I have a motorola 21T16 that has 2 diodes instead of selenium rectifiers. They are in a voltage doubler set-up. My set will work correctly for a few minutes than the picture shrinks down about half size and stay there. the voltages from the diodes does not stay consistent.from the first one it will check at about 125 or so(DC). but after the set is not working right I can't seem to get my digital voltmeter to check it. It seems to be only AC from it.Also the first diode runs very warm, the second one is cool. Does this tell me I need to replace them or is something else in the set wrong? If it is the diodes, and I put new in, which way does the cathode band point in the circuit? Thanks, Doug.

One of the electrolytics in the doubler circuit could be shorting. But, it might only be the diode breaking down. I don't have a schematic for your set so all I can say is to put the new diodes in, in the same orientation. Aren't the bands visible on the old diodes?

John

If you haven't replaced the associated electrolytics you need to do so pronto. You may have to replace them again if they've already been so; this can be a tough circuit on its associated components, and it may take the trial of a few before you find some caps that will rest easy in that task.

An additional R&R of the diodes may not be a bad idea either. Voltage multiplier circuit power supply arrangements are not a good design for the sake of reliability; they are an arrangement made strictly for the sake of low cost.

I have replaced all paper and electrolytics already.

Double check to be sure the electrolytics were installed with the polarity correct. If one was backwards it would short and could cause the symptom you see.

If the diodes weren't chosen with a high enough PIV they could break down. Most doubler circuit designs have high PIV requirements. If one does short for whatever reason, it makes for a high current through one or more of the electrolytics and probably damaging them.

Diodes are cheap, electrolytics not quite as inexpensive, but it sounds like you need to replace both in the circuit.

I think this generalization is correct, but I make no guarantees. The diodes in any voltage multiplier circuit look like they're are connected in series. They might have a resistor between them in one version for in-rush protection, but the orientation is still the same.

The banded end is the cathode. If the supply is a positive supply, the banded end of the one closest to the output will be the output. All the other diodes will precede it pointing in the same direction.

I've attached a simplified schematic of one version.

John

Are these old silicon diodes from the 60's? I have had a lot of trouble with old diodes, probably just as much as bad electrolytics if not more. A lot of the time they end up shorting which just causes a cascade of trouble as AC ends up going into the electrolytics through the shorted diode.
I just replaced some diodes (along with all capacitors) in a voltage doubler circuit on a Harman Kardon tube type amplifier...used 1N4007 (1 amp, 1000 volt) and they seemed to work fine.