The machine had been getting flakier for weeks. Random reboots under load, the occasional refusal to start until I flicked the switch on the back twice. The kind of fault that makes you suspect RAM, then the board, then your own sanity, in roughly that order. It was the power supply, and the power supply was dead because of a single capacitor that cost forty pence to replace.
I nearly binned it. A modern ATX unit is cheap enough that an hour of your time is arguably worth more than the part. But it was a decent semi-modular 650W unit, only a few years old, and the wasteful part of throwing a kilo of perfectly good copper and steel in the skip over one failed component had started to bother me. So I opened it up. With appropriate caution, because the inside of a power supply will hurt you.
The safety bit, which is not optional
A PSU stores energy on its primary-side capacitors, and those big ones can hold a lethal charge long after the unit is unplugged. The bleed resistors are supposed to drain them, but you do not bet your heart rhythm on a bleed resistor you have never met.
Unplug it. Leave it an hour if you can. Then, before you touch anything on the primary side, discharge the main bulk capacitors deliberately, through a resistor, with one hand kept firmly in your pocket. I use a 10k resistor on insulated probes and watch the voltage fall on a meter. It is dull and it feels like overkill right up until the day it is not.
If that paragraph made you uncomfortable, good. That is the correct response, and it is the reason most people quite reasonably just buy a new one.
Finding the culprit
With the cover off and the caps confirmed dead, the fault was almost embarrassingly visible. One of the secondary-side electrolytic capacitors, on the low-voltage output rails, had a domed top where it should have been flat. Capacitor plague is still alive and well: the electrolyte ages, gases build up, the can bulges, and the safety vent on top splits. Sometimes you get a little crust of dried electrolyte to confirm the diagnosis.
The giveaway is always the top. A healthy electrolytic cap has a flat or very slightly concave top scored with a cross or a K-shape, the vent. A failing one domes outward as the pressure builds. Once you have seen one you spot them across the room, and you start eyeing every cheap appliance in the house with suspicion.
This one was a 1000µF 16V capacitor on a rail that, when I poked it with the scope, was carrying far more ripple than it should. That ripple is exactly the kind of dirty power that makes a motherboard reboot at random under load. The board was not faulty. It was being fed rubbish.
The repair
The replacement is not glamorous. You want a like-for-like part: same capacitance, the same or higher voltage rating, and crucially low ESR rated for the temperature, because the cheap general-purpose part will just fail again in a hot PSU. I match the ripple-current and temperature rating too. A 105°C low-ESR part is the floor for anything living next to a power supply's heatsinks.
Desoldering is the fiddly part. These boards have a thick ground plane that wicks heat away from your iron, so the joint refuses to melt with a normal tip at a normal temperature. I turned the iron up, added fresh solder to the old joint to improve thermal contact, and used a desoldering pump to clear the through-holes. Observe polarity on the way back in: electrolytics care which way round they go, and fitting one backwards turns a quiet repair into a small, loud event.
While I was in there I gave the rest of the secondary caps a hard look, because where one has gone from a bad batch the others are often not far behind. Two more were faintly domed. I replaced all three for the sake of another quid and another twenty minutes, on the principle that opening a PSU twice is twice the risk for no extra reward.
Was it worth it
Strictly on money and time, no. The capacitors cost me about a pound delivered, but they came in a bag of a hundred I will never finish, and the repair plus testing took the better part of an evening. You can buy a new 650W unit for the price of the evening.
But the machine has been rock solid since, the unit went back into service instead of into landfill, and I now own a meter reading that tells me, when the next mystery reboot arrives, to check the power before I blame the RAM. That last part is the real return. Knowing what a tired PSU looks like on a scope is worth more than the forty pence, and you only learn it by opening the thing up rather than driving it to the recycling centre.
The bag of capacitors will outlive me. I am at peace with that.