911 Tachometer schematic Options

[timothy_nd28]

I have been helping Mepstein with some of his 911 core tachometers. It was needed to keep these original, so updating the tach drive was not an option. After searching the web I failed to find anything relevant, I even came across a few schematics that were wrong (they drew it with PNP transistors). It seems that VDO was tight lipped about publishing anything about the internal workings.

My uncle reversed engineered the VDO tachometer, and even found suitable replacement components as some these 1970's parts are obsolete. This model is proven to work with the TINA Spice program, which I can't seem to post the file to share, so PM me and I can email it if you want to play with it. I'm hoping this may clear things up and help others with fixing their own tachs.



Original transistors Q1,Q2, and Q3 are BC170D, and are obsolete. Simulation with TINA uses 2N2222A, which works great as these are excellent general purpose replacements. The inductor L1 is shown as 10mH, just to have some value. I did not measure it and do not know what its inductance is. It is used to filter the current so the needle does not bounce as much. I do no have a part number or value for the thermistor (TH1). It has a resistance of about 50 ohms (green-black) at 25C (room temperature).