MicroVAX 3100/95 PSU Weirdness

[Toby Thain]

On 2020-04-22 6:19 PM, Rob Jarratt via cctalk wrote:
> 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? 

You should be able to see by inspection of the PCB.

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?

This unexpected effect is due to dielectric absorption.
https://www.robotroom.com/Capacitor-Self-Discharge-5.html

http://slot-tech.com/interesting_stuff/sencore/LC103/TT105%20-%203759.pdf

--Toby

> 
>  
> 
> Thanks
> 
>  
> 
> Rob
>