MicroVAX 3100/95 PSU Weirdness

[Rob Jarratt]

I am on a mission to fix a bunch of power supplies and now I am looking at
my MicroVAX 3100/95. A few days ago I mentioned that the big smoothing
capacitors on the primary side might need replacing. I have done that now.

 

However, in doing so, I have discovered that *one* of the capacitors does
not get discharged after the power has gone off (this applies both to the
original ones and the brand-new replacements). Furthermore, after
discharging them with a resistor and checking that the charge had gone,
several hours later, the one that does not discharge, has some charge again,
that was not there before!

 

Does anyone know if these PSUs have a bleed resistor to discharge the
smoothing capacitors? Why would only one be discharged, is there normally
one bleed resistor per capacitor? Why would the capacitor acquire charge
again when it hasn't been powered on?

 

Thanks

 

Rob