unknown machine

There are some who just feel it's fun to flame, I guess (or at least that's the only exxplanation I can find...). But htere have been more serious reasons for not doing it : It takes time (but not half as much time as rebuidlign a machine where the PSU has gone crazy!), you have to get some parts (the dummy load bulbs nd a multimeter, at least), and there's the possibility of damaging the machine by taking it apart (all I can say to that is that I've dismantled a couple of hundred classic computers in my time, and haven't don't permeanent damage to any of them). Probably most fo the time it _is_ safe to just power up and see what happens, just as mosto f the itme you don't need dual-circuit brakes in a car, you don't need seat belts, you don't need a tension band or an implopsion screeen on a CRT, etc. It's the other times that you're glad of them. And yes I have had PSUs that were faulty. Most recently was one of the cable PSUs in an HP11305 disk cotnroller, the 723 regualtor was defective and the output was sittign at about 12V. I don;t know what would have happened if tyhe crowbar had been connected (it's on a separate PCB, along with some of the logic), and I certainly wasn't goign to find out. Most likely it would have just been another burnt resisotr and a blown mains fuse. But it could have been worse... THe 2 reasons being (a) I haev a 'scope and (b) I know the trick anyway? Sure, I do that soemtimes, it's quite handy if you are working on a large machien and can't eaily get the 'scope to it. What you need is that the impedance of the capacitor at the mains frequency is small compared ot the input resistnace of the meter. Most DMMs are 10M Ohms (but soem are less o nthe AC ranges -- check the manual). Aanlogue meters are considerably lower. A 1uF capacitor should be fine for most cases. Even if you cna't get an accrate measure of the ripply, you cna at least see if it's higher than you might expect. One advantage the 'scope has though, is that it lets yo see the ripple frequency. On linear PSUs, ripple at twice the mains frequency is likely to be dried-up smoothing capacitors (jsut about all PSUs have full-wave rectifiers). Ripple at the mains frequency often means a defective diode in the rectifier. For SMPUSs, ripple at twice mains frequency leads you to the smoothing capcitors o nthe mians side. Ripplye at the stiching frequency (10's of kHz), particulalt sharp spikes, leads you to the output capacitors. -tony