Thanks for your reply.
I popped off the back. all capacitors look fine in my opinion. No sign of bulging or goop. Actually, maybe I have a good batch of capacitors, because the logic board was replaced in 2006 after I had a random shut-off problem.
My status now: After I reset PRAM (control/option/P/R) and SMU (o/n unplugged) the screen came back to life. But the system got stuck during startup. After I forced shutdown/restarted the OS came up and running, but now I have no video. Unfortunately I could not repeat this effect.
Does anyone have an idea what else I could try? I would very much appreciate it.
Cheers, Darko