G4 Quicksilver power light dimms asa I release power button

I recently bought a used Power mac G4 Quicksilver. Unfortunately I was not able to boot the machine. When I press the power button, the power light lit up, but it dimms as soon as I release the button. The fans of the machine are spinning and there is a red light burning on the main board. Based on that, I concluded that the Power unit should be OK. The connected display remains dark however.

I've tried the following to fix this:

- Reset PRAM and installing new battery.

- Install a new working HD with a fresh install of Leopard. that drive is working in an other MDD mac I own.

- Used different start up option to force boot from CD.

Nothing helped and now I'm out of options.

Hope that someone from the vintage Mac experts can do a good suggestion to help me further.

Thanks anyway for reading!

Regards, Ruud