21-CT-55 power supply failure

[miniman82]

Originally Posted by Penthode

1. Which +285V is being pulled down: the +285V IF or the other +285V supply feeding the vertical centering control?

Here's a rundown of voltages:

Mains input set at 116 with RCA Isotap.

Main 400v: 380
IF (coming from pin 2 of the ballast): 300
285 supply going to vert centering control: 260

As read at the filter cap of each respective line.

2. Did you change the rectifiers to silicon?

Yes I swapped some in briefly for a test, made no difference so I changed it back. Stock seleniums are still connected.

3. Have you tried putting back the old electrolytics for a test?

You mean the chassis originals? They are totally shot (open circuit), so no point.

4. What did you last do before the problem appeared? (Did you see the problem before changing the electrolytics?)

I didn't change or do anything really, just moved it from one room to another and it worked fine in the new room for a spell then this popped up for no reason.

5. What basis do you think the ballast too hot? (Between the two resistors, the ballast is going to dissipate close to 50 watts which will make it pretty darn hot!)

Hot enough to produce smoke, I'm not willing to let it go any longer than that...

My long shot guess is that if the capacitor impedance has changed maybe you are seeing the rectifiers switch on the screen?

I don't think it works like that with seleniums, but I could be wrong. Something about this set changed in a bad way to make this start happening, it was working fine one minute, the next it's all jacked up. I just haven't been able to track down the source yet.