A little class-D amplifier board died on me. No smoke, no smell, just nothing: power LED off, no output. The honest thing to do with a £15 board is bin it. I wanted the practice, so I sat down with a multimeter and a magnifier instead.
The rule I try to follow is don't touch the iron until you've found the fault. Soldering at random is how a slightly broken board becomes a thoroughly broken one. So: meter on continuity, check the fuse first (intact), check there's voltage arriving at the input (there was), then trace where it stops. The 12V was present at the barrel jack and absent two pads later. Something in between had gone open circuit.
Two things, as it turned out. The first was a cracked solder joint on the power inductor, the big one that takes mechanical stress every time the cable is wiggled. Under the magnifier it had the tell-tale ring fracture, a grey hairline circle around the lead. That's a classic for anything heavy or anything near a connector: thermal cycling and vibration slowly work the joint loose until it's a joint in name only.
The second was a pair of electrolytic capacitors on the input rail with domed tops. Bulging caps are the most obvious failure in consumer electronics, the vent on top giving way after years of heat. They read nothing useful in circuit, but a domed top is enough to condemn them.
The fix is unglamorous. Reflow the cracked joint properly: clean tip, fresh flux, a touch of new solder to get it flowing, hold the iron until the whole joint goes liquid and reforms as a shiny cone rather than a dull blob. Flux is the thing people skip and shouldn't; it's the difference between solder that wets the pad and solder that sits on top sulking.
The caps came out with the usual two-pad dance: heat one leg, lean the cap, heat the other, walk it out. Clear the holes with desoldering braid, drop in two replacements of the same value and voltage rating (and mind the polarity, electrolytics are very particular about which way round they go, and emphatic about it if you get it wrong), and solder them in.
Powered up on the bench supply with my hand near the off switch. LED on. Audio through it, clean. The whole job was maybe forty minutes, most of it the diagnosis rather than the soldering.
I keep coming back to the same lesson with this stuff. The iron is the last ten percent. The meter and the magnifier are the job. Most "dead" boards aren't dead, they've got one bad joint or one tired capacitor, and finding it is far more satisfying than the £15 it saved.