I recently had my early model EV tach slowly fail over a six month period. I had run an extra earth and checked all the connections... But one day it finally gave up all together. So I sent it off to;
http://www.ottoinstruments.com.au/restoration.htm
He said the motion had open circuited... and repaired it with parts sourced from a Ducati supplier. The good thing about this is the needle now sits on zero at rest (not 500 rpm).
John Gorman
coll33nn<at>optusnet.com.au