I have a fairly old iMac - I believe it's an '08. Anyway, it's am aluminum 24" that has run like a champ for years. Last Friday, it was turned on and after the chime, the iMac does nothing and isn't responsive. Just a black screen I've tried to zap the PRAM, check the brightness, boot from a Snow Leopard install disc ... nothing. You can turn it on/off via the switch on the back, but that's it. Attempting to zap the PRAM, or anything via the keyboard does nothing. So all we get is the chime, and then it just "stops". Is this a dead motherboard issue? Or does anyone have any suggestions on what to try? Thanks in advance.