Rogue Resistors.

   Just a little racontage here.  I acquired as I have many such "knick knacks", a PC (AT) "clone" power supply from the flea markets in the "not working when installed" category (warranty rejects).

   The problem was easily tracked to the one ohm five-watt wire-wound resistor that commonly substitutes for a NTC thermistor to limit the primary capacitor charge current through the rectifiers in these switch-mode supplies.   I simply swapped it over for one from my parts box and turned the supply on (with a "dummy" load - a motherboard with a chequered past and no real future).

   Surprise!   It still didn't work!

   Perhaps more surprising, perhaps not, the fault was the same; the newly-replaced 5W wire-wound resistor.   Actually, I had replaced the resistor with an almost identical part even to the fact that neither was brand-marked, presumably sister products from somewhere in Asia (or to be fair, the Americas).

   Now I should point out that I had checked the switching transistors and bridge rectifier, and at least resistance-tested the capacitors (insofar as that makes sense).   I was a bit short on resistors, so had to visit the local outlet (actually, not Dick Smith but Jaycar) in whose drawers I found a couple of similar resistors plus a couple of branded, Australian-made IRC (the R within an omega symbol) resistors of the same ratings.

   Fitting one of these resulted in a couple of years of reliable operation of the supply in my "work" computer until the fateful day that, trying to make it boot properly, I switched power off and on again within a couple of seconds to be greeted by an orange flash from the power supply perforations and a bad stink (the flash and smoke puff were visible since I frequently operate my machines, as I suspect do many who will read this, "naked").   The resistor had blown, but it went out in style, sending half of the casing explosively across the supply cage.

   In case you wonder, the bridge rectifier had shorted.    "One of these days", I'll probably replace it.

   The moral?   You get what you pay for, of course.    Buy decent components; the "imitation" ones don't handle two hundred amp surges, and they don't even whimper as they depart!