On Mon, Aug 28, 2017 at 12:49 PM, Rob Jarratt via cctalk <cctalk@classiccmp.org> wrote: > 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?
If you havr 230V mains input and you full-wave rectify it and smooth it (say using a bridge rectifier) you will get 230*sqrt(2) or about 325V. There is no easy way to get 163V DC from a 230V AC input (you could do it with a step-down transformer or a voltage regulator circuit, of course, but not with just diodes and capacitors). The common input circuit for PSUs with a voltage selector switch is quite ingenious. With the switch open, it's a bridge rectifier and smoothing capacitor (the latter is actually 2 capacitors in series, with bleeder resistors, each charged to around 163V). With switch closed, it becomes a voltage doubler. Two of the diodes do nothing, but again each capacitor charges to around 163V (with a 115V AC input). A lot of older auto-sensing power supplies used that circuit, with an automatic switching component (triac or similar) in place of the switch. The first thing to check is if the 2 capacitors are in series. Does the +ve terminal of one go to the -ve terminal of the other? What voltage do you measure across each capacitor separately? What is your mains voltage? 115V or 230V? If the former then it's possible the automatic switching has failed and that it's acting as if it is connected to 230V mains. -tony
