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 >
