Solved! Laptop screen is black but running, works with external monitor. Changed LDC cable and screen - no dice >:

[franklintheturtle]

I have an ASUSx551m whose screen would go black while it was turned on. I bought another LCD cable and screen from a used but working ASUSx551m laptop. I tried the new lcd + cable, the old lcd + new cable, the new lcd + old cable and the actual laptop screen remains black. Plugging it to an external monitor works but isn't the best for mobility. What could be the issue? Thanks in advanced.

 

[AlHuneke]

There is also circuitry on the motherboard that drives the LCD and supplies power. It may be the problem.