G4 FW800 won't start up

I had many of the problems others have reported and am in the middle of fixing the problem, I believe.

Symptoms: Mac just stops running, in sleep mode or even when lit up; Upon restart after normal shutdown, pressing the start button(upper left) no longer starts up the Mac. . . the button light temporarily lights then goes out as soon as fingere releases, the fans try to run for a fraction of second then stop. When the problem started happening I could get lucky sometimes by pulling the power cord and re-installing. . . sometimes only once degrading to having to do so multiple times before I got lucky and it started fully and ran. Eventually, I was forced to pull and try many times with no effect other than the light popping on and then out and the same with the fans.

Solutions tried: new PRAM battery. . . it was out of juice but the new one measured full 3.6 volts and did not change outcome, Resetting the main logic board per std methods suggested by the pros here(pull the cables and battery, reset the PMU, wait 30 min, install battery and cables and retry)-- no joy there either, I took off the optical drive and the power on board and testing revealed the button switch was OK. Eventually I found Japamac's suggestion to test the Power Supply (PSU). That seems to be the problem: only have 25 volts, not the other voltages coming into the board.

I have not yet tested the Hard Drive for failure, and until replacing the PSU, will not bother. However, that appears another major failure mode with similar symptoms.

My plan: try to find PSU's cheap, like less than $50, failing that I may pull the PSU and try to find a failed component. Not likely without an elec diagram I may just pull the hard drive and drain the data for use on another Mac.

Thanks to all the experts explaing the details for troubleshooting the PSU, the PMU, hard drive, and the power on board. All were useful in eliminating variables.

Regards, Brian in Oakdale,MN

any suggestions for cheap PSU replacement, repair.