I recently bought a second-hand PowerMac G5 Dual 2.0GHz June 2004 model. The guy that sold it to me said he was selling it because of a power supply issue. He told me that an Apple Genius noticed a PSU failure and that a PSU replacement would cost him about 350€ (I'm in Italy), so he decided to sell the computer. Meanwhile he bought a replacement PSU from an eBay shop. When I first plugged in the Mac (with the original PSU), there were no life signs. I thought that was a real PSU problem and I decided to test the new PSU before putting it into the computer. I connected the PSU to the power and... it literally exploded. Lucky I didn't put that PSU into my G5.
Anyway, I'm in the same situation as before. When I first tried to plug in the computer, I noticed a very short blink from the power LED. However, the computer won't start at all. No fan noise, no lights, nothing. I then tried to reseat RAM, video card, fans... nothing. I then tried to replace the PRAM battery and resetting the PMU, same results. I decided to reseat the processors being extremely carefully, but nothing. At this point, I decided to test the original PSU on the bench and it seems to be outputting all the correct values (all the 5V, 3.3V, 12V, -12V, 25V) on the correct pins, even under a small charge. I then tried connecting pins 14-23 and the PSU fan started running. I then tried to manually give the OK for the voltage to come up on the logic board by connecting those pins with the PSU plugged into the logic board. All the fans started running very slowly, but that's it. Nothing else happened (Note: I tried this without RAM, CPUs, video card etc. I then read that this model won't start without even one CPU, they both have to be on the logic board, that's probably because it didn't proceed with the boot sequence).
Any idea? I don't want to spend 950€ on a new logic board, and I may try to repair the components on the logic board myself if it's necessary (I have a lot of experience in soldering etc, I already replaced 6 caps on an iMac G5 logic board and now it works perfectly). Anyway, reading on the web, I think I have a bad switch on the front board (the one with the power button). The button itself works, maybe the circuitry on the board died. I'm not going to buy any component before I'll find out what the exact problem is BTW.
I rely on your help! Thank you (sorry for my english)!