Has anyone had an isue with their vintage gauge tachometer?

Mine has sudennly stopped working and will no longer let me go thru the calibration process.
When the car is running the tach looks like a second hand on a watch clicking up and down the gauge and after a few cycles of this stops on either 0 or 2500 marks.
Wiring checked and is getting good signal from the coil.
Suspicious may be the tach since will not let me do the callibration anymore.

Any thoughts??

Thanks
Mike