Screen goes to sleep immediately after boot

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?

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