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Technical Getting a Sun Football Tach Working

Discussion in 'The Hokey Ass Message Board' started by 6-volt, Jun 26, 2021.

  1. 6-volt
    Joined: Jul 6, 2009
    Posts: 12


    The old Sun Tachometer has been on a shelf for over 40 years and I figured it was time to get it working. The problem was the Mercury batteries had failed long ago and corroded the inside of the transmitter box. The transmitter is the Achilles heel of these old tachs. The transmitter circuit consists of batteries, capacitors, and a relay, all of which are prone to failure over time. The meter movement is very well made and if you have ever held one in you hand you can tell it's a quality piece. This tach will look great on the steering column of my '56 Chevy.

    My plan was to put together a solid state circuit to replace the electrical/mechanical guts of the old transmitter box. I used a monostable multivibrator circuit using two common NPN Bipolar Transistors to generate an adjustable square wave signal to drive the meter movement. The timing of the output is controlled by an RC network. This video is a test of the circuit with the components mounted on a breadboard. The test was a success so I ordered a printed circuit board that will mount inside the original transmitter box. I used EasyEDA software to lay out the printed circuit board. EasyEDA is free and after playing around with it I found it easy to use. When finished I generated a Gerber file that any PCB manufacturer can use to build a circuit board.

    JLCPCB in China is building 5 circuit boards for $2 plus $9.80 shipping. $11.80 for 5 circuit boards is a good deal. The rest of the parts consist of 8 resistors, 4 capacitors, 2 diodes, 2 transistors, and 1 trim pot. these are common parts and dirt cheap. The PCB's should arrive from China in two weeks. I will let you know.

    I used timing light to calibrate the tach by adjusting the trim pot. Hope you enjoy the video.

  2. vintage6t
    Joined: Jul 30, 2007
    Posts: 351

    from CT

    Very cool project. That circuit board looks great and amazing how cheap it is to get them commercially made. Huge advancement over etching your own like you used to have to do and cheaper too.

    Assuming the transmitter box is going to be mounted under hood, it will be interesting to see how well your electronics hold up to the heat. If that becomes an issue, you can always use the transmitter box as a wire junction box and move the circuit board under dash.
    stillrunners and Elcohaulic like this.
  3. Budget36
    Joined: Nov 29, 2014
    Posts: 6,919


    Very nice! Years back in an electronics class we took an existing circuit and had to lay it out with similar SW, we didn’t design the circuit, just the PCB. Back then I was amazed at how inexpensive the cost to make the PCB was.
    Elcohaulic likes this.
  4. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,291


    The action of that meter is very smooth... I would gladly pay to have one of my boxes converted. I don't have the tach yet.. If your interested in building these for us please count me in.. I need a tach for my el Camino, I've been shifting the 454 by ear and my hearing isn't so great anymore...

    I'm an electrician and can solder but I'm not an electronics tech
    like you guys are.
    stillrunners likes this.

  5. Dennis D
    Joined: May 2, 2009
    Posts: 756

    Dennis D
    Member I used them for the conversion on my football tach. Could not be more satisfied with the service or people I dealt with! D
  6. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,602


    What Dennis said, I had 3 tacos and senders done by them and as far as I'm concerned it's worth every penny.

  7. 6-volt
    Joined: Jul 6, 2009
    Posts: 12


    According to the data sheets all of the parts in the circuit are good to 150°C (302°F). Time will tell.
    I'm not interested in selling this. I just wanted to share with any "do it yourself guys" that might be interested in the circuit.

    All Resistors ½ Watt

    R1 - 330 Ω

    R2 - 3K3 Ω (3,300 Ω)

    R3 - 220 Ω

    R4 - 3K3 Ω (3300 Ω)

    R5 - 470 Ω

    R6 - 100 Ω

    R7 - 220K Ω (220,000 Ω)

    R8 - 22K Ω (22,000 Ω)


    C1 - 10uF Electrolytic Radial 25V

    C2 - 3.3nF Metalized Polyester Film (Mylar)

    C3 - 1nF Metalized Polyester Film (Mylar)

    C4 - 82nF Metalized Polyester Film (Mylar)

    D1 - 1N4004 Diode

    ZD1 - 1N4737A Zener Diode

    Q1 - 2N3904 Transistor

    Q2 - 2N3904 Transistor

    VR1 - 3296W-1-202 (2K Trimpot)
    Last edited: Jun 27, 2021

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