I got a new power supply to replace the one that got broken by jitter (two, actually, but one of them doesn't seem to work). This one is from an IBM PC 330 server that I got for $5 from the local second-hand store. I can tell that it works, as it actually spins up a little even when no power is drawn. This is the label:
However, it's a big and special power supply with connectors I haven't seen described in the usual literature.This connector P3 looks a bit like the two old AT style connectors rolled into one, but the wiring color is all wrong.
Front look of P3.
There are also two huge connectors P1 and P2 (not connection to bananas) that I have never seen before. Serious mofos with thick wires (standard harddisk connector shown for comparison - look at the size of those things!). Probably can't use these easily, but I feel certain that it can give the output I need. It also has not 4, not 6, but 8 standard harddisk connectors plus assorted small bits.
Here is a better view of the wires for the non-standard 10-pin connector. The white would be -5V, the gray a power on indicator, the black ground, the yellow 12V, the blue -12V, but I don't know what the brown would be, and the green is a power on input. Maybe the brown is 3.3V sense (only brown I've seen mentioned), but that makes no sense with no 3.3V output.
Update: Tried with shorting the green to black, and with a 7W 1 Ohm resistor between a 5V and ground. The power supply hums merrily, but there is nary a blink of power output. Strange.