Update. The G4 is not working. When the power button is pressed, the computer comes on, but no chime and no dings to indicate problems. The cooling fan runs on the Sonnet 1 GHz upgrade card. The red LED stays lighted and the computer stays on (or at least there is power of some form enough to spin the fans and turn on the re LED).
Attempts to start the computer using DW 3 or DW 4 have not worked. Attempts to use TechTools 3 and 4 have not worked. The DVD drive starts. The green LED flickers a few times while I hold down on the C key. After a while I press the disk eject button and the disk is ejected (a good startup will not eject until the startup is complete, or fails).
I pulled the FirmTek/SeriTek card and the SSD drives. No change. I pulled the add on USB FireWire card. No change. I pulled the Stealth GeeThree card. No Change.
I checked the PMU battery. 3.66 volts. So that's not the problem.
I disconnected the power cord waited 10 seconds. Pressed the PMU button (not a CUDA switch on an AGP) one time, waited 10 seconds. No change.
I bought a guaranteed to be good rev. 2 (uni-N 7) motherboard, installed it and have had the same results. That board was squeaky clean and came with the correct original processor. I've not tried the computer with that processor.
I checked the internal speaker at two locations (I get no internal chime, since about a month ago when there was a load pop. External speakers worked fine, just no startup chime. Now no sound at all). I checked the internal speaker on the contacts on the speaker 8.0 Ohms and at the connector 8.0 Ohms as well. Spot on.
Without drives, if proper power up was to happen, I would get the question mark.
It's possible that I scrambled the PMU. I did handle the board with the battery in the board (both the old and the new before I realized that I was not supposed to do that).
It's also possible that one of the diodes on the bridge rectifier in the PSU blew out and that that's what I heard. I'm not quite certain how to check voltage on the PSU or where to check it. I have had older PowerMacs (one 7100 in particular) that limped along on three diodes for several weeks until the second one blew (out of four) and shut down the PSU.
I looked at the pins on the underside of the Processor daughter card and the little clips the pins go into. Everything is absolutely straight.
It is possible the processor is dead, but I should still get some sort of beep or chime.
I had the display (an HP 24N) hooked up with a DVI cable. I replaced the cable with a known good VGA cable. No change.
The Apple troubleshooting guide says to check and reset the PMU, check the PSU, and then to remove and reseat each of the cards that are absolutely necessary, and as a last resort, to get a new CPU or logic board.
I've not checked the PSU voltages and not pulled and reseated the AGP card (it was good the last time I booted the computer successfully). Nor have I put in a supposedly good CPU. As I said to start, the logic board is one that I got on eBay and which was guaranteed (money back) to be good.
Right now I'm waiting on a new latch panel for the right side access panel (it won't stay closed without tape, because the "claws" are all broken. I have the broken latch panel on my desk and cannot put in the logic board until the new latch panel is back in place.
Each time I have tried to start the computer, the cable that goes to the CPU has been properly seated. Again, if that cable were not properly seated, I should still get some chime or beep. The IDE cable does not have to be connected since the drives were on the PCI bus.
I'm not certain how to reset the PMU if it has lost all of its settings. The PMU walks the computer through all of the startup routines until the drives can be engaged. If that got well and truly scrambled, could it reset itself?
Thanks for any suggestions.