Hope you can help with a stumper.
Machine: iMac G3/350 slot loader (Summer 2000; Indigo, no Firewire)
Problem: Machine powers on with green light, hard drive spins, but no startup chime, no video, no reading/loading OS from HD.
More info: Got the machine like this from a friend. Removed a stuck CD from the CD drive, played with it a bit, then after a few tries of resetting the PRAM, booting into Open Firmware, etc., all of a sudden the video came up in Open Firmware after I pressed the programmer's switch on the side. I typed "Mac-boot" as per the OF prompt, and the machine booted. All seemed well.
But then: I tried to do a fresh OS install (OS X). Booted from the CD, got to the "Use English as the Main Language" screen, then clicked the continue button, and the machine froze, hard - not a kernel panic, just a frozen screen and frozen mouse and keyboard. I turned the unit off by pressing the power button, and then the original problem resumed - green light and HD spinning on power on, but otherwise dead. PRAM reset, OF key combo, and pressing the programmer's and reset buttons haven't worked at all.
I then tried opening the unit up and resetting the PMU, but that didn't help either.
This unit HAS had the firmware update applied prior to updating to OS X, so that's not the issue.
The Apple Service Manual describes Troubleshooting procedures if the unit won't power on at all, or if it will power on and fails POST, or if it powers on to a grey screen.
But this situation is in-between - it powers on, but it doesn't even get to POST - no startup chime, and no error beeps.