Vintage Dixco Tachometers

The input section consists of R1, C1, R2, C2, R3 and D1.  The purpose of this part of the circuit is to turn Q1 on when the points are closed and to turn Q1 off when the input is pulled up to battery voltage via the coil primary winding.  R1/C1 and R2/C2 make up a two-stage low-pass filter that takes any ringing or noise out of the signal so it is a nice square wave.  R3 limits the current through the transistor’s EB junction when the points are closed to protect Q1.  D1 prevents any high voltage transients from the coil primary winding from damaging Q1.

The output section is designed to put a measured amount of charge through the meter for each ignition pulse.  When the points open and Q1 turns off, current flows through Z1 and R4 (which limits the current through Z1.)  The 6.2V drop across the Zener diode sets the collector voltage at 6.2V below the battery voltage.  Current flows through D3 until C3 is charged to 6.2V.  D4 prevents current from flowing backwards through the meter and calibration resistors R5 & R6.

When the points close and Q1 turns on, the collector and meter + goes to battery voltage, but the bottom of C3 is still at 6.2V below battery voltage, so current flows through the meter and calibration resistors R5 & R6 until C3 is discharged so both sides are at battery voltage.

As the frequency of the ignition pulses goes up, the average current flowing through the meter goes up, increasing the needle deflection and as the frequency of the ignition pulses goes down, the average current flowing through the meter goes down, decreasing the needle deflection.  R5 and R6 act as current dividers with the meter.  By removing R5 and/or R6 from the circuit, the amount of current flowing through the meter will be increased.  This is how the tachometer is recalibrated for engines with 6 or 4 cylinders.

Here are photos of the 797 substrate with the components identified:

Common Failures and Repair

The illumination bulbs are number 53 bulbs and they do burn out.  They are socketed in some of the tachometers and soldered in others.  Just pry the socket out, replace the bulb and push the socket back in for the socketed units.  You will have to unsolder and re-solder the bulbs in the other tachometers.

A lot of the tachometers have issues with the meter.  The needle will consistently stop at one point on the scale and go no further without manual intervention.  I have not been able to find where the mechanical issue is to this point.  Unfortunately, if you can’t get the meter to cover the whole scale, I do not know how to fix it.  Alex Miller has suggested that it may be that bits of ferrous material may have been magnetically attracted and are in the gap between the armature and the pole pieces.  He says it may be possible to remove those bits with a needle if you can get access.

Several of the transistorized tachometers I have analyzed have had failed Zener diodes.  It is very easy to diagnose a failed Zener diode without even taking the tachometer apart.  To test the Zener diode, first put a DC voltmeter across ground and one of the meter terminals on the back of the tach.  Power up the tachometer with battery voltage and ground and then put battery voltage on the input wire to turn Q1 off.  At this point, the voltmeter will read close to 6.2V below the battery voltage if the Zener diode is working.  If your battery voltage is 12V, then you should see about 5.8V on the voltmeter.  If it reads close to 0V, then the Zener diode has likely failed.  If it reads close to battery voltage, Q1 may have failed.

If the Zener diode is functional, you can then test Q1 by grounding the tachometer input with the same test setup.  The voltmeter should go to battery voltage.  If it doesn’t, Q1 has probably failed.

When I have replaced the Zener diodes, I have had to change the calibration resistors to recalibrate the tachometers, so be sure to check your tachometer’s calibration after you have repaired it.  I use a function generator set to 12V square wave mode.  I set the function generator to 7000 RPM and use a pot across the meter terminals to move the needle to point at the 7000 mark.  I then read the resistance of the pot and select a standard resistor that is closest to the pot setting and put that across the meter terminals just like the OEM calibration resistors were.

Variations

I have seen a number of variations of the same a similar designs.  Most of the other tachometers have cap C3 as a 0.2uF cap rather than a 0.15uF cap in the Model 797.  But the Model 47 unit with a transistor also had a 0.15uF cap.  In two of the Model 97 units, R4 was 580 ohms rather than the 680 ohms in the rest of the units.  In the two Model 47 units, the meters were 117 ohm meters rather than the 500 ohm meters in all of the other units.  The calibration resistors have been different for every tachometer that I have examined.  

One of the tachometers that looks like a Model 97, but was not marked, had a much simpler input circuit with only a single stage low-pass input filter:

One of the Model 47 tachometers had a very different circuit in it, one with no transistors:

The input circuit uses an RLC filter to clean up the input signal.  But the R is also the current limiting resistor for the Zener diode.  When the points are open, the coil primary winding pulls the voltage at the input of the cap up to 6.2V.  D2 puts the other side of the cap at one Ge diode drop (~0.2V) above ground, so the cap charges to about 6V.  When the points close, the input to the cap goes very quickly to 0V.  Since the charge in the cap can’t change immediately, the voltage on the other side of the cap goes to -6V.  Current flows through the meter,  calibration resistors R2-R4 and D1 until the cap is discharged, moving the needle.

Note that all of the power needed to drive this tachometer comes via the input wire from the coil primary.  This means that the power wire coming into the tachometer is only used by the illumination bulb.  With this tachometer, the bulb can be used with the car’s dimmer if it is the type that controls the positive side of the dash bulbs.

I have not seen a failed tachometer of this type, so I can’t say what common failures might be.  By grounding the tachometer and putting 12V on the input, you should see 6.2V at the top of the Zener diode.  If you see close to 0 or 12V, then replace the Zener diode with a 1N4735.

If you would like to download this information plus the schematics for each of the variations that I tested, click here.

The Model 793 tachometer uses a very similar circuit to that in the no-transistor Model 47 tachometer, but the component values are a little different.  However, the theory of operation is the same.

The fully functional Dixco Shift/Tach Tachometer that I analyzed has a date code of “32 90” on it which implies that it was made in work week 32 of 1990.  The meter on the inside has a date of 12/89 on it which is consistent.  Here is what this tachometer looks like:

I did not disconnect the meter from the circuit, so I do not know the meter transfer function. 

This tachometer requires a high voltage (>100V) signal that comes from a coil/points ignition system.  Modern ignition systems that do not provide such high voltage signals will not drive this tachometer without a voltage boosting module such as the TachMatch V-Boost from Technoversions, LLC.

It was shipped with calibration resistors for 8 cylinder operation.  Two calibration resistor’s leads are exposed from the back one marked with red and the other marked with blue.  For 6 cylinder operation the user must cut the red lead.  For 4 cylinder operation, the user must cut both the red and the blue leads.

Shift/Tach Tachometer Schematics

Theory of operation

D1 protects the tach from reverse polarity which can happen if someone attaches jumper cables backwards,  R1 limits the current to Z1 which regulates the tachometer power supply to 8.2V.  C1 filters noise from the power signal.

R2 and R3 cut the input signal by about 95%.  R4 limits the current to Z2 which limits the input signal to 8.2V which is the power voltage of U1, a quad 2-input NAND Schmitt Trigger gate.  The 4 gates are used with an RC network to create a one-shot.  The first gate inverts the signal.  The second gate accepts the trigger signal plus the feedback  signal from the third gate.  The third gate input is slowed down by the RC network of R5-8 and C2 which is fed back to turn the output off.  R8 is adjusted to calibrate the one-shot pulse-width. The 4th gate inverts the signal to get the correct polarity output for driving the meter.

R9, R10, D2 & D3 seem to limit the voltage and current from the logic to bias the PN2222. R11 appears to be a current shunt resistor in the emitter circuit that drives a constant current through the meter and also sends a low voltage pulse to the shift light module.  With a functional tachometer, you should see a short 1V square pulse on the emitter starting when the points open.  

The collector is connected to the negative side of the meter and drives it.  C2 appears to be there for filtering.

Shift Light Module

The shift light circuit is a separate PCB module that is connected to the main tachometer PCB with 4 small wires, Lamp Power (LAMP+, Battery Power), Tachometer Power M+, (regulated 8.2V), Ground and Input Signal (Q1-E).  The shift lamp is designed to light at and above an RPM that is user selectable via a pot.

Theory of Operation

The shift lamp gets battery voltage to power it to maximum brightness.  It has leads that are soldered to the PCB and the bulb is held vertical with white silicone.  I could not see a part number on the bulb.  The main component is an LM358N Dual Op Amp.  It is powered by the regulated 8.2V power supply from the main tachometer PCB.

R1 and C1 make a crude frequency to voltage converter that drives the first op amp which is configured as a unity gain buffer.  The output of the first op amp connects to the positive input of the second op amp which is configured as a comparator.  The negative input of the op amp is driven by the wiper of a pot at the bottom of a voltage divider connected to the regulated 8.2V power supply.  In a functional shift lamp circuit you should see a DC level of a few hundred mV that goes up with rising RPM and down with falling RPM.


The output of the second op amp goes through current limiting R2 into the base of a PN2222 NPN transistor.  The collector of the transistor is connected to the shift lamp.

By changing the voltage applied to the negative input of the second op amp the user sets the RPM threshold where the shift light turns on.

Component Locations

Here are the component locations for both the main PCB and the shift light PCB:

Theory of operation

With no input signal, Q1 is off, Q2 is on and Q3 is off.  When a pulse greater than about 122V is on the input, Q1 will turn on.  The input of C2 is grounded driving the base of Q2 to about -8.4V, turning Q2 off.  That turns Q3 on which grounds the negative side of the meter causing current to flow through it.  Current continues to flow through the meter until C2 charges through R6, 7 or 8 until Q2 turns back on.  That turns Q3 off and the circuit then waits for the next ignition pulse.

After reconnecting the broken wires, I determined that the Zener diode had failed with a hard short to ground.  Fixing that repaired the tachometer.