How it Works: Suppressors - Removing Static From The Radio

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As cars became more complex, so too did the many electrical circuits built into it, the most important being the ignition system. Any make or break of one of these circuits (as in a switch or an electric motor or the distributor) is likely to generate interference on radio frequencies, which shows itself as an annoying whine or crackle. These days the technology has improved to the point that interference is a thing of the past, however for the classic car enthusiast it can remain a problem - particularly when keeping a car "stock" with original AM band radios.

The answer most car manufacturers used to enable static free AM reception was to fit suppressors to the ignition system. These operate by eliminating the pulses in the circuits which lead to the formation of electra-magnetic waves. Resistors fitted in series with the high-tension leads, as near to the spark plugs as possible, reduce the effect of the high- voltage impulses without lowering the voltage significantly.

These usually had a resistance of 10,000 ohms, and were made of carbon so that they were non-inductive; in most cases where wire leads were used, these suppressors were built into the plug cap. Many engines made use of high-tension leads with a graphite rather than wire core and these would act as suppressors themselves.

In the distributor, a carbon brush carried the electricity from the coil lead to the rotor arm; this, too, acted as a radio suppressor. The low-tension side of the ignition system (interference is generated by the contact-breaker points) was taken care of by a capacitor fitted between the ignition coil and earth. A capacitor would not pass direct current, but would pass alternating current, so that any stray impulses will be short-circuited to earth and thus removed.

The generator was probably the next most effective cause of radio interference, emitting more of a whine than the harsh crackle of the ignition. How you fix this depends on whether the generator is a dynamo or an alternator; if it is the former, a capacitor fitted between the output terminal and earth should remove the interference. An alternator has to be treated differently if it does not have a built-in rectifier, because a capacitor connected across the output would short-circuit the unit.

Even if the rectifier and regulator are incorporated in the alternator, it is quite common to supplement the normal capacitor by fitting a special filter in series with the output lead. This filter consists of an inductance and a capacitor connected together in parallel, and it can match the exact frequency of the interference, which is dependent on the speed of rotation of the alternator. Other electrical items, such as the windscreen-wiper motor, the heater fan, the petrol pump and even the clock, will probably require capacitors - all of 0.1 microfarads value - connected between their live terminals and the chassis or body, in order to obtain peaceful radio listening.

Even the horn and the indicators need to be dealt with in extreme cases. With a steel car body it is quite easy to prevent any interference from reaching the radio, because the steel body screens the receiver from the potential trouble-makers - but this is assuming that the components mentioned have been suppressed and that the aerial is mounted correctly with a good connection between it and the body. A glassfibre body presents a different and more taxing problem; it is almost impossible to suppress the electrical equipment of a glassfibre car to the point where there will be no interference, especially when the radio is off station.

If normal suppression methods are efficient enough, then the first thing to do is to line the engine cover with metal foil and then earth the foil to the chassis. If this does not do the trick, then a cover will have to be fabricated to enclose the spark plugs as much as possible. It is possible to obtain special graphite paint, which is electrically conductive; this can be sprayed on the engine cover in place of the metal foil. It will not work, however, if it is not earthed.

One final source of interference on steel or plastic-bodied cars may be the brakes - particularly the drum set-up found on older cars: it is sometimes found that static electricity builds up in the brake drum when it is spinning, causing a faint crackle which stops only when the brakes are applied. This is very difficult to eliminate, probably the most effective means being to fit special metal brushes to the chassis or body so that they rub on the brake drum and earth it. If you have any other tips you can pass on, please send an email to us here at Unique Cars and Parts.
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