Rebuilding AH-BJ8 Smiths Elect. Tach.
Found this a www.healey6.com. Link is to the pdf. If you visit the site there is a lot of other technical articles. But turn off your speakers. The site is full of obnoxious sound effects (imho).
AUSTIN HEALEY BJ8 SMITHS
ELECTRONIC TACHOMETER
INSTRUCTIONS FOR REBUILDING THE
ELECTRONICS