Hardware :: Cause For IBook Dead / Blank Screen?

I just got this iBook 12" in the mail been to a mac shop before I bought it where the owner was told the screen had died. Motherboard was said to be Ok by the repairer. I replaced the screen with a know working one and the screen was dead. Then after the fiddle of pulling it apart again wondered if I could use the hard drive on another ibook. The hard drive I put in another iBook which loaded up fine with display. Later I tried again but it was a dead screen? Is it that both ibooks have faulty motherboards? Motherboard determines a display? Or could it be the display module? What else could cause dead screen? Cables good.