Amiga 500 not booting

[bitman]

Recently I got an Amiga 500 (with commodore key), kickstart 1.2.
 
When powered up - the kickstart screen shows - and the disk-drive starts waiting for a disk, but if I put in a disk it task a few spins/reads for 2-3 seconds, and the returns to the kickstart screen (without rebooting).
 
I've tried replacing the drive with a drive from an A600 and A4000 - but the same scenario keeps happening.
 
Any suggestion`?

[bitman]

Tried replacing most of the chips - no success. Left it on for a few minutes - screen turned green. Now it's green evey time I turn it on - guess it's a ram problem then.