Broken iMac G3 DV/SE 700MHz logic board

[Gustav Pihl Bohlin]

I have an iMac G3 700MHz (Graphite, 2001) that will not boot or even chime. It just "crackles", and I see a green LED turn on on the mainboard near the RAM slots, that goes out if I hold down the power button or pull out the power cable. I bought a separate, similar iMac G3 blue 400MHz (also slot loader) so I can swap parts and see what part is broken. The blue iMac boots fine, so it is a good testing machine.
I have eliminated the typical suspects:
Both PRAM batteries work in the blue iMac.
Both Down Converters work in the blue iMac.
The blue iMac boots and chimes with or without every permutation of ramsticks from the graphite.
At first I suspected the flyback transformer (which I have no idea how to fix), because the graphite's display quality deteriorated in its final years before dying 2 years ago. But... that can't be it - because the graphite's Mainboard does NOT boot in the blue chassis (which has a known working flyback transformer). So - the issue is the mainboard. Does anyone know where to begin figuring out what can be wrong with the mainboard? An error that leads to:
No chime
No boot
Green LED ON
No HDD or CD Drive action

I've read the service manual but I can't find instructions for troubleshooting above case - only if there are chimes or error sounds which I do not have.

 

[Gustav Pihl Bohlin]

I've probably overheated the CPU - my iMac had intermittent freezes in its last years, a problem that I think is due to overheating after browsing these forums. But I want to know for sure that that's the problem - and that it can't be fixed.

 

[Slix]

Did you try the RAM from the blue iMac in the Graphite one? Kinda doubt that's the issue but it's worth trying. I've had weird RAM issues on G3 iMacs before that can't really be explained.

 

[Gustav Pihl Bohlin]

Yeah I've tried the ram sticks.. The thing is, I'm supposed to be getting an error beep if I attempt to boot without ram sticks, that's what happens on the blue mainboard, but I get no beep on the graphite.