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MAINTENANCE OF SOUND EQUIPMENT 653
long time and much patience. In very rare instances it may prove necessary to borrow duplicate parts in order to determine by comparison just where the trouble lies.
The volume indicator is a valuable instrument in this type of trouble. Knowing what the volume of the amplifier's output should be, with the test film previously used, the projectionist can tell by means of this instrument whether the cause of low volume is in the projection room or in the speaker circuits. If it is in the projection room and not confined to one projector either the amplifier or its transmission lines must be responsible. If there be both a voltage and a power amplifier the output meter (by comparison with previous results) may be able to determine which of the two is the faulty one.
Poor Quality of Sound
(13) This type of trouble will sometimes be found readily, perhaps with the help of headphones. Sometimes it proves more obstinate than any other.
The ways in which each separate portion of the sound equipment can cause poor quality have already been described in detail. In extreme cases it may be necessary to investigate them all, one at a time.
Fortunately, it is very often possible to determine the cause of poor quality by spending a little time in the auditorium and listening carefully. Once the difficulty has been classified, as, for example, flutter, wows, loss of high frequencies, loss of low frequencies, the work of finding its cause is immensely simplified. The various conditions that can cause these different troubles have all been explained. Flutter and wows are due to imperfect functioning of the drive system. Loss of high frequencies may be caused by nothing more than bad focus of the exciting light, or yellow or insufficient exciting light. However, an open-circuited by-pass condenser in an amplifier can also be responsible. Or the power supply to the high frequency speakers may be faulty. Loss of lowfrequency response may be due to a similar defect in connection with the low-frequency speakers.
Since bad sound quality is not a single trouble, but may