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

http://forums.pelicanparts.com/uploads13/vdo+spice1215459220.jpg

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.