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Old 07-15-2006, 06:29 PM
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drh4683 drh4683 is offline
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zenith TVs and the selenium focus rectifier

There has always been a problem with most zenith tube sets that use the selenium focus rectifiers like the 212-48, or 212-85 in the tube type chassis (20X1C38, 20Y1C48, 16Z7C50 etc). Im sure many of you who have zenith sets find that the set will focus only at the very end of the range of the 15meg focus pot. Sometimes, the focus will be almost perfect but just not quite there yet while at the maxium end. These zeniths generally have a focus voltage of 4.3KV. Every so often I will run across a zenith in which the focus is perfect and the setting is somewhere in the mid range of the focus control.

Ive decided to finally look into this problem and attempt to correct it. That space command 400 color set I picked up last week does not have the best of focus and like I said, it seems to be a common problem with zenith tube sets. This TV is a 20Y1C48 chassis. First things first, I removed the selenium rectifier, which by the way was a mallory replacement back in 1973 or so (date code on rectifier). This was a universal replacement and Ive seen ALL universal replacement rectifiers have a PIV of 6500 volts. I really have no way of testing these so all I can do is substitue.
I did conduct several resistance tests across the 4.7 meg 2w resistor that is from the focus pot to the solder lug that attaches the focus pin of the CRT. That tested 4.6meg, no problem. There is also a 1.5pf 6KV cap from the focus rectifier cathode to ground. Ive heard in the past that sometimes this cap will have enough leakage and become a 5K resistor and kill the focus. That was not the problem, with my simpson 260, I read nothing, which I would expect from a cap like this. Last is a 220 ohm 1/2 watt from the focus pot to a spark gap to ground, all that was OK (spark gap clean too), the resistor tested to be 210 ohm but that was acceptable as there was a footnote that indicated that both the 220ohm and the 4.7 meg could use a +/- 20% resistor so obviously everything was within specs.

I happend to have just one selenium rectifier, a universal but the package said it would replace the zenith 212-48 (earlier rectifier) the 20Y1C48 used a 212-85. I went ahead and subbed it. Powered up the set, this time the focus was darn near razor sharp but still the focus pot was all the way over at the end of its range. From my understanding, it probably should work this way considering the more I turn the pot, the more voltage I give the CRT.

Anyone have any input on their experience with zenith and the focus issues?
The biggest question, where can I find replacement universal focus rectifiers, and parts suppliers carry anything that would work? The one I have tacked in the set did work, but not the correct p/n. Id like to stock up on some replacements. Any info greatly appriciated. Thanks

Last edited by drh4683; 07-15-2006 at 06:33 PM.
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Old 07-17-2006, 07:02 PM
roundscreen roundscreen is offline
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drh4683
Here are some ideas on that focus problem. First replace the cap and resistors. They are cheap and then you will know for sure that they are not the problem. I have seen resistors check ok untill they are put under load. If that does not fix it go after the high voltage.
Set the focus pot in the center and adjust the hi volt control up and down to see if you get good focus. If you do Then check the hi voltage and see if it is correct.
I think you have to check your ac line voltage and then set the hi voltage to the proper amount for what ever the ac line voltage is. {please correct me if I am wrong}.
I have sets with the focus tube tube {1v2} and they did that. It was resistors or weak tubes. 6hf5, 6cj3,3a3,6bk4,1v2. Hope this will help
ED
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