First, I think you mentioned the power button (on the computer) doesn't work. Does that mean it doesn't light (even when you do eventually get the machine up) and it just doesn't work to power the machine?
It lights but it has a broken button so I can't power on the G4 with it.
If the machine is actually booted and running, does pushing the button cause the machine to go to sleep (takes a few minutes) or provoke other response (sleep is the default although it cna be changed).
Using the button from the Apple ADC display it goes to sleep normally,and wakes up on mouse move .
Since you are starting from the monitor anyway because it isn't powering on the machine (I'm basing this on an earlier post and trying the verify it's still true and what I understand), then you might want to disconnect it completely from the logic board just in case it is stuck in some weird state and causing the logic board in turn to behave oddly.
I have disconnect it from the logic board but I still get the same issue.I have also replaced the switch ,from my other G4,but I get the same behavior.
Moving on you say you get the chime but no screen. Do the disks spin up properly now?
I hear every time the chime but not always the disk spinning.If after the chime sound I hear a "click" sound from the speaker then I know that the machine will boot up normally.Most of the time it fails.
It will boot up after several attempts.
If not, have you tried removing all of the disks (or disconnecting the IDE cable at the logic board) and seeing if you at least get video and blinking question mark/folder? Can you boot into open firmware consistently and get a video signal (cmd-opt-O-F)? What about target disk mode?
I have to try all these and report back
Here is what I have done since yesterday:
1) Left is without battery overnight to reset the logic board.
Result=NEGATIVE
2) Remove all the disks and left it with system only HD.
Result=NEGATIVE
3) Connect the HD to the 66mhz buss.
Result=NEGATIVE
4) Connect the disk to the DVD buss(33mhz?)
Result=NEGATIVE
5) Put the disk in master mode…just in case.
Result=NEGATIVE
Here is a strange observation…When I hit the startup button and I don't hear the HD to spin ,I remove the ribbon cable from the disk right after the chime and put it quickly back.Then the HD is spinning and the G4 booting up normally.