Vintage Smiths Tachometer Repair Issue

Thread Starter


Joined Apr 10, 2012
I wonder if I could get your opinion on what might be wrong with a Gen 2 Smiths tachometer from a late 1960s Volvo.

There is a local guy here who sells a lot of vintage Volvo parts all over the world. He gave me 4 gen 2 tachometers to go through. One tachometer was functional and only needed calibration. Three of them would slam to a high RPM reading when they got warm or hot, but worked normally when cold. I heated the circuit with a hair dryer and then cooled components with freeze spray. The transistor was the only component that needed to be cooled to get the circuit working.

For reference, here is the schematic for the Gen 2 Smiths tachometer:

The meter is a D'arsonval ammeter. The circuit is a monostable blocking oscillator. The triggering signal is a 1A audio frequency square wave through the primary winding.

I put one scope lead on the calibration signal and the other on the base of Q1. When the circuit is operating normally, you can see the negative going one-shot pulse on the upper trace and the reference signal on the lower trace. The one shot is triggered by the rising edge of the reference signal:


However, when the temperature sensitive units got warm to hot, Q1's base shows that the circuit starts to oscillate until the falling edge of the reference signal:


I replaced the 2.5uF, 16V caps with 2.2uF, 50V electrolytics and two of the units started working properly over the full temperature range. I also ran an experiment with different values of capacitance in one of the tachometers and learned that the circuit will start to oscillate at about 0.5uF and below. The circuit will work properly up to 3.2uF, but the one-shot will not trigger at 4.4uF.

The value of the cap does not have any effect on the one-shot pulse width.

As a precaution, I replaced the cap in the functional tachometer as well. At this point, I have 3/4 tachometers ready to go back to the customer. However, the fourth one still oscillates at higher temperature.

In that unit, I replaced the transistor with an NTE176 that I had sourced from eBay. The new transistor did not fix the issue, but I didn't really trust the eBay sourced transistor. I called NTE Direct and manage to get 6 units of NTE176 from them and put one of those transistors into the unit, but that had no effect either.

I tried adding capacitance to the cap up to 4.4uF, and all it did was reduce the amplitude of the oscillation and increase the oscillation temperature threshold a little bit. I also checked the resistance of the 39 Ohm resistor with an accurate Agilent DVM and it read 38 Ohms.

I am far from an expert in blocking oscillators. At this point, I am at a loss as to what could be causing this tachometer to fail at higher temperatures.

Any thoughts as to what is happening to this particular tachometer?

Thanks in advance,


Thread Starter


Joined Apr 10, 2012
Thanks for the response.

I am trying to minimize the amount of soldering I do. These old (mid 60s) PCBs delaminate pretty easily. But to test if the thermistor was causing the problem, I simply jumpered across the thermistor/resistor combination. Its only purpose is to compensate for thermal changes in the meter movement windings, so it varies the resistance by less than 2 ohms as it works. I checked the resistance of the resistor/thermistor combo in circuit, and it was at 1.8 ohms at room temperature, so I figured it is working ok. But I jumpered it out to check anyway. I am also not sure what the value of the thermistor is, so I am not sure what to replace it with.

I poked and prodded all of the components looking for intermittent issues and saw none. Nor did I see any issues when I flexed the PCB. In spite of that, I resoldered all of the solder joints in the tachometer to ensure I had no cold solder joints.

I re-ran the hair dryer/Freeze spray tests using shields to isolate the components being sprayed, and the transistor is the only component that will stop the oscillation when it is cooled.

An interesting thing to note is that every day I work on this thing, the temperature threshold needed to get it to fail has been going up. It took a lot of time for the hair dryer to get it to oscillate this morning. I am getting nervous that this thing will stop oscillating when hot. I am not sure the customer will be able to trust it...


Joined Jun 22, 2012
All you can do is swap components between the working one and the faulty one , there is only a transistor , inductor, , capacitor, meter, thermistor...