Additional mod to the tach:
1. Added ST L7812 voltage regulator to the circuit. Stabilize the input voltage to 12V
2. Added a 220uF cap on the regulator output.
3. Cleaned and assembled
Test will be carried out on weekend on my VWs.
Found the circuit schematic from Internet. it is somehow different version as our tach. but the basic concept is the same:
Spare transistor parts: BSX68 is a 40 years old transistor. Common replacement is NPN 2n3904 that I have a few at home.
This is the precise Signal Generator from my company Lab.
Technicians claimed it worth twenty thousand HKD back 10 yrs ago.
I tried 10v to 14v square wave, range 1khz to 10khz. No response. 🙁
Then tried to put a 3v DC across the pointer rotor motor. It turns. 🙂