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OPERATING NOTES 177
much an exact one as might be thought by the above remarks. It is ultimately one of taste as regards the tone, and if, therefore, you need an extra loud speaker the best thing to do is to connect it in parallel with the existing one. If you find the tone satisfactory it is best to leave well alone. On the other hand, if you find the reproduction unpleasant a new transformer can be substituted by trial and error or alternatively by working back in the above formulae, or a multi-ratio transformer may be used.
Loud speaker buzz is usually caused through some part of the diaphragm becoming loose. Usually the fault will be so small as to be imperceptible to the eye and, therefore, it is practically useless endeavouring to search it out. The best way of curing a buzz of this character is to run over all the joints on the diaphragm of the speaker with a good quality cement, preferably of the celluloid variety.
If, when the set is working, the sound comes feebly through the loud speaker which happens to be of the field excited type, you may suspect that something is amiss with the field supply.
Sometimes loud speakers buzz or rattle due to the speech coil becoming out of round or out of centre. Usually this can be corrected by slackening the centre screw which holds the spider retaining the apex of the cone. Considerable care should be exercised when making such an adjustment, for it must be appreciated that there is only a few thousandths clearance between the speech coil and the centre core of the magnet.
Some types of loud speakers have their cone supported round the outer rim by means of leather segments. After continued use this leather sometimes contracts, tightening up the cone and reducing volume. This can usually be corrected by carefully kneading the leather round the outside by means of a rounded instrument, such as the back of a propelling pencil which breaks up the fibres of the leather, causing them again to stretch.