Hi, contango!
Bypass any surge protector or UPS and connect the power cord directly to the wall outlet. If the power cord has a clear/transparent plugs, you should see a spark inside the plug as you connect it to the Mac, which will tell you that the Mac's power supply is indeed getting power through the cord.
Remove the battery from the Mac, do a PMU reset and hold the PMU button down for a five-count, depress the front power button for a five count, wait thirty minutes or so, and then reconnect the power cord, keyboard, mouse, and monitor, and try starting up without the battery installed. If successful, reinstall the battery and test startup again.
If the above is unsuccessful, and the MDD/FW800 Mac's power button doesn't light momentarily while you're depressing it, I'd check for a trickle voltage output from the power supply, or have someone who is knowledgeable about testing live circuits do this for you. Using a voltmeter to probe from the wire side of the power supply connector plug at the logic board, connect the positive probe to pin #1 (purple/+5.5v) and the negative probe to pin #12 (black/ground). Insure that the probes are fully inserted into the plug alongside the wires to insure pin to probe contact is made. The voltmeter should read +5.5v DC if the power supply is supplying the required trickle voltage. If the power supply is receiving power through the cord but is not supplying the trickle voltage, it's defective. If the voltage is present, there are more tests that can be done to check things further.
Here's a link to a color-coded
pinout diagram which shows locations for these connector pins.
Gary
1GHz DP G4 Quicksilver 2002, 400MHz B&W rev.2 G3, Mac SE30 Mac OS X (10.4.5) 5G iPod, Epson 2200 & R300 & LW Select 360 Printers, Epson 3200 Scanner
