Quite often the fuse will blow, resulting in nothing happening when you try to start the computer. No surprise, right? The puzzle is just what caused it to blow. Obviously, you must replace the fuse, PLUS the thing that failed. I am now working on a PC power supply that failed - one of the large filter capacitors went out by blowing a hole in it's side! This also cooked a power transistor that was right next to it. Since the damage is so visually obvious, and all the parts are accessible, I think I have a real good chance of getting it working. BUT -- I am probably pretty lucky, too. Considering the price of used power mac G4's, you may be better off buying another one for parts. There are sellers on eBay that sell refurbished power supplies for about $140.