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.
I had the same problem with my B1260 although only after I'd removed the Mediator and was connecting the card directly to the motherboard. I found that it worked fine at it natural resting point when angled towards the right hand side of the case.
