Blue light on both screens blank.

My DSI XL when turned on the blue indicator light comes on, but there is no audio or video. The screens are blank, there is no flash or flicker, nothing but a blue light indicating power. I replaced the power board, and after that didn't work I removed the mother board and put it in another DSI XL that was working. Still blue light/ blank screens. I can handle the repair, but I need help with the diagnosis

Thanks,

Ryan