So I have a PowerBook G3, Wallstreet series. 233mhz PowerPC chip, 256MB of RAM, a 10GB Toshiba hard drive, OS X Jaguar, and even a 14.4" TFT.
History: It was working well, for the most part... it had some boot issues once (couldn't find startup disk), which I solved (temporarily) by replacing the logic board (from my collection, not new). Worked like a charm, and was up for a week straight. I had to pack it up, and when, after an airplane ride, I again tried to power it up, same thing - no startup disk found (blinking ? in a floppy). Reinstalled OS X and it booted, then powered off after a few hours with no error message and wouldn't boot again. Went through that routine about ten times, trying to get all my data off the HD in anticipation of that possibly being the problem.
Recently: Found a site online that described those exact issues and said the problem was the PRAM battery, so I got a new one. Installed it today. So now there's a brand new 40GB HD in there and a new PRAM battery... neither of those should be the problem. This time, though, when I tried to power it up (holding C to boot from the OS X CD), it would dong happily, show a grey screen with the Happy Mac, initialise the CD, and then either reboot or go black and do nothing.
Today: Someone suggested the problem was the RAM. So I got out my collection of parts and replaced the entire RAM/processor daughter board (RAM included, of course). Powered it on (holding C after the dong); it donged, initialised the CD, rebooted, donged, initialised the CD, read the CD for about two minutes, and turned itself off.
I walked away, came back a few minutes later, and it was running the fan with full power (louder than I've ever heard it run before), and the indicator LED was fully lit... which of course it's never supposed to be. I unplugged all power sources and the fan cut off, but the LED is still shining brightly, and the computer will not power on at all now... pressing power does nothing at all.
I've tried swapping out yet another RAM/processer daughterboard, with no results.