It uses an external mosfet. I can't quite see what it is.
I had some time this morning, so I removed all five electrolytic caps and tested them. There are two big caps near the mains input that are 10uF 400V. They both test good. Then there is a 10uF 50V cap that's somehow involved in the feedback circuit, and it also tests good. And finally, there are two caps on the output in parallel, both 680uF 16V. I believe those are bad, one twice as bad as the other, but both bad.
I don't have much test gear, so I have to make do. I use my DSO150 battery powered kit scope, which has a built in signal generator - a 3.3V square wave at 1KHz, which has an output impedance of about 1K ohms. I set it to AC, then simply connect the capacitor across the signal generator output, and attach the scope leads as well. A large perfect capacitor produces a straight line across the scope display - it's essentially a dead short. But a capacitor with a lot of ESR will display the typical capacitor charge/discharge curves.
And I use an analog multimeter set to 1K resistance to measure capacitance. Well, "measure" may be too strong a word. Basically, I just see how far the needle deflects as compared to a good cap with a known value.
Below are three scope display captures with the same settings on all three. The first picture shows a known good capacitor - a 470uF 35V that I got from Digikey a while back. Sorry the picture is so crappy, but basically all good caps look pretty much like that - either a flat line, or a very low amplitude square wave.
Then we have the first orignal cap, and the second original cap. Both look pretty resistive to me, the second being about twice as bad as the first.
I also desoldered and tested the output diode, and it looks good. It's an SB540, which is a big honking 5A Schottky. All the other circuit diodes are also good.
I don't have any 680uF 16V replacements. I have 1000uF's, but they are only 6.3V, and are too tall. So I'll have to add new caps to my next Digikey order. Meanwhile I have rigged up an alternate supply that will work for now, but isn't a permanent solution.
If replacing the output caps doesn't fix it, then I think I will have reached my limit as to how much to invest to revive this thing. I may put one of my 1000uF caps in temporarily just to see if that fixes it - before I bother with ordering the replacement caps.