The bench supply went dark mid-afternoon. No fan, no click from the relay, nothing on the front. The obvious move is to bin it and order another, and I nearly did. Then I remembered it owes me nothing, it was cheap, and the worst case is I learn something.
Lid off, and the diagnosis took about thirty seconds. Two electrolytics on the primary side were domed at the top, the tell-tale bulge that means the can has vented. One had wept a little brown crust down its side for good measure. The mains fuse had also gone, which is usually a symptom rather than the cause, so I made a note not to just slap a new fuse in and hope.
I had a couple of 105°C caps of the right value in the parts drawer, which is the only reason this was a five-minute job and not a parts-order job. Desoldering the old ones took longer than fitting the new, mostly because someone at the factory was generous with the flux. New fuse, a careful look for anything else scorched, and a long stare at the board before reconnecting mains.
It powered up first time. Fan spun, relay clicked, output came up clean on the scope. Total cost: about 40p of capacitor and a fuse I already had. It is not always this tidy, sometimes the thing that killed the caps is still lurking and takes them out again, but for a unit I was about to throw away it was well worth the half hour. Keep a parts drawer. It pays for itself the first time something like this happens.