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.

Average Rating: 4.9 out of 5 based on 163 user reviews.

Average Rating: 4.4 out of 5 based on 276 user reviews.

Average Rating: 5 out of 5 based on 175 user reviews.

Found the circuit schematic from Internet. it is somehow different version as our tach. but the basic concept is the same:

Checking detail:

7W Z1 Zener diode ouput 7.1 V … okay , working
3 transistors BSX68 … okay, de-soldered and checked
3 IN4148 diode, … okay, checked working
3 capacitors de-soldered.. checked okay, some leaking and weaken capacitance, but still working

Spare transistor parts: BSX68 is a 40 years old transistor. Common replacement is NPN 2n3904 that I have a few at home.

Average Rating: 4.7 out of 5 based on 239 user reviews.

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. 🙂

Average Rating: 4.8 out of 5 based on 280 user reviews.