I've just got hold of a 550mhz TiBook which refuses to turn on. It's in reasonable shape, scratched and worn, but complete. The PSU ring glows orange and it is definitely charging the battery. If you press the power button, there is a faint pop from the speakers and the ring changes to green. However, that's it, no chime, no drive activity and the screen remains dark. If you press the Caps Lock key, it lights up. If you then hold down the power button, it shuts off again with another faint pop.
I've pressed the reset button on the back, which didn't make any difference, and it doesn't do enough to try resetting PRAM or anything.
It's got 512mb RAM in two DIMMS installed, which I've tried swapping out, and apparently a 40gb hard drive. I've reseated everything I could see under the keyboard, but haven't had a look underneath the bottom cover yet.
Any suggestions would be much appreciated!