm in no hurry to pull the chassis.

While I am at it...where (physically) is the jumper for measuring hv current? The chassis design on these is very thoughtful, I thought it would be marked & on top of the chassis but I see nothing. Sams makes reference to it in the schematic & the setup page but does not show it. I have to admit that I have never done this before but the other thread on here has been discussing it & I do think the flyback gets too warm on this Zenith. I don't think there is any connection between this & the focus...

Have you checked the focus voltage with the CRT socket unplugged?

No, Andy, didn't think of that. Good point.

But for the latest wrinkle...the last time I subbed the hv tubes I did not sub the horiz. out. because it looks brand new & seemed to be fine. Last night I swapped damper/regulator & h.o. & the focus stayed fine. I tried putting just the old h.o. back & after some time it did get worse but not nearly as bad as before. For now I'm just leaving the all new tubes in there & letting it roll. As it is the focus is very good & stays there. I am at the end of the travel on the control, though. I notice in Sams that the resistor I replaced is listed as "not on all models" so maybe I could just play with that, try some different values. I still want to set the efficiency for the sake of the flyback.