The tach gets 12V from the printed circuit and ground through the gauge housing. You did not state what kind of distributor you have. If you have the original point distributor the tach should work. If you have HEI or some other aftermarket distributor the factory tach may or may not work. There are some tach filters available.
The most common cause I have found for a factory tach to not work is bad or broken contact for the circuit on the tach. Vibration tends to pull the terminals off the board and break connections.
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