Just something to look at. I had an A4000D a couple of months ago that had no video output. At first I thought it was the motherboard but it ended up being a bad power supply--specifially the +12 volt out was bad.
This is assuming the screen is totally blank. If it is black, the motherboard is probably good and the CPU card is not working. In the A4000D I'm referring to, I got the video to go from blank to black which told me the motherboard was ok by putting in a good power supply.