The router had been rebooting at random for weeks. Not a software fault: it would drop dead under load, come back, drop dead again. The kind of thing you blame on the firmware until you put a multimeter on the input rail and watch the voltage sag every time the wireless ramped up.
So before binning a perfectly good little box, I opened it. The smell told the story before my eyes did. One of the electrolytic capacitors near the barrel jack had vented, that faint fishy whiff of dried electrolyte, and the top was domed where it should be flat. A 470µF 16V part, cooked by years of sitting next to a warm regulator.
The replacement cost about eight pence. The work cost a careful twenty minutes. Desoldering electrolytics on a cheap two-layer board is the easy bit; the trap is the ground plane sinking all your heat so the joint never quite flows. I cranked the iron to 360°C, added a touch of fresh leaded solder to the old joint to get it moving, and the pad let go cleanly. New cap in, correct polarity (the stripe is the negative leg, and getting that wrong makes its own little bang), and while I had the iron hot I reflowed the barrel jack too, because those through-hole power connectors crack their joints from years of plug-and-unplug stress.
Powered it back up on the bench and the rail held flat through a sustained download. No sag, no reboot. It has been up for a fortnight now without a hiccup.
None of this is clever. It is the most basic repair there is, and that is exactly why I keep writing it down. We have quietly decided that consumer electronics are disposable, and most of the time the actual fault is a part you can buy in a bag of fifty for a couple of quid. A magnifying lamp, a decent iron, and the willingness to look before you reach for a replacement on Amazon. The board did not want to die. It just wanted one capacitor.