27” iMac 2009
When you power it up, everything is completely normal during the boot process. Seconds after it reaches the login screen, the screen goes to sleep. Completely asleep; not just backlight. The computer itself is still running. I can login to it from another Mac via Screen Sharing and operate the iMac normally. If I put the iMac completely to sleep then wake it back up, the screen comes on for a few seconds then goes to sleep again. If I boot the iMac from an external HD with a different version of the OS, the behavior is exactly the same (not a software issue). SMC reset. PRAM reset. No change. Boot in Safe Mode: exactly the same problem. Suggestions?