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I've put it back together, everything works except the backlight - keyboard, trackpad, screen, system boots if I light external light to it I can log in etc.
Seems like U8400 is not being told to turn on the backlight and not drawing down the voltage on R8400 - Q8400  ?
Maybe the enable signal should be debugged somehow ? I don't know how. From other posts I could see there should be 3,3V on U8400 - I have 0 there.
Traces look, fine, I have connection from U8400 to R8402. R8402 is the one I had to solder on a broken pad, but I checked and it has connectivity both ways and it's not touching GND anywhere. Seems like 12V goes from F8400 > R8400 > R8401 > R8402 > U8400 but it's not switched to GND in U8400.