My recently acquired DECstation 5000/240 has had a PSU failure. After replacing a couple of small capacitors with high ESR, I have been doing some basic checks to see which stages are working. My first step has been to check the output of the input rectifier, which has two large capacitors to smooth the output, they are each rated at 200V. When I check the rectifier output with a basic DMM I measure about 136V on the AC range, and a voltage ranging around 160-170 on the DC range.
I know that some PSUs double the voltage here, and the presence of two large capacitors would suggest that happens here too. However, from what I have read, the doubling is done on PSUs with a voltage selector switch. This PSU does not have an input voltage selection switch. I don't know if on auto-ranging PSUs the rectifier output should be 325V or 163V. However, given that each capacitor is rated at 200V then it seems to me that the rectifier output is expected to be 325V. Is that right? For the record, both of the capacitors do feel like they might have a slight bulge at the top, and on the underside of the board of one of them there is some mottling, which suggests there may have been a leak at some point. The bases of the capacitors are in full contact with the board, so I can't really see if there is any leakage. They seem to have been glued in with epoxy too. Testing them in-circuit with the multimeter, one appears to charge slowly, the other is immediately open-circuit. So clearly there is a problem here and they need to be replaced anyway. The supplementary question is about finding replacements. It seems that almost all capacitors of the exact spec (820uF, 200V) are of the snap-in variety. I don't have experience of these compared to normal leaded ones. The leads seem to be 11mm apart, but the snap-ins I find have them 10mm apart, if the leads are effectively very short then I am not sure if snap-ins are a wise choice. Can anyone advise? Also my usual supplier (Farnell) has a limited choice, if I went for a slightly higher valued capacitance (1000uF instead of 820uF), is that likely to cause a problem? Regards Rob
