First thing, does a different accelerator work in the same Amiga? Also do you have an Amiga with a different revision mobo? I had severe problems with certain cards using a particular revision A1200 mobo.
Also, do you have different RAM you can try? Tried cleaning the contacts? Short of that, it might have to be off to France :-( I think a black screen with not even any coloured flashes probably means the CPU isn't doing anything - either because it's dead itself, or some other part of the card is.
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moto
