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MAINTENANCE OF SOUND EQUIPMENT 613
Line-frequency hums may be caused by amplifier conditions described in connection with the construction and functioning of amplifiers. They may also be caused by exposure of the photo-cell to the projection room light, if that is created by a. c. Another cause may lie with the ground connections, and this is a more tricky matter, in which trouble-shooting becomes largely experimental. The earlier sound systems often used a number of grounds, and sometimes line-frequency hums could be cured by adding still others, and sometimes by removing some of those already in use. The modern tendency is in the direction of a single ground connection for all the equipment. Corrosion or bad contact at this or any connection may easily create line-frequency hum in the sound.
Hums of other than line frequency are rare. They may, however, (as already explained) be caused by feedback in an amplifier. Or by loose connection in a grid circuit.
The crackling, buzzing or fizzing or rasping sound caused by most imperfect contacts can sometimes be run down by headphones, which may indicate what amplifier, supply line or other part of the system is the source of the trouble. Sometimes, however, much patience is needed before trouble of this kind is traced to its ultimate cause, which may be, perhaps, no more than a defective fuse.
Intermittent Troubles
(17) Any of the above troubles becomes infinitely harder to find if it does not represent a permanent condition, but one that comes and goes. In such cases tests have no meaning unless they are applied at a moment when the trouble is present. The fact that an experimental remedy seems to work likewise means nothing with a trouble that constantly disappears of itself.
One very helpful procedure in these difficult cases is to attempt to make the trouble worse, rather than better. Wires are tugged, ground connections are rattled, rheostats and switches are operated rapidly and jerkily, tubes are tapped and panels tapped or shaken in an effort to