I just had my tach repaired at APT instruments in Minnesota
They did a complete rebuild for $160. I drove it once this winter and
tach seems well behaved, agrees now with my handheld dwell/tach.
They said they only did complete repairs, argued that I'd be less than
satisfied if it broke again in 2 years. And seeing what a joy it is to
get in and out of the dash I had to agree. I also got good reviews from
the list for Palo Alto Speedometer.