I would be grateful for any clues or a course of action with a recent attempt to upgrade the logic board of a slot loading iMac. I have a Blueberry S-L 350 iMac - one of the original slot-loaders (ie the generation before the indigo 350 came out). I recently bought a 400 logic board/down convertor on ebay so I could have that tad extra speed + firewire + VGA out. After fitting it onto a known good power supply (my current iMac) it chimes on startup, I hear brief disc activity, it briefly lights up my USB hub and the caps lock key and then I can hear the disk spin down and the front led starts to pulsate. The screen doesn't kicks in at all. The on-board LED at D511 goes dim after the disk spins down. If I try to reset, I won't get a chime - unless I unplug and plug the AC cord. I reset the PMU. This 400 board has a daughterboard just behind the Airport socket - I would guess (only a guess) that is part of the video subsystem. Anyone have any ideas?

Either a bad P/A/V Board (Power/Analog/Video) or a bad Logic Board.

Well the PAV is fine as I'm typing this using it and my old 350 Mobo. So the problem is somewhere on the Down Converter or logic board. One thing occurs: the PRAM battery is completely dead - would this affect start up? Would the logic board even chime, the light come on and the first murmur of disc access occur if the logic board was kaput? The other thing that happened was that the cd-rom drive would not accept a cd - it took it in really slowly then spat it back out really slowly. Does this ring any bells with anyone? I'm obviously keen to explore any avenues still open...

I have read that a dead pram battery can cause startup problems in imacs. It is worth a try replacing the batt. Radio shack carries them.