I had exactly the same problem and the problem was a burnt line on the back of the motherboard. Check out around E561C (near serial connector) and C311-C314. There should obviously be some visible damage, but it is pretty easy to fix by soldering some wire to bypass the damage. Also, on the front side of the motherboard, check next to RAM chips.
I got this damage by shorting 12v/5v lines when fiddling around to power a 3.5" hard drive from floppy power and as the end result lost 5v supply (or it was around .5v). As someone has toyed with the hard drive mounting area, I suspect you should follow this path to floppy power connector.
If there's no visible damage, then it's time consuming multimeter work awaiting.
Mikko.
