Fan goes on but won't power up
I have a iBook G4 that had a bad power plug. Prior to the plug getting damaged, the comp worked fine. I replaced the DC in-board following the excellent instructions on this website and plugged in with a known working adapter. But when I push the power button, I hear a humming noise and a fan whir, then nothing. I tried resetting the PRAM and PMU to no avail. The battery is charging , and the whirring seems to indicate power going in, but I get no startup chime and no screen flickers. Any ideas on what might be wrong?
Addition - when I press shift-option-command-power all together for a few seconds, I get a long boooop and the light on the front flashes.