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Hate to say it.... your tach board is fried. Sorry...
The link on the 75-77 board won't be up until Tuesday... But if you hardwired it and it is still doing the same, the problem is the board.
I'll post again when it's up and running.. We did the final test on the new boards a week ago Friday.. Had to make a move in one of the Caps... But they should be ready on Tuesday for shipment.