Tachometer trouble shooting

[John Middlesworth <jape@email.unc.edu>]

The tach on my 72 B has never worked in the few months I've owned it and
I'm wondering if there's an easy way to tell if the unit itself is
defective.  

It's electric, and I confess that even after studying the wiring diagram,
I don't understand the principle by which it works (or doesn't work in my
case).  If someone could tell me a little about that it would help.

John Middlesworth