ok.. I think I have done it this time. Yesterday I installed a new Combo drive in my powerbook g4 12"867MHz 640mb. I followed the ifixit.com instructions to the tee. i successfully installed the optical drive and reassembled the machine. When i tried to boot up, I heard the normal chime a heard the HD start up, but the screen stayed black. After a couple minutes the fan started reving and then the system shut down. I reset the PMU and Pram already.... Does anyone have any idea as to what I may have done or how I might fix this problem?