I have this same exact issue with a PowerBook Titanium G4 867
Tried everything to get it to boot:
Took out battery
Took out RAM
Took out PRAM Batt.
Actually I dismantled the whole machine at one point looking for broken connectors or fried boards (nothing).
I was hoping that the suggestion of wrong AC was it. But alas that was not the case either it seems?
To bad as it was a good platform for running my Network Monitoring Apps