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Be sure D7701 is not shorted A-K.
Backlight circuit is enabled, but doesn't boost for some reason.
Did you try other display assembly?
Try few PRAM reset.
I still doubt about U7701, or its solders.
PRAM reset done several times but nothing. The screen is the only one I have but it works perfectly with other motherboard. I have to watch if short circuit and if not replace U7701.