Motion Picture Herald (Sep-Oct 1935)

48 MOTION PICTURE HERALD September 21, 1935 TECHNCLCeiCAL The BLUEBOOK School By F. H. RICHARDSON BLUEBOOK SCHOOL QUESTION NO. 283.— (A) What care should be given sound-three head drive belts? (B) What effect will a dirty sound screen have on the quality of sound reproduction? (C) What care should be given sound screens to insure good sound transmission? Answer to Question No. 277 Bluebook School Question No. 277 was: (A) In equipment where the power supply for the energising coil of the relay is obtained from a rectified a. c. source, a "hum" may develop. What is its cause and what can be done in an emergency to eliminate itf (B) In equipments employing a separate voltage and power amplifier, provision is usually made for operating the stage loudspeakers from the voltage amplifier in case of failure of the power amplifier. How would you proceed to accomplish this purpose? The engineers who prepared this question answer : (A) "Usually such a condition is caused by a breakdown or open circuit in the filter condenser which forms a part of the rectifier circuit. In emergency the show may be kept going by disconnecting the power source from the relay coil and operating the relay manually. (B) "In case the power amplifier fails, disconnect the link circuit between the voltage amplifier and power amplifier. Next disconnect the stage loudspeaker line from the power amplifier, reconnecting it to the proper impedance tap of the output transformer on the voltage amplifier. It is good practice to go through this procedure after show hours, once a month, in order that the projectionist become thoroughly familiar with all the necessary wiring connections. Manufacturers' representatives will be very willing to supply projectionists all necessary information with regard to such emergency procedure." Might I butt in right here to suggest that the representatives of sound equipment manufacturers most certainly will be very willing to help projectionists by providing full explanations, coupled with demonstration where practicable, of emergency or other procedures upon request? May I offer a suggestion that projectionists will do well to jot down things they may require information about, so that when a manufacturer's representative calls they may be reminded of the various points? I was indeed surprised by the few answers received to this question. Of those that came, but very few were within hailing distance of correctness. Therefore, in order to avoid embarrassment to some most excellent students of the "school," I will omit names. Here is the gist of some of the answers to Section A : 1. "Hum may be caused by reverse current relay stuck in open position. I can think of but two causes, (a) accumulation of dirt, weak spring tension, poor contact between the magnet and its corresponding pole piece. Under this condition it is possible there will be sufficient ripple in the power supply, even though recti fied, to cause vibration and thus set up a characteristic 120-cycle hum." 2. "In equipments where the power for energizing the relay coil is obtained from rectified a. c. and a hum develops, the usual cause is sticking of the relay switch. In case of emergency, short circuit the switch and operate manually." And here are two answers to Section B : 1. "In equipments employing separate voltage and power amplifiers, provision is usually made so that in event that the power amplifier goes haywire, the show may be continued by using the voltage amplifier only. In such emergency turn the power amplifier switch to off position and leave it there. Connect the output of the power amplifier to the input of the voltage amplifier, using proper jumpers. Turn the voltage amplifier to operating position and continue the show, with the fader at higher position." 2. "In a sound system where the voltage and power stages are separate it is an easy matter to operate the loudspeakers by using the voltage amplifier only, in case the power amplifier fails. However, to accomplish this requires a rather intimate knowledge of the circuits involved. There are really but two things one can do. The first is to connect the plate terminal of the output transformer in the power amplifier to the terminal of the plate of the last output tube in the voltage amplifier. This serves to feed the output of the voltage amplifier directly to the speakers through the pri F.H.RICHARDSON'S NEW BLUE BOOK OF PROJECTION .™_^^/ A new edition of this famous Vjj^ >«• y IjqqI; ij pQy, Qn f|,g pfBss. 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I submitted these answers to the engineers, who have replied as follows : "Probably the reason for so few answers is because such troubles rarely occur. However, the more important emergency situations often are those that rarely occur, and the projectionist who is on his toes wants to know just what to do when they do. "While the answers to Section A might be considered as right in some instances, usually symptoms of a sticking relay is that sound is not faded to the proper projector when the fader key is depressed. When some part of the filter circuit in the power supply circuit for the relay becomes defective, there is usually some ripple supplied to the energizing coil in the relay, which, being in close proximity to the signal circuit, is picked up at that point, thus producing hum in the loudspeakers. "The first answer to Section B is correct, with the exception that the output or speaker connections should be connected to the output transformer of the voltage amplifier. "Whoever submitted the second answer to Section B apparently had in mind a voltage amplifier, which does not have an output transformer. As he says, when such a condition exists an intimate knowledge of the circuits is required, and this is, I believe, beyond the scope of emergency procedure. The type of amplifier considered in framing the question was one in which an output transformer is used, the same having a proper impedance tap to match the loudspeaker load." Publishes Second "Mickey" Issue "Mickey Mouse Magazine," issue numtvvo, reached the newsstands this week from Hal Horne, fonner advertising director of United Artists, in a revised format, the large size having been abandoned in favor of an eight-by-eleven-inch overall. Running some 44 pages, the magazine is published in conjunction with Walt Disney Enterprises and his Mickey Mouse motion picture interests.