If the PSU on you’re powermac g5 has gone, or you think it may have, you aren’t the only one (see macintouch forums). When I was reading around on the subject I read the failure rate is around 11%, I can’t remember the exact figure but hopefully I’ll find it again later. Luckily Apple did acknowledge the problem and has extended the warranty on the power supply, which you can claim here: Power Mac G5 Repair Extension Program for Power Supply Issues
A relatively new customer phoned us up last week in a panic, claiming his server (the powermac) had stopped working and wouldn’t even switch on. We’d recently set-up a remote backup solution for him and installed an extra internal hard drive in the powermac for local backup. I’m guessing the extra load of the hard drive took it over the edge. Needless to say he needed the server back up asap.
Phoning around we found one supplier with a spare powermac psu in stock. None of the local apple stores did and most companies sounded confused, as if I was asking a silly question. We ended up paying £150-£200 for the psu, a little on the expensive side but desperate times call for desperate measures.