Those are cool old tachs. We used them all of the time until bigger stuff came out. Rotunda also marketed aftermarket add on Air Conditioning. The truck guys like them. We used them in plenty of street high performance stuff too. They are chrome, with a chrome face, and a black background and a white needle? And the facia has a D shaped opening ?
OK, I too have fixed my share. I just did a few for OEM tachs on Mustangs, like the Mach 1 style. Those die when a diode fries, easy fix, You solder a new one in, heat sinked, to the board and then calibrate.
But like the kind tech said above, you have 2 pots to zero. The idle pot gets fixed first, then calibrate the higher RPM. I have seen the needle slip too but often to a lower RPM due to the tach having been pegged a lot. Pegging it a lotcan leave it at a lower RPM then true.
Both the links sound like a nice resource, but you can do it yourself I bet. The step by step help above is great. If you do not want to fry the points during a long test session in the on position,pull the primary wire , or black wire from the Distributator.....lol to the coil. Slide/bolt it back on to run it later..
Leaving the positive wire on will help simulate that actual voltage your device sees, thus the observed RPM. Leave the hot connection on. You know, the resistance wire can gain OHMS over time too, so? Check it out. I wonder if that is throwing you off? Lost volts? I would measure it running, and in the on position, compared to the actual battery voltage. Just an aside. Takes a second....but you are high , so....
Good luck !