Why is my screen or backlight turning off in seconds?

Hello everyone,

I am a little outdated in apple repair. I am a certified apple repair technician and used to do work at an apple authorized repair center, now I do mostly PC's instead.

I have a problem that has me stumped and I feel like it is something simple I am just missing.

I have an Imac that originally came in with a blinking folder indicating a likely failed hard drive (which I confirmed with a hard drive test)

When replacing the hard drive, the sata jack became seperated from the board. I was able to slide it back in and it fell into place fine. However, upon turning the computer on the display will come on for roughly 3-5 seconds and it either turns the blacklight off or the entire screen goes off. The diagnostic LED's are all on fine. If you boot to a CD it will boot fine, I know this because if you boot to a CD and let it go to sleep and click the mouse the screen will turn on again for 3-5 seconds and turn back off.

With this issue I have:

Reset the SMC

Performed an NVRAM Reset

Replaced the LCD Cable with a known good cable

Replaced the inverter with a known good inverter

Replaced the logic board with a known good logic board

The Imac is an Imac 20" 2.16 Ghz Intel, MA589LL/A and W870846TVUV

I suppose the only part I can replace at this point is the LCD Panel, but I feel like this problem is much more simple than I am letting it on to be... Any advice would be appreciated