Chosen Solution
After turning it on, I hear the dong, then it keeps rebooting & donging every couple of seconds. Screen stays dark. 3 LEDs lit on the motherboard. (screen is disconnected, so not 4) LEDs don’t change during the reboot cycle. Tried different power supply. Replaced the EFI chip with a new pre-programmed one. No dice so far. Any thoughts? Thanks!
It could be the case that the temperature sensor attached to the monitor is broken, causing the back light to malfunction. What you could try is to reset the NVRam and the Pram a turorial on how to do that can be found here: https://www.macworld.com/article/2881177…
