Answer to: iMac 24" Intel won't start black screen first 3 LED onHi Ori ! I had exactly the same Problem on my iMac. Finally helpful was the replacement of the Power Supply! I bought it at a Store in UK, www.applemacparts.co.uk but im sure u find the Part also in US or elsewhere. Mine was a used Power Supply, repaired by them and i sent mine in exchange... Works perfect until now. Take care that the part u buy fits exactly for ur type of iMac, there are many diffrent Powersupplies in Apple Computers : ) Greetz and good luck, G.