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Thread: New life for an Old Tach

  1. #1
    mad scientist
    Join Date
    Apr 2007
    Location
    Austin, TX
    Posts
    537

    New life for an Old Tach

    I had a 911E tack from a 1970 coupe I parted many years ago. My ST conversion was running a later tach since it was originally built with a stock 6-pin CDI setup. Since that ignition is long gone with the EFI system I decided it was time to go back to the early style, silver button tach. I opened up the parts bin and plugged in the new tach. HMM it doesn't work. This is a problem.

    I decided to open this up and see what ails it. Since I had previously pulled the face out to clean the glass and repaint the needle dis assembly was no issue. Once open I connected the tach to a function generator and oscilloscope such that I could trace the circuit.

    The basic circuit functions as such. There are two sides to the voltage and they are compared in the middle. One side is the power input. This is regulated down to 6 volts. The second side converts the incoming tach signal (12V square wave) into an inverted 6 volt signal. The ratio of these two signals drives an electromagnetic coil that the needle is attached. The higher the frequency of the input, the greater the voltage signal to the magnet, the higher the needle swings. Cool.

    Once open you can see the really old fiberboard. The tach was dated 8/69 and is full of early transistors, resistors and capacitors. The capacitors are electrolytic and are known for drying out after 15 years or so. These tested OK but were a little high. Not the source of the failure but would only cause the system to be off at certain rpms. The caps still needed to be replaced.


    Existing board/circuit


    Next the transistors read open. Ah ha! I soldered in a modern replacement and there was still no signal. You can see the transistor sticking up in this picture.



    I was talking to one of our electrical engineers at work who is an expert in analog systems. He took one look and promptly said. "Don't waste your time. Build a new circuit". Five minutes later he hands me a sheet of paper with a circuit on it. "Build this".

    It took a week or so of lunch breaks to get this done. First, I cut a scrap section of circuit board to fit in the stock location. This was as simple as drilling 3 holes. here is the test if of the new board.



    Next I mocked up the main components. Yes, there are only two chips and a power regulator. I love modern simplicity.




    The remainder of the parts are a few resistors and capacitors to tune the frequency to voltage filers built into the two chips. You can see the circuit in the back ground.



    AFter some soldering I managed to get the circuit complete. when I hooked it up it worked perfectly. Change the frequency on the generator and the needle would swing. HMM The output was not quite correct. Oh, didn't you make this one a trim potentiometer? No that wasn't on the circuit. A few moments later with the soldering iron all was right.



    The blue rectangular part is a 10 turn potentiometer. I used this to fine tune the output of the tach. The tach receives 6 signals for every single turn of the distributor. If you do the math, this means that the frequency in kHz is 1/10th of the rpm. This is a simple relationship to verify the tach function.



    The final calibration was done on the bench. I had to reset the needle and then fine-tune the pot. Everything is working great. Even in the car.

    Sorry for the dark pictures. I only had the phone with poor dynamic light resolution. It doesn't work so well under bright work lights.

  2. #2
    Amazing! Electrical circuits have always been like hieroglyphics to me. Thanks for sharing a cool technical story.
    Peter Kane

    '72 911S Targa
    Message Board Co-Moderator - Early 911S Registry #100

  3. #3
    You sir, are a bonafide American Badass...Kid Rock ain't got sh-t on you!
    Kenik
    - 1969 911S
    - 1965/66 911
    - S Reg #760
    - RGruppe #389

  4. #4
    mad scientist
    Join Date
    Apr 2007
    Location
    Austin, TX
    Posts
    537
    Kenik, I don't which is more disturbing. The fact that you think I am a badass or that you listen to Kid Rock. Yep, definately the fact that Kid Rock was involved.

  5. #5
    Quote Originally Posted by jpnovak View Post
    Kenik, I don't which is more disturbing. The fact that you think I am a badass or that you listen to Kid Rock. Yep, definately the fact that Kid Rock was involved.
    I don't listen to Kid Rock (well, almost: while his 1989 debut album "Grits Sandwiches for Breakfast" is a hip hop masterpiece, his nouveau country rock rock is shyte), so it was meant in jest, since "American Badass" is what he likes to call himself.

    BTW, you have a ride from the airport on Thursday? Email me if you need a pickup. I think Scooter may already have you covered...
    Kenik
    - 1969 911S
    - 1965/66 911
    - S Reg #760
    - RGruppe #389

  6. #6
    Jamie,

    I posted a circuit diagram of the reverse-engineered VDO tach, it's a monostable multivibrator or "one-shot."

    I was working on a LM2917 tach in breadboard, never had time to finish. What's the circuit logic in your photo? That almost looks like the internal logic design vs. the pinout of the chip.

    THANKS! Here's a link to my previous work.

    http://forums.pelicanparts.com/porsc...eter-guts.html
    Attached Images Attached Images  
    1966 911 #304065 Irischgruen

  7. #7
    mad scientist
    Join Date
    Apr 2007
    Location
    Austin, TX
    Posts
    537
    John, I think your post was part of the inspiration. I tried to track this tach and had issues. The overall layout and function appears to be the same as what I found. The difference could be with individual component values and locations.

    There are two chips in this current circuit. The 4 NAND gate Schittdt Triggers come on a TI CD4093B. I am only using two of the 4 channels to flip the input pulse.

    http://focus.ti.com/docs/prod/folder...t/cd4093b.html

    The driver is a 2 channel MOSFET TC1426. The power input is regulated to 10V using a TO package regulator (top of circuit diagram).

    I can scan the circut if you want a copy.

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