The machine under the desk went dark mid-afternoon. No fans, no beep, no click of a relay, just the small disappointed silence of a thing that was working a second ago and now isn't. My first instinct, and probably yours, was to order a new PSU and get on with my life. Instead I spent an evening with a multimeter and a soldering iron, and I want to walk through it, because fixing the thing turned out to be both cheaper and more satisfying than binning it.
Confirming it's actually the PSU
Before you open anything, rule out the obvious. I swapped the kettle lead, tried a different wall socket, and reseated the 24-pin. Nothing. The classic next test is the paperclip trick: unplug the supply from the motherboard, bridge the green PS_ON wire to any black ground on the 24-pin connector, and see if the fan spins when you switch it on at the wall.
24-pin ATX, PSU end:
green (PS_ON#) -> bridge to...
black (COM/GND)
Spin means the supply at least starts. No spin means it's refusing to turn on, which is where mine was. Dead at the first hurdle.
A safety word, said once and meant: a PSU holds mains-level charge in its primary capacitors long after you unplug it. Leave it off the wall for a good while, and do not poke around the big caps on the primary side unless you know how to discharge them safely. I'm comfortable in here; if you aren't, this is a fine place to stop and buy a new one. No shame in it.
Opening it up
Four screws, lid off, and the diagnosis was sitting there in plain sight. Two of the electrolytic capacitors on the secondary side, the low-voltage output stage, had domed tops. Capacitors are supposed to be flat across the top. When they bulge, the electrolyte inside has cooked and vented, and the cap can no longer hold the value printed on its side. This is "capacitor plague" in its everyday form, and it's the single most common reason a mid-life PSU dies without drama.
I noted the values off the cases before anything else: a 1000µF 16V and a 2200µF 10V, both general-purpose 105°C parts. The 105°C rating matters. The cheaper 85°C caps are exactly what got the supply into this state, so I wasn't about to replace like for like and book a return visit in eighteen months.
The actual repair
Desoldering on a double-sided board is the fiddly part. The ground plane sucks heat away, so I bumped the iron up, added a little fresh leaded solder to the old joints to get them flowing, and worked the legs free one at a time with a solder sucker. Patience beats force here; lift a pad and you've turned a 30p fix into an afternoon of bodge wires.
Then the bit that's easy to get wrong and costly when you do: polarity. Electrolytics care which way round they go. The board marks the negative leg with a shaded half of the silkscreen circle, and the cap marks negative with a stripe down the can. Stripe to shaded side, every time. Get it backwards and you'll learn exactly how loud a capacitor can be.
New caps in, joints reflowed until they were shiny little cones rather than dull blobs, legs trimmed, and a good look under a loupe for any whisker of solder bridging to a neighbour.
Moment of truth
Paperclip test again first, on the bench, well away from anything I cared about. The fan spun. I let it idle for a few minutes with the multimeter on the 12V and 5V rails: 12.06 and 5.04, rock steady, no creep, no smell. Only then did I trust it back into the machine. It booted first time and has been up ever since.
The replacement caps cost me well under a pound for the pair, out of a drawer of bits I keep for exactly this. The total outlay was an evening and a pleasant hour with a soldering iron, against forty-odd quid and a guilty little box of e-waste by the bin.
I'm not romantic about repair for its own sake. Plenty of dead hardware is genuinely dead, and your time has value too. But a PSU that won't power on is one of the higher-odds repairs in the box. The failure mode is visible, the parts are pennies, and the fix is squarely within reach of anyone who can solder a tidy joint. Pop the lid before you pop it in the bin. More often than you'd think, the problem is staring right back at you with a swollen little head.