G4 1.25 iMac dying-Help w/diagnosis

[spork183]

G4 1.25 iMac 20in. Been running leopard. Classroom computer. After extended break of about a month, booted, then froze, then wouldn't boot. No Chime, no screen, just a faint hum. Took it home, it deigned to boot. I reinstalled the OS (dropped down to Tiger and updated to 10.4.11). Now back to the no chime, no screen (black) no boot, faint hum. Suggestions?

What I've tried:
Zapped the PRAM (while it was still booting) and reset the PMU via button on the board (after it stopped booting). Unable to boot off of utility disk or firewire since I never get a chime or screen now. Ran TTP on it when it was booting, negative results.

Would a bad battery do this?

Background-Purchased it used in 2006. upgraded ram, HD, Superdrive in 2007. Used intermittently until now.

 

[666sheep]

Do you hear the fan running? If not, most likely PSU has died. If fan works, you can try to replace battery. Theoretically iMac should run without or with dead battery but in real world this could cause some boot issues.
Another possibility is that graphics card has died. It's common problem of FX5200 mobility in various Macs that were equipped with it (PB G4, Imac G4 and iMac G5).

 

[spork183]

Do you hear the fan running? If not, most likely PSU has died. If fan works, you can try to replace battery. Theoretically iMac should run without or with dead battery but in real world this could cause some boot issues.
Another possibility is that graphics card has died. It's common problem of FX5200 mobility in various Macs that were equipped with it (PB G4, Imac G4 and iMac G5).

If the graphics card went, would it have been intermittent? I'm hearing a hum, but no fan, so I think your PSU diagnosis may be spot on.