Minimum setup to boot Dual 1.42 FW800 MDD (vid card req'ed?)


I've got a Dual 1.42 FW800 MDD motherboard that won't boot up.

When I press the power switch, only the white power LED lights up. Nothing else lights, spins, or otherwise makes noise. Not even the tiny, red, diagnostic LED on the motherboard.

I've done the obvious and not so obvious:
  1. Tested the battery in the board. OK. Replaced it just in case.
  2. Reset the PMU as described in this apple doc.
  3. Tested the power supply by "jumping" the power-on line so that all lines go live... all lines measured the correct voltages.
  4. Tested the power supply further by "jumping" it while connected to the motherboard. Doing this forced power to the motherboard and all drives & fans so that things did spin up and the diagnostic LED lit up... but still no "bong" from the speaker nor any image from the video card.
  5. Replaced the power switch and associated ribbon cable.
  6. I even pulled out the motherboard and tried it on a whole different setup (2nd power supply, 2nd set of drives, 2nd power switch, 2nd speaker, etc). But still the same behavior... just the white LED on the power switch lit up. Nothing else happened. The ONLY device shared in common on this second setup was the AGP video card. At the moment, I only have one video card, an ATI Radeon 9000 Pro.
  7. I sent the motherboard back to Mac-Pro, who tested it and said it worked for them... and subsequently sent it back to me.
So all that makes me think that the culprit could be the video card... the only device I didn't have two of.

BUT... if I pull out the video card, the computer still won't respond to the power switch.

Therefore, my question is... what is the minimal configuration that will boot up a FW800 dual 1.42 motherboard? In other words, what devices MUST be connected to it in order for the motherboard to boot, and what devices can I leave disconnected?

Starting with the power supply and power button... what other devices are absolutely required to get the motherboard to turn on? By "turn on," I mean for the diagnostic LED to light... and if the speaker is attached, for the speaker to make ANY noise.

I figure I should start with the absolute minimum connections so I can narrow down whatever suspect device is preventing the motherboard from booting, whether it's the video card or not.

(Edit) Yes I tried the bare minimum (power supply and power button only) and a few less-than-minimum configurations (adding first memory, then speaker, then fans, then one drive). But none of the configurations I tried would do anything (no noise, no motherboard LED). Therefore, if a video card is required for the motherboard to turn on, maybe I need to go out and buy a 2nd video card?