International projectionist (Oct 1931-Sept 1933)

SOUND EQUIPMENT SERVICING BY PROJECTIONISTS Aaron Nadell IN A Middle Western city, a few \ears ago, a new theatre burned out a plate power transformer just a few hours prior to a muchadvertised "Grand Opening." No duplicate transformer was to be found in that city, and the consensus, after considerable discussion, was that the opening would have to be postponed. The new theatre stood to lose about two thousand dollars in admissions and perhaps twenty thousand dollars in prestige. But, everj^one said, there was no help for it. Entered, at this point, a gentleman whose knowledge of sound apparatus was not as strong as his common sense. He asked : "What does that transformer do?" The answer was: "It supplies high voltage a.c. to the rectifier." "All right," came the next question, "what does the rectifier do?" "Well, it changes that a.c. high voltage to d.c. high voltage and passes it on to the amplifier." "And we can't work without the amplifier, is that right?" "That's right." "And the amplifier can't work without high voltage, is that right?" "That's right, too." "How much voltage does the amplifier need?" "Seven hundred and fifty." "D. C?" "D. C." "Well, why don't we go out and buy seven hundred and fifty volts of B batteries to last till we get a new transformer?" There was no reason M'hy not. Fifty dollars' worth of B batteries were bought and a two-thousand-dollar show went on. It was an expensive emergency repair, but justified by the circumstances. Repair or Replacement There are two general methods of emergency action whenever it is not possible to do a proper repair immediately. One is to readjust or to modify the defective part or apparatus, to enable it to function temporarily. The other is to replace the defective part with something else, even something radically different, that will, by any round-about way, result in achieving the same fundamental purpose. The case of the B batteries cited above is especially interesting because those batteries did not replace merely the damaged transformer but substituted for the entire rectifier, of which that transformer was only one part. Such broadly effective emergency treatment is impossible unless every part in the projection room is thought of in terms of w/ial it is there for rather than in terms of what it looks like or of what work it performs in a more narrow and limited sense. A narrow and exact interpretation of the function of that power transformer would be that it existed to step-up the voltage of A. C. Only from the broader viewpoint that it was there to help supply high voltage direct current to the amplifier could the possibility of substituting B batteries for it become apparent. The imagination and knowledge of the projectionist are in the last analysis the only limits to the possibility of emergency replacements. Imagination, of course, sometimes runs wild, and it is never desirable to undertake freak remedies until every normal possibility has been exhausted. And, of course, repairs of any kind are impossible until the nature and extent of the trouble have been accurately run down and are thoroughly understood. But when this has been done, and when it has been clearly shown that normal repair methods cannot bt helpful for some time to come, then the ordinary repair man holds up the show and the better-than-ordinary man does something unusual. The possibilities of unusual action are so great that it is almost impossible to exaggerate them. Consider any simple apparatus — a resistor, for example. What is its function? To limit the flow of current. What can be substituted for it? Almost anything. If the resistance is relatively low, a few arc carbons or even a glass of salt water may do. If the resistance is very high, running into megohms, a few pencil marks scratched [20] on paper may serve. These are extreme suggestions. But any conductor in the projection room possesses resistance. Whatever conductors are available, including electric light and vacuum tube filaments, may be connected in parallel or in series or in series-parallel to secure the desired resistance and current-carrying capacity. (If filaments of any kind are used, remember that their resistance will change with heat. If they are to be used hot, determine their resistance by their rating ; if they are to be used cold, determine it with an ohmmeter). But even this is an extreme suggestion, intended chiefly to emphasize the extreme possibilities. In common practice such heroic steps will seldom be necessary. As a practical matter, resistors of any imaginable value can be found at the nearest radio repair shop, and the real problem is not obtaining a resistor but only finding out what value of resistance in ohms and currentcarrying capacity in amperes is required. Sources of Supply Much the same is true of all other simple parts. In the case given above, the same radio store that furnished rhe B batteries could probably have supplied a suitable transformer, or transformers, that would have served admirably to replace the one that had burnt out. Yet even that would have been a practice to some degree extreme. The same radio store nnght also have been able to furnish an ampiiiler, complete with its own power supplies and with proper input and output impedances, that would have rented for much less than the cost of the B batteries. While the possibilities of emergency methods are unlimited, their practical application should be drastically restricted to the easiest, quickest and simplest procedure — therefore to the one that will mean the least interference with the apparatus in the projection room, and to those methods which will come nearest to ap