A machine in the corner of the office stopped powering on. Fans twitched, nothing came up, the usual sad half-second of life followed by silence. I'd half-decided it was the motherboard until I swapped the PSU for a spare and everything sprang back. So the power supply was the culprit, and the easy thing would have been to bin it and buy another.
Instead I unplugged it, left it sitting for a while to let the big primary capacitor discharge (this matters, those things hold a charge that will genuinely hurt you), and took the lid off.
You can often spot a failed electrolytic capacitor by eye. A healthy one has a flat top. A dying one bulges, sometimes domes right up, sometimes leaks a crust of brown gunk out of the vent scored into the top. Two of the secondary-side caps were clearly unwell, both rated for the low-voltage output rails, both domed.
This is the most common failure mode in cheap power supplies of a certain age. A bad batch of capacitors from years ago, the so-called capacitor plague, means a whole generation of kit dies the same way. The capacitor's job is to smooth the rectified output, and as it ages its ability to do that falls off, the ripple climbs, and eventually the supply can't hold a stable rail long enough to pass its own power-good check. So it refuses to start. Not dramatic, just tired.
The fix is genuinely cheap. A handful of replacement caps from the usual suppliers costs less than a coffee. The trick is matching them properly: same capacitance, voltage rating equal or higher, and crucially low ESR parts rated for at least 105°C, because the ones in a PSU live a hot, hard life. Get a 85°C bargain part in there and you'll be back inside the case before long.
Desoldering through-hole capacitors out of a multi-layer board is the fiddly part. Those ground planes suck heat away fast, so you want a hot iron and ideally some flux. Watch the polarity, electrolytics care which way round they go, and the board is marked. New ones in, reflow, snip the legs, double-check nothing's bridged.
It started first try. No drama, no smoke, which with mains-adjacent electronics is exactly the outcome you want.
The wider point: an awful lot of "broken" electronics has failed in one small, replaceable way. Not everything, and not safely for everyone, mains kit deserves respect and a healthy fear of the primary side. But a domed capacitor is one of the most fixable faults there is, and there's a quiet satisfaction in a fiver of parts and twenty minutes saving something from landfill. The spare PSU went back in the drawer for next time.