Making the most of your receiver Eddystone Stratton VK7AG 0012
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ee ea ie Pee, RET a SS, ee oom on @ latter usually form part of water-heaters, electric blankets and refrigerators— also give rise to intermittent noises. Experience is necessary to diagnose the type of electrical equipment produc- ing noise in wireless receivers, and often considerable patience is necessary when tracking it down. Listeners in the United Kingdom, whether they use communications or broadcast receivers, should obtain the appropriate form from the Post Office and send in details, when the matter will receive the attention of the Post Office engineers, who have been trained in the location and prevention of interference, and who are properly equipped to carry out the work. It is probable that the same procedure is available to overseas listeners—in any case, it is worth while making enquiries. before tackling the work oneself. The general advice which follows is provided mainly for the benefit of those using short wave broadcast receivers on board ship, but it is also applicable to others. It is necessary to emphasise two points—great care should be taken when carrying out tests, since even 110 volt mains can cause a nasty shock under some conditions, and secondly, only very reliable components should be used for reducing or preventing the interference. It should be noted that any electrical noise heard may be emanating from a comparatively weak near-by source, or from a machine at a distance but pro- ducing severe interference. Further, the sensitivity of the receiver comes into the picture. Maybe noise is weak or inaudible on a domestic type of receiver, but becomes serious on a set possessing high sensitivity, as is the case with Eddy- stone receivers, particularly communications models. Another point to bear in mind is that the interference created by a machine is likely to vary over the frequency range used for radio communication and although, after fitting a suppressor, the interference disappears over one par- ticular range on a receiver, it may appear again at other frequencies. It may happen that some equipment in use has already been fitted with suppressors by the manufacturers—for instance, a rotary converter, as frequently employed for transforming D.C. to A.C. or vice versa—but often such suppressors are designed for cutting out interference on medium frequencies (that is, the medium wave broadcast band) and they may not be fully effective at the high frequencies used for long distance short wave broadcast transmissions. There are two ways of tackling the problem: (a) to endeavour to eliminate the interference at the source, and (b) to prevent it entering the radio receiver. It is more satisfactory, and, in the long run, more economical, to suppress the interference at the source, since, ina bad case, the noise may be affecting recep- tion over a comparatively wide area. On board ship, efforts to cure the trouble at the source will probably meet with considerable sup- port, since both the ship’s communications installation and receivers used for broadcast reception will benefit. When the offending machine or apparatus has been located, the first thing to do is to check its condi- tion. All connections should be examined for tightness and, where a metal frame exists, atest should be made to ensure that it is properly earthed. Quite likely the equipment has been in use for a long time, and renova- \ ONDENSER CONNECTED : : tion may be required. Ifa ACROSS TERMINALS. motor, perhaps the brushes are badly worn or the commutator may be rough, causing severe sparking and Fig. 14. Showing how a fixed condenser is connected across the terminals of a fan, for suppressing interference. Page Eleven