PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Fri Jul 17 13:42:46 CDT 2015

> > It is generally a good idea to re-form electrolytic capacitors in power
> > supplies, and to bench check the power supplies (under some kind of
> > load) before actually applying power to the whole unit.
> It is always a good idea to replace electrolytic capacitors in power supplies.

Could you, please, explain why? And how often should this be done? Every
week, every month, every year, or what?

FWIW, the number PSU elecrtrolytics I have replaced can be counted on the fingers of
one hand -- in unary. Well, perhaps both hands. But it's <1% of all the PSU electrolytic
capacitors I own. 

Only 2 cases spring to mind :

The PSU in my 11/44 had a high ESR capacitor on the +36V rail (all other caps in the machine
were fine)

I changed the 2 mains smoothing capacitors in my HP120 not because they were electrically 
defective (they tested fine) but because one was bulging a little on top and had it exploded it would
have hit the neck of the CRT with all the problems that would be likely to cause.

I do find this witch-hunt against capacitors to be curious, given how few I've found to have 
failed. I suspect a lot of it comes from audiophools who think this is the way to fix anything...


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