Smiths Tachometers

 Years ago, Mark reverse engineered the first generation Smiths electronic tachometer in his Sunbeam Tiger, and published the results on the internet.  He also developed a calibration tool for diagnosis and calibration of  those tachometers.  He began repairing and calibrating them for others as well as selling his calibrator.

Smiths tachometers were used in many British cars of the 60s to early 70s.  They were even used in Swedish Volvos of the same era.  Mark's work on the first gen tachometers led others to send him second and third generation Smiths tachometers for reverse engineering.  And the Volvo community did the same for the Gen 0.5 Smiths and later optional German-made VDO tachometers for Volvos.

A great resource for a lot of information on Smiths tachometers is Alex Miller's "A Gentleman's Guide to Smiths Tachometers, V4.1".

Thanks to Tom Hayden,  we know that the Vishay BC NTCLE100E3331JB0 (Digikey BC2418-ND) is a good replacement for the thermistor in Gen 1 tachometers.

Electronically, the Gen. 1 Smiths tachometer is a relatively simple two Germanium transistor inductively coupled electronic tachometer.

The two transistors together form a monostable multivibrator, or one-shot.  Normally, the collector of Q1 is at 6V.  An ignition pulse couples through the transformer to trigger a one-shot voltage pulse to 12V on the collector of Q1 for a set amount of time.  Every time an ignition pulse is detected through the transformer, the collector of Q1 will pulse from 6V to 12V for a fixed amount of time.  While the collector of Q1 is at 12V, the top of the meter is held at 6V by the Zener diode, so current will flow through the meter, causing the needle to deflect.  The width of the voltage pulse is determined by the combination of the 0.25uF capacitor C2 and the combination of resistors R3, R5 and pot R4.  The one-shot is triggered by every ignition pulse, so the voltage waveform looks like a series of pulses when the engine is running.  Since the pulses are fixed in width and the frequency of the pulses is determined by the engine speed, the ratio of the time the waveform is at 12V vs. 6V goes up with increases in engine speed and down with decreases in engine speed.  The way the meter works, the more time the waveform is at 12V, the more the needle is deflected and the less time the waveform is at 12V, the less the needle is deflected. 

To calibrate the meter, you want to drive the tachometer with a very accurate, known signal at the correct frequency for the RPM reading that you want on the meter.  Once you are driving the tachometer with the accurate frequency, you can adjust the needle deflection to the proper place by turning the calibration pot. R5.  The tachometer can only be calibrated at one RPM.  After that, all you can do is check to see how close you are at other RPMs.

It is possible to recalibrate the 7,000RPM Alpine 4 cylinder tachometers to work in V8 Tigers using this method.

The most common failure modes I have seen are failure of the main timing capacitor C2.  Failure of this cap. has caused tachometers to be erratic, temperature sensitive or just plain dead.  If you suspect your tachometer to have this problem, locate the capacitor, carefully unsolder the capacitor, and replace it with a new one.  I have not been able to locate a supply of 0.25uF capacitors, but available 0.22uF to 0.27uF capacitors work fine.  I have yet to come across a tachometer with a failed transistor, but they are rumored to cause problems as well.  The other major failure mechanism I have seen is a broken meter spring.  The meter needs to be replaced in this case.  Replacement meters must be gotten from a parts tachometer.  As a matter of course, the tachometer will have to be recalibrated if any of the parts are replaced.

 All of the Smiths tachometers of this era use the same electronic and mechanical design, regardless of number of cylinders, positive or negative ground or the make of the car.  The power and ground wires are reversed between tachometers for positive or negative ground cars.  The only other thing that changes is the face used on the meters.  So don’t throw away any vintage Smiths tachometers or tachometer parts.  They can be used to resurrect any other one.

Trevor Goodenough of the UK developed a clever method to reverse the polarity of this tachometer for his Aston Martin.  He reversed the polarity of the Zener Diode, main timing cap (replacing it with a new one in the process) and the meter movement wires.  Then he replaced the germanium PNP transistors with Mullard AC127 germanium NPN transistors (NTE103A is an equivalent.)  His tachometer is working perfectly in his Aston Martin that was converted to negative ground.

Here are the 3 Youtube videos he published regarding his conversion:

Part 1
Part 2
Part 3

You can identify a Gen 1 Smiths tachometer by the wire loop/plastic block interface on the back: 

Sometime around the late 1960s, Smiths went from a two transistor tachometer design to a one transistor blocking oscillator one-shot tachometer.  I was sent a tachometer from a 1972 6-cylinder Jaguar for analysis and repair. I reverse engineered the circuit, and figured out how to repair the one transistor tachometers.

This tachometer runs the ignition wire inside of the tachometer case where it is wound with 3 turns around a toriod core. The signal is coupled into the tachometer circuit via a winding with about 90 turns.  One side of that winding is connected to battery voltage, and the other side is connected to a series RC network, with the capacitor also connected to battery voltage.  The middle of the RC network is connected to the base of a Germanium PNP transistor marked MT59/A.  The emitter is connected to battery voltage and the collector drives the meter, a 30 turn (approx.) winding on the same toroid core, and a resistor to ground.  The 30 turn winding provides feedback that keeps the one shot going until the RC network times out.

There is a 50 ohm trim pot in the RC network that allows calibration of the pot by varying the one-shot pulse width.

Note that the electrolytic capacitor polarity is upside down from what you might expect.  The reason for that is that the emitter-base junction of the transistor prevents the base of the transistor from going below about 0.5V less than battery voltage.  But the voltage on the base can go well above battery voltage as it couples through the transformer.  Be careful not to reverse the polarity of the capacitor when replacing it.

I learned that the NTE176 is a good replacement transistor for repairing these tachometers.  Here is the schematics I have drawn for this circuit.

I have also learned while repairing a 70 Volvo P1800 tachometer that, if corrosion occurs at the point where the calibration pot contact presses against the PCB trace, the tachometer will work intermittantly or fail completely.  I recommend lossening the calibration pot nut, spraying the area there the pot contacts the PCB under the pot with a good electrical contact cleaner and then retightening the nut to ensure the pot makes good contact with the PCB.  the tachometer will start working again if that is the problem.

I have measured a Gen 2 tachometer thermistor that looks to be in very good condition at exactly 25 degrees C and measured 10.6 Ohms.  I was able to find several 10 Ohm NTC through-hole thermistors from a number of on-line electronics distributors.  If you need to replace a thermistor, any of those will probably work.