Try booting up with the keyboard disconnected - it could be a faulty chip on the keyboard. Can't remember off hand what it's called, but you'll find it adjacent to the LED's. Usually a faulty keyboard chip will generate a flashing sequence through the caps lock LED and prevents the Amiga from booting up. This kind of problem seemed to affect A500+'s more than A500's. From what I remember, the chip is soldered directly to an A500+ KB and socketed on an A500.
