Dead G5 Power Mac: This is what I've tried so far

I have an identical G5 to yours, and I've done similar troubleshooting on it and similar machines.

When you test the PSU (at least under Apple's instructions in the service manual), you're only testing for 24V trickle charge. This voltage is present anytime the PSU is plugged into the wall, to provide power to the PMU and SMC circuitry. If you have no trickle charge, the PSU is dead. If you have trickle charge, the problem is elsewhere.

I presume you've plugged it into a working wall socket? (You may think that's a stupid question, but ...)

Was the charge on the battery under 3V? If so, try without the battery, and/or replace it.

Does the front LED power light come on at all? Does it blink in a particular pattern? (Three flashes, pause, two flashes? Two long, one short? One if by land, two if by sea?)

Check that RAM is paired properly. Memory should be installed in matched pairs. Beginning with the centermost slots, install one stick from each matched pair. Add additional sticks in matched pairs, working outward in a mirror-image from the center pair.

To clarify, if you have eight slots ABCD EFGH, where A is at the top and H is at the bottom of the logic board, the first matched pair should be in D and E, the second in C and F, the third in B and G, and the fourth in A and H, sort of like playing "Chopsticks" on the piano. There should be no unmatched pairs, or gaps between sticks.

If you have only four slots AB CD, you should match B and C, then A and D.

Have you tried disconnecting unnecessary components one at a time (FireWire devices, USB devices, secondary HD, primary HD, optical drive [note: disconnect both power and data cables from drives], AirPort Extreme card, Bluetooth card, modem card, all but one pair of [preferably Apple-branded] RAM sticks, PCI and AGP cards) to see if any one of them is interfering with the power-on sequence?

Have you tried replacing the front panel board (the one that contains the power switch, USB, FW and headphone ports)?

Beyond this point, you're down to troubleshooting the CPUs and the logic board. You'll need to make sure you have the proper diagnostic utility for this machine in order to run the necessary thermal calibration if you remove or change position on the CPUs. Without thermal calibration, all fans will run at full bore all the time: you'll think you've moved next door to a DustBuster factory. Try all the other troubleshooting steps *first*, and report back.

You've double checked all connections to the motherboard including the front panel connection? With new PSU installed, plug the G5 into the wall and leave it plugged in overnight and then try it in the morning--has worked for me with older towers. It also could be the video card. It's a stock video card? A bad video card kept at least one G4 tower I had from any sign of life at all. Replaced the card and it booted right up. My approach to checking would be, battery and PMU first, ram seating second, video card third, and then PSU. Gotta have another video card to check, of course. I've got a flashed PC Radeon 9800 Pro in my G5 and in a couple of G4's. They're the most easily flashed and compatible PC cards for AGP Macs. They work great. I guess you don't have extra comps to switch out and test parts with the G5? Usually, you really have to stress out a video card or a PSU to make them go bad. RAM, on the other hand, goes bad sometimes for no apparent reason at all which always makes it the primary suspect for startup problems, but the comp will usually turn on, although often not boot, with bad RAM. Going straight to the PSU and investing money in that right away is jumping the gun from my experience. Like I say, if you've got another video card handy, that might be the first and easiest to check off the list. Try booting without a video card installed. I don't remember whether or not you'd get at least a panel light blink without a video card, but any blink would be a real sign of hope.