The motion picture projectionist (Nov 1929-Oct 1930)

October, 1930 Motion Picture Projectionist 17 233-C INPUT TRANS. T, OUTPUT TRANS T2 V.,.. M SOO^OWT Schematic of the Western Electric 42-A amplifier. This amplifier is A. C. operated. Great care must be exercised to prevent the secondary of the T-4 transformer from burning out. Shorted vacuum tubes have been responsible for this occurrence panel. This switch was replaced, which eliminated this trouble." Most of our troubles are with vacuum tubes. To get the most life from vacuum tubes connected in push-pull it is most important that they be as evenly balanced as possible. These tubes work in pairs, and a tube with low emission on one will throw the load on the other which, in due time, burns out and at the same time gives poor tone quality. It is always good policy to give the sound reproducing system a daily inspection and test. This inspection consists of cleaning panels, checking tubes, exciting lamps, p. e. cells and batteries, and checking the entire reproducing system. Voltage Fluctuation The Fox U. C. Theatre, Berkeley, California, reports the following: "Our main trouble seems to be with power amplifier tubes burning out, which is due to a high A. C. voltage supply averaging from 110 to 121 volts. The projectionists have installed a small bank of resistance in the A. C. line, which keeps the voltage at 110 volts maximum." The Fox Imperial Theatre, Alliance, Nebraska, reports the following: "A recent trouble with the motor control cabinet was caused by a burned out resistor, which caused the projection speed to increase. This caused no serious trouble or interruption, as the motor control cabinet was set on variable speed until the new resistor was installed." We believe U&flAMfl/ fooolooftf feooooool oooooooaooooooo' -OGN0 , -PLATE] that one of the causes for the resistor burning out was due to the fact that the manufacturer places a metal tag around the resistor with the resistance value stamped on it. This tag causes the heat to concentrate beneath it and thus retains the heat in one spot, which causes deterioration of the resistance wire. We believe this trouble may be eliminated by removing the metal band from around the resistors. Defective Grid Leaks The Fox Riverside Theatre, Riverside, California, reports the following: "The Movietone amplifier on the white projector became very noisy. We checked all tubes, photo-electric cell, dry 'B' batteries, etc., but could not locate the trouble. The service engineer was called and, upon his arrival, stated that the photo-electric cell was the cause of the trouble. He also condemned the spare photo-electric cells. New photo-electric cells were obtained, with the result that we still encountered the same trouble. Finally the service engineer replaced the amplifier and still we had the same noise. The dry 'B' batteries were also replaced and still we encountered the same trouble. Finally the grid leaks were tested and one was found to be defective, which caused all the trouble." The Fox T. & D. Theatre, Oakland, California, reports the following: "Considerable fluctuation was noted on the plate current meters on the main amplifiers whenever rewind or projector motors were started. This was remedied by placing the rewind and projector motors on a separate feed from the main switchboard, leaving the amplifiers and charging panel on a feed by themselves. Chief trouble with the Movietone amplifiers is an occasional microphonic tube. Several power amplifier tubes were lost due to high A. C. line voltage. A resistance was placed in the line, bringing the voltage down to 110 volts — thereby effecting considerable saving in vacuum tubes. Rectifier tubes do not give as many burning hours as amplifier tubes. A. C. Hum "We experienced difficulty with A. C. hum from the battery charger particularly at a high fader level making it practically impossible to use the charger during the performance. This was eliminated by re-arranging the lead covered 12-volt Movietone amplifier filament leads as far away as possible from other wires surrounding the charging panel. Popping, due to throwing the Movietone switch in and out, was reduced considerably by turning the exciter lamp rheostat down before pulling the switch." Keep Contacts Clean The Fox California Theatre, Pomona, California, reports the following: "We had considerable trouble with the film disc transfer switching panel cutting off, during the presentation of Movietone subjects, due to the cover of the switching panel shorting against the switching keys." It is advisable to keep the contacts of switches in the film disc transfer switching panel clean at all times — otherwise, loss in volume will result. Condenser Testing The C-l coupling condenser .006 M. F. in the W. E. p. e. c. amplifier has given very little trouble — however it may become shorted, and if so, it may be tested in the following manner:— During the testing of a condenser, always remove or disconnect apparatus in circuit, so that there will be no shunt path around the condenser. In testing the .006 condenser, remove the first vacuum tube in the p. e. c. amplifier, VI, and then remove the two leaks, % meg ohm and the 10 meg ohm. This provides a clear path for testing. The condenser should first be completely discharged by holding a wire or piece of metal across its two terminals. This will prevent an erratic test caused by a residual charge. The two tips of a headphone tester, with a C-battery connected in series, are then touched to the condenser terminals. There will be a click resulting from the sudden formation of a charge on the condenser plates and the accompanying rush of current through the headphones. One of the testing tips is tapped on the same condenser terminal several times.