Motion Picture Herald (Jan-Mar 1932)

February 13, 1932 Motion Picture Herald 29 MODERN PROJECTION PROJECTION • SOUND REPRODUCTION • ACOUSTICS REPAIRS MADE BY THE PROJECTIONIST THEIR PRACTICAL EXTENT AND LIMITS By AARON NADELL MANY OF THOSE portions of the sound equipment that are customarily returned to their maker for adjustment can — with proper precautions — be repaired in the projection room. This is often easier and quicker, and nearly always less expensive. However, such repair should not be undertaken except after careful study of each individual case. In some instances special apparatus Avill be necessary if proper results are to be secured ; in others, projection room repair, although technically practicable, will be economically inadvisable because of the cost of the time involved. It is in the third very large group of instances that projection room repair will be found technically practical as well as economically advantageous. Motors Managers and projectionists are familiar with the basic necessities of motor maintenance and repair. In extreme cases it may be necessary to have the commutator turned down by the local machine shop. Badly worn bearings may or may not be replaceable locally — often they are. An open or short circuited winding should be inspected very carefully before any motor is returned to the factory, since the trouble may be easily accessible to local repair. Motor Speed Controls Speed control devices fall generally into three classes: the mechanical governor, the centrifugal contact and the electrical "control box." Trouble with the first is nearly always traceable to defective action of the friction shoe, while dirty or pitted contacts are possibly the most frequent source of difficulty with the second. Neither should be beyond projection room relief, in normal circumstances. Trouble in the electrical circuits of a "control box" is run down and treated like any trouble in a circuit. Electrical Speed Controls The presence or absence of the proper current and voltage at any point of any electrical circuit can be determined by suitable meters. Meters — or headphones-andbattery — will reveal any suspected break in the wires, or any short-circuit. What is necessary, is that the person attempting to find any electrical trouble understand the circuits with which he is dealing and the functions of each separate part. He must possess and know how to read a circuit drawing of the apparatus in question, and he should have a very good idea, if not perfect knowledge, of what values of voltage and current are the proper values at every point. With such knowledge and proper equipment (circuit drawings and meters, e. g.,) no "control box," amplifier or rectifier should ever be sent out of the projection room for repairs until every local effort has been exhausted. Very often it will be found that the trouble is no more than a loose connection or a grounded binding-post — something that can be repaired at really no cost at all. In other cases, some part— easy to replace, even if it cannot be repaired — may be at fault. Only rarely will the trouble be so extensive as to involve several separate parts of the apparatus. As between making some minor repair to a loose or grounded connection, or repairing or replacing some small part ; and shipping a heavy and bulky panel to its manufacturer for overhauling, the advantages would seem to lie entirely with projection room adjustment — with proper knowledge and equipment for testing. Without those aids, the time lost in running down the trouble ruay be more expensive than outside repairs. Without proper knowledge, local repairs may be inadequate to restore proper operation, or to provide sufficient safeguard against repeated trouble of the same nature. A number of speed control circuits are now in common use. Two fundamentally different principles are employed, while a number of circuits are based upon the same principle but differ materially in detail. Where intimate and detailed knowledge of the operation of their own motor speed control is in the possession of the projection staff, or can be obtained from a service engineer, local repairs should certainly be attempted. Where such knowledge has not been secured, it will probably be most advisable to return a defective control cabinet to its maker for adjustment. Much the same considerations apply to the projection room repair of amplifiers, rectifiers, chargers, or any equipment of more or less complex wiring. All rectifiers operate upon a common principle. The same is true of amplifiers. Details, in each instance, may vary enormously, and especially so in the case of amplifiers. Detailed acquaintance with local equipment is the only reasonable assurance that local repair will not take so long as to cost more than factory repairs. It is the only possible quarantee that local repair will be of safe and satisfactory quality. Repair of the individual parts that commonly are used in amplifiers and rectifiers, as well as in motor speed control panels, will be reviewed later, under the heading of "amplifiers." Drives REPAIR OF THE drive portion of the sound equipment requires not so much electrical as mechanical competence. The details of drives vary far more widely than the details of any other portion of the sound equipment. Belt, chain and gear drives are employed, and sometimes combinations of all three. Friction clutches, "viscous damping" devices, "mechanical vibration filters," flexible couplings and a number of other rare or unusual mechanical contrivances, in almost every possible combination, are used in sound drives. Mechanical troubles are nearly always easier to find than electrical troubles. Action and troubles of the apparatus are physically present to the senses of sight and touch. The mechanical background of the generic projectionist is (because of the requirements of the projector head) at least comparable to his electrical background. Mechanical repairs are nearly always within his competence.