The machine wouldn't power on. No fans, no lights, the dead-silent kind of failure that usually means the power supply has given up. The easy answer is to buy a new one, and the supply in question owes me nothing. But it was Boxing Day, the shops were doing nothing useful, and I had a soldering iron, so I decided to have a look before reaching for the bin.
Safety first, and I mean this. A PSU holds charge after you unplug it, and the primary side can hurt you. Leave it unplugged for a good while, and discharge the big primary capacitor through a resistor before you go poking about. If that sentence made you uneasy, that is the correct response, and it is a fine reason to just buy a new unit instead.
the usual suspect
With the lid off, the fault was visible before I had even reached for the meter. Two electrolytic capacitors on the secondary side, domed on top where they should be flat, one with a faint crust of dried electrolyte around the vent. Bulging caps are the single most common way these things die. Years of heat dry out the electrolyte, the capacitance drops, the ripple goes up, and eventually the supply can't hold its output and shuts itself down.
I read the values off the casing: 1000µF 16V, low-ESR types. The replacements need to match the capacitance and meet or beat the voltage rating, and for a switching supply you want proper low-ESR parts, not whatever generic capacitor happens to be in the drawer. ESR matters here, and the wrong part will run hot and fail again in months.
the repair
The actual work is unglamorous. Note the polarity (the stripe is the negative leg), desolder the old caps, clean the pads, fit the new ones the right way round, solder, snip the legs. Desoldering braid and a bit of patience beats brute force; lifting a pad on a multilayer board turns a ten-minute job into an evening. Take a photo before you start so you can check the orientation against it afterwards.
Before reconnecting anything precious I bench-tested it with a cheap load and a meter on the rails. The voltages were back where they should be and steady, no sag, no flicker. Then, and only then, did I trust it with the actual computer. It powered straight up.
Total cost was about a pound in capacitors and an hour of my time. The wider point isn't the pound saved, it is that an awful lot of "dead" electronics are dead in exactly this way: a couple of failed caps that announce themselves visually if you take the lid off and look. Not everything is repairable, and plenty of modern gear is sealed and glued specifically so you can't. But a PSU usually isn't, and there is a real satisfaction in keeping a working thing working rather than feeding it to the recycling and ordering a replacement that will fail the same way in five years. Have a look before you bin it.