Welcome to the Wonderful World of Magic Mirrors…
I'm pretty sure you're the guy I recently commed with on FB. Your situation sux BUT it could be worse. You at least still have a complete working MDD.
Start here:
http://www.jcsenterprises.com/Japamacs_Page/Blog/D275729F-09DA-4FBB-96B3-BEEEB2C04619.htmlYou use this guide to test the PSU pin-by-pin.
If you don't have a voltmeter, buy one. They're mandatory when using 20+year-old hardware anyway. It doesn't have to expensive.
If you find a PSU issue - and you very likely will - you can get it rebuilt here:
https://www.ebay.com/str/baltimora2000?_trksid=p2047675.l2563This guy has repaired many, many MDD and QS PSU's, is reliable and does good work.
HOWEVER, since it's the holidays, his turnaround time is longer than usual so you can:
1) Send it to him and wait patiently OR
2) Swap in the PSU from your 1.25 THEN Send the bad one to him anyway because your working one is just as old and they all can and do fail without warning.
If the PSU tests goodDid you get a monitor with that FW800? A bad monitor can fry the +25v out on a PSU (OR on the GPU card) and cause a no-start condition.
You can get fooled by that mess if you have say, a DVI monitor on the 1.25 and an ADC monitor on the FW800 because the broken one can sometimes continue to work via DVI.
…… just one of the little perks of Apple's ADC scheme.
Havin' fun now, right?