A spare desktop in the corner of my office stopped powering on. No fans, no front-panel light, nothing. Pressing the button did precisely nothing, which is the most boring failure mode there is, because it tells you almost nothing about where the fault lives.
The motherboard or the PSU were the obvious suspects. The cheap, satisfying test for the PSU is the paperclip jumper: short the green PS_ON wire to a black ground on the 24-pin connector and see if the supply spins up on its own. It did not. The PSU was dead, the rest of the machine was probably fine, and a new supply is twenty-odd quid. By every sensible measure I should have binned it and ordered a replacement.
But I had a soldering iron, an idle evening, and a strong suspicion about what was wrong, because dead PSUs of a certain age nearly always die the same way.
I opened it up, and there they were. Two electrolytic capacitors on the low-voltage secondary side with domed tops, the metal vent on the crown pushed up into a little dome instead of sitting flat. That bulge is the textbook sign of a capacitor that has had enough. They dry out and fail with age and heat, and an old PSU run warm for years is exactly the environment that kills them. This is the tail end of the "capacitor plague" era hardware, and it is a genuinely common fault.
Before doing anything else, the safety bit, and I mean this seriously. A PSU has large primary-side capacitors that hold a mains-voltage charge long after it is unplugged. You unplug it, leave it, and discharge those big caps through a suitable resistor before your fingers go anywhere near the board. Get that wrong and the supply will bite you hard. I am happy to poke around inside one because I know which bits will hurt me; if you do not, this is not the project to learn on.
The repair itself was unglamorous. Desolder the two bulging caps, note the polarity (the striped side is negative, and electrolytics very much care which way round they go), and drop in replacements of the same capacitance and voltage rating, ideally a low-ESR type rated for 105°C rather than 85°C so they last longer next time. Two caps, a few minutes with the iron, a tidy-up of the joints.
Paperclip test again. The fan spun. Back in the machine, front-panel light, POST, the lot. It has been running fine since.
The honest accounting: a new PSU would have cost me twenty pounds and ten minutes. This cost me a couple of pence in capacitors and the better part of an evening, so on a pure time-and-money basis it was a terrible decision. But I did not do it to save money. I did it because there is a particular pleasure in finding two visibly knackered components, replacing exactly those two, and watching the thing come back to life. The throwaway path is fine. Sometimes the repair path is just more fun, and you end up understanding your hardware a little better for it.