Which part of the motherboard is responsible for display?

I have 2 vitas, one that works and one that does not. I used one mother board on both screens and they both turned on just fine but when I used the other motherboard, neither screen turned on. However, even though the display doesnt turn on, the touch controls still work fine. How do I fix the part of the mother board that is responsible for display or just which parts of the motherboard do what? Thanks!!