I agree, most likely the tach board. Mine went out, replaced it. The old one had a small burnt speck on the board. It's a pretty big job to pull the tach, replacing the board is not that big of a job to do. good luck.
John thats a good question all I can say is start the car but may be some of these smart guys will have better suggestion. When I had my dash out I started the car to make sure everything worked, lights, tack, etc before I did the final install.