I have had the same problem...another tach. could not be found... find an electronic expert that can take a look at the platin behind the tach. it´s easy to demount...you will most likely find one of the hair thick connections broken through the years of vibration.  if you are lucky the one inspecting the platin can also micro solder and repair the fault... and you don´t have that big load of questions when cruising with someone..."is the tach. broken!?" and youn don´t have to rely on your ears when shifting.