I had the same problem and couldn't get the PSU repaired, so I managed to find and old non-standard ATX power supply with the -5V line and it fitted perfectly inside the A3000 PSU case. I had to resolder the IEC socket, but that was a doddle.
Then I used an ATX-to-A3000 adapter cable - which used to be made by Nathan (Amiga.org = Amigamaniac?) to connect it to the A3000 MB. Worked like a charm.
Here ya go - I just found my old thread:
http://www.amiga.org/forums/showthread.php?t=52357
Mike.
