[Info-vax] SBB power supplies failing slowly

[Scott Dorsey]

gah4  <gah4 at u.washington.edu> wrote:
>
>The popular design some years ago (as in when I was actually looking at
>how they were built) is a bridge rectifier in 240V countries, and voltage
>double in 120V countries.  So the primary side capacitors run
>about 300V in both cases.  That is, for ones with a 120/240 switch.

Right, you don't see that very often anymore.  You will still see a switch now
and then, but these days the PWM range is so enormous that there is no need.

>When working with the computer museum, we would replace all the
>electrolytic capacitors before powering up the first time.  That is,
>for ones that might be 30 or 40 or 50 years old. 

This is often a good plan.

>They used to make "computer grade" capacitors.  I don't know
>if they still do that.

Computer grade capacitors hardly ever fail, but they also have incredibly
high ESR and very poor high frequency performance.  They are designed for 
60 Hz ripple filtering and they do very well for that... they can handle a
lot of ripple at 60 Hz.  They are also expensive and died out pretty much
with linear supplies in the eighties.
--scott

-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."