Read this link. I only paraphrased the repair procedure - I have no idea if the information is valid:
http://howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm
The author claims that this type of tach failure is due to a failed resistor chip on the tach PC board. The only part required to do the repair is a 12-turn potentiometer (about $5).
After the tach has been removed from the cluster and is laying on the bench, the actual repair takes only about 5 minutes. It involves first removing the failed resistor from the circuit. It isn't physically taken out, it is just isolated - by cutting an etched trace on the back of the board.
When the old resistor chip has been disconnected, it only a matter of soldering the new pot across the appropriate pins on the resistor chip. He shows great detail about which trace to break and which pins to solder the pot to.
That is the entire repair. But, as he acknowledges, the tach will still need to be calibrated.
He uses an oscilloscope. But just in case your oscilloscope is broken, he suggests using another known good tachometer instead. Reconnect the factory tach and also piggyback the test tach onto it. Start the engine and compare the reading on the repaired tach with the test tach. Trim the pot as necessary to calibrate the repaired unit.
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