PowerBook G4 won't power on / turn on

I have a PowerBook G4 1.67 that shut itself down sometime during the day yesterday, and will not power back up. It's completely unresponsive to the power button, with no signs of life whatsoever.

I've tried resetting the PMU.
There has been no new hardware or software installed.
This has been tried with 2 batteries, and 2 AC power supplies.

There is inconsistency with the color of the AC power supply tip, as it doesn't seem to indicate the strength of the battery. Regardless of which battery or how much charge is left, the tip is Green, and will flicker Yellow very quickly once, when plugged in. It was Green following the initial shutdown, Yellow when I placed a dead battery in it, and then eventually turned Green. Now, it seems permanently in the Green / flicker Yellow state.

It did fully charge one battery while like this.

Thanks in advance

Is that it, just that both batteries are gone? (shouldn't it work even w/o a battery, but with the AC adapter alone?)