Electrical problem?

Are you SURE you ruined the box? The tach box should be accepting an input driven by the MSD box. How did you determine this?

Anyhow I'd "unwire" the thing and hook it up "theft" style reduce it to the bare minimum wiring. Hook the big red and small red "jumper" direct to battery. Double check the big black is ground. Unhook ANYTHING else, tach, etc.

Also do you have ANYTHING connected to the coil except the two MSD wires? There should be no radio cap, no nothing, only the two MSD wires