Do you still get a black screen with the accelerator card removed? I suspect the mediator's adapter is thinner than the edge connector on the A1200 and CPU cards don't sit right on it. I have to put the Apollo in at a very specific angle to work. The fans go and the power seems okay on the accelerator but I think the power to the card is a bit unstable.
In the end I built and fitted a bracket to keep that card in alignment and most times it boots from power-on, but sometimes (if the tower is moved) then the accelerator needs to be re-seated.
I think if you remove the card and boot to the early startup (hold down both mouse buttons at boot) and you get the early startup screen that may be the problem. Black screen usually means the A1200 cannot access the accelerator's CPU.
