Screen shuts off immediately!

I have the metioned iMac here where i replaced the optical drive. I tested it (with the front bezel off) and everything was great. The suddenly the screen died.

Now, when I switch it on i see the grey startscreen for a second or two and then it gets dark again. I zapped pram and SMU. I tried to boot it from CD. No change. I haven't hooked it up to an external monitor but will do so in a minute and post the results. It appears to start normally (although no picture), it chimes, the fans and HDD operate as usual.

Any idea?