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Measuring the local oscillator in a superhet

Is it possible to use a frequency counter to check the local oscillator frequency of a transistor radio, ie 455kHz? If so, is it possible to achieve this without physically connecting the counter to the local oscillator circuit? (V. K., Newcastle, NSW).

  • This is certainly possible. The way to do it is to use a sniffer coil connected to the frequency meter. This is simply several turns of wire connected to the frequency meter probe and positioned close to the oscillator coil.

    The local oscillator in a superhet is always above the incoming station, so the local oscillator frequency will be 455kHz above the tuning frequency. So for example, when the radio is tuned to 1269kHz the oscillator will be at 1724kHz.

    Fridge controller
    is not working

    I have a question to ask regarding the Fridge Temperature Controller in the June 2005 issue. I built the kit and find that the unit is permanently on. I’ve double-checked the construction (I bet everyone says that ) and can’t find anything wrong.

    What I am getting is 8.9V at pin 3 of the comparator chip and if I measure the resistance of the LM335 over a temperature range of 2°C to 20°C, it only varies from 35.7kΩ to around 36.5kΩ. I’m guessing that this small change isn’t enough to give a 2V to 3V drop across pin 3; in fact, the voltage stays at 8.9V. If I remove the LM335 and replace it with a 2.2kΩ resistor, the LED goes out.

    My question is where could my circuit be wrong or is there some instruction missing and should I be adding some extra resistance to the LM335 part of the circuit to help with the voltage drop? (J. P., via email).

  • If you are getting a voltage of 8.9V more or less permanently on pin 3 of IC1, this suggests that the connections to your temperature sensor are either open-circuit or perhaps reversed. That’s about the only reason why the voltage at pin 3 would remain very close to the 9V supply line.

    Measuring the resistance of the LM335 is not very meaningful, because it’s very non-linear. In fact it behaves very much like a zener diode, whose reverse breakdown voltage varies in direct proportion to the absolute temperature in Kelvins. And at any particular temperature the dynamic resistance is very low, so that the voltage drop hardly changes over a fairly wide range in current (100μA to say 5mA). This is why a resistance measurement is not very meaningful.

    Speedo Corrector Affects Odometer Readings

    I built and installed the Speedo Corrector Mk2 from the December 2006 issue in a rebuilt 1977 Porsche 911 which had the wheels changed from 15-inch to 16-inch. I have calibrated the speed against a GPS unit and it is accurate at 100km/h and a little under at 50km/h (48). The odometer reads under by about 300 metres on a measured 5km stretch.

    Is this normal when the speed is corrected? If it is to be expected, how can I correct the problem? (D. B., via email).

  • This whole issue was thoroughly canvassed in the Mailbag pages during 2007. The problem is that most cars have an odometer which is highly accurate but a speedometer which is optimistic. This means that a typical car’s speedo will read high at any speed and may read as much as 108km/h at 100km/h.

    Since the speedometer and odometer use the same pulse stream from the wheel sensor, there is nothing you can do about it. If you want the odometer to be accurate, you will have to live with the speedo reading being high.

    In practice, we think it would be better to have the speedo reading slightly high at 100km/h (eg, 103km/h) so that if you have the speedo sitting on a particular speed limit, you know that you have a safe margin and cannot be pinged by a speed camera.

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