It is a separate board It is located behind the screen at the top left.
No the screen looks fine just turns off. I have it displaying to a external monitor right now I don’t know if the rules out the gpu.as I can see its an LED driver board on the display in this iMac,
try following this guide iMac Intel 27" EMC 2390 LED Driver Board Replacement - iFixit Repair Guide
see how you go it could be that the display itself is going bad maybe some of the led array
do you have any bright or darker spots on the screen when it does come on ?
also it could be the GPU going bad as ive seen this happen then also.
That means the GPU is okay. You might be better off replacing the entire display assembly which should also include the backlight board. Of course depending on much you want to spend..... $$$No the screen looks fine just turns off. I have it displaying to a external monitor right now I don’t know if the rules out the gpu.