Nothing on computer screen but yes on external display

I have a PowerBook 1.5 GHz computer. I don't get any picture on the computer screen. However, when I hook it up to an external display, I get the desktop on the external display.

Is there something wrong with the logic board? I bought another screen for it, installed it and I didn't get any picture on that display as well.

I might have the opportunity to buy another logic board for it. Therefore, the question is this: Is it possible to get external video and not internal if there is something wrong with the logic board? I remember people once telling me to test the machine and see if I get external video. If I did, then it was the screen. However, after changing the screen and not getting the picture, that theory is ruled out. So, if I swap the logic board, will this problem be solved? Btw, I swapped the display for another one and that didn't solve the problem.