No fix. Applying pressure worked at first, then more tries between the successes, now nothing at all. Resoldered many components near the power connector and power section, only got the chime once and it stopped playing it midway.

I start to suspect broken traces inside the multi-layer PCB, or microcracks under some BGA chip...

I think it is a good time to start a spare parts hunt

Does anybody here need 3 "For parts only" G4 iBooks? All disassembled.