3 years ago



TROUBLE-SHOOTING TIPS Continued from preceding page Check fuses in amplifier or power supply to amplifier. Are the tubes lit?—the rectifier tube lit? If power supply is okay, and you know the tubes are all right, filaments lit I in glass tubes) . then the trouble simply boils down to locating the unit in w-hich the defect occurs. It may be in the preamplifier, the power unit, the stage speakers, crossover, etc. If you are getting sound from the booth monitor and not from the stage speakers, your trouble is back stage. EXAMINE THE EXCITER LAMPS Have you checked the exciter lamps? If exciter lamps are okay, remove film and @^ ^&. s> uper Snaplites are better indoors or ^ ^' out. Sharper Pictures, Greater Contrast, More Light, Better Definition all add up to happy patrons. Make your Movies Better Than Ever; use Super Snaplite lenses. Super Snaplite Projection Lenses give a true speed for f/1 .9 in every focal length up to 7 inches. Ask for Bulletins 207 and 209. Also Series II SNAPLITES with a speed of f/2.0 KOIiLiWOIU.'^ PLANT: COKIMIKATIWK NORTHAMPTON, MASSACHUSEnS NEW YORK OFFICE: 30 CHURCH ST., NEW YORK 7, N. Y. SAVE MORE ON CARBONS ^^^^^^^^^^^^^^_ CALI CARBON COUPLERS -^^N. Let You Burn All the Carbon .^^ \ "They're Expendible" Full Refund If not 100% Satisfied Tht most popular carbon saver. Used bv more i_ j j »• .1 . theotre. than ALL Improved other mokes COMBINED production methods create greatly At oil progressive supply houses. REDUCED PRICES Per Hundred, postpaid: Not Pocked In Mixed Sizes Some high quality-Patents pending. 6 or 7mm $2.00 8mm or 5/16-inch $2.25 'mm $2.50 No worrvino about injury to hijh priced carbon uvpr^. Burn 'em up. you still profit. A4oif •conomfca/ carbon tavr you ever uied.' CALI Products Company 3719 Morjori* Woy Socramento 20, Calif. The WORLD'S LARGEST Producer of Corbon Savers check, with a white card, see if light is coming through to the photocell. This gives you a check on the exciter lamps for correct alignment and if there is any obstruction, if sou7id is oil on one machine only, cutting light beam from photocell. Check connections to photocell socket and to amplifier or pre-amplifier: check P.E. cell voltage. Operate the fader or changeover switch several times in order to eliminate any possible poor connection or switch contact. Check fader connections and see that they are clean. Check each sound head and see that all connections are making good contact and electrically firm. Check sound head mirrors that reflect light beam to photocell, etc. If any particular trouble persists, turn up volume, gently tap the input tube. This should produce a thump in the monitor, indicating you have sound, but it may not be coming through from P.E. cell. This cell may be defective or have no voltage, making poor contact, etc. If no sound is heard in monitor, by tapping input tube, then the trouble probably exists in the amplifier and its components, such as low voltage, defective tube, no voltage at tube sockets, defective resistor or condenser. Check ALL the tubes, or replace with new one in each stage, until you find one defective, if tubes check out okay, power supply to amplifier is all right, you have an open circuit, defective part or bad connection. To check voltages you need a good volt-ohmmeter. You will need a table of the correct voltages at the grids, screen-grids, plates, TROUBLE IN cathode, etc. THE SPEECH LINE In general, trouble in stage speakers or crossover network will be evident by normal signal from monitor only. If your monitor and stage speaker or speakers are all dead, your trouble is in your speech line going to your stage speakers, in the crossover, or amplifier. If you use a field supply for your stage speakers and only stage speakers are dead, check this field supply first, of course. The thing to do is to try to tie the trouble down to some specific unit in the sound reproducing system. Very important are voltage and current measurements. These can be measured with any good make of volt-ohm-ampere meter at the socket of each tube. Measuring voltage and current of a circuit using 38 The MODERN THEATRE SECTION

I : a , a three-element tube, you should measure: 1. plate voltage; 2. plate current, 3. grid voltage, grid current; 4. filament voltage. When checking an indirect heater type, screen grid, or pentode tube circuits, the following additional measurements are made: 5. cathode voltage; 6. screen-grid voltage; 7. screen-grid current, and suppressor-grid voltage. All these voltages and currents are checked to see if they are same as recommended by the manufacturer of the amplifier. THERE MAY BE A SHORT When using voltage-current method for analysis of theatre audio amplifier or preamplifier defects, incorrect voltage or current at the tube sockets is, of course, an indication of a probable short in the circuits or component; a high resistance ground, defective or shorted condenser, resistor, choke or transformer defect, causing incorrect voltage or current reading. Each stage should be carefully checked, starting with the output stage, checking until you find the stage giving the trouble. This method of point-to-voint troubleshooting points out the stage which is causing the trouble. How to correct many amplifier troubles you will find in back issues of Modern Theatre. Each month we try to present socket connections of all the types of tubes used in amplifiers and TV. In order to check voltages, it is necessary that you know which "pin" is the plate, grid, filament, etc. Diagrams or schematics of amplifiers show the bottom view of tube sockets, and that is the way we are presenting these tube sockets, showing plate cathode, grid. etc. Be sure that all the tubes are in good condition before starting either voltage or current measurements. TURN OFF THE POWER Those of you who have studied radio and audio amplifiers will know it is wise to be cautious when operating voltages are on and you are checking these voltages. Voltages used in many audio amplifiers in theatres are high enough to be dangerous to life. The best plan to follow, for safety's sake, is to turn the power off at the supply panel before changing tubes or arranging amplifier chassis for checking operating voltages. Of course, when ready to check voltages at sockets, power pact, etc., it will be necessary to have amplifier on, using insulated test probes for testing voltages. It is a good idea to stand on an insulating board of some kind and touch no grounded metal objects while checking circuit voltages of any make of amplifier. This advice also applies to checking photocell and exciter lamp voltages. Before checking voltages, be sure to let your tubes warm up several minutes. Before starting the show each day, the amplifier should be turned on at least five or ten minutes. When checking capacitors ( condensers ) be sure to "short" these with a heavy insulated screwdriver. This is a safety precaution that should be kept in mind when Continued on Page 42 ^lSfAl»^l3.'^°" ECONOMY/i llUXURIOUS comfort, smart appearance and the economy of both moderate initial cost and the lowest maintenance cost of any chair^ on the market — all these are yours when you reseat with the IRWIN "COMET". Write for complete details. Reseating with this superb chair is a powerful means of boosting Box Receipts— an investment that soon pays for itself in larger profits. iRwiw: ?AP»0^ Truly the leader in march away from black masking. — • — Be the first in your community to give your patrons something new. Patrons like this screen—Tests prove. RWIN SEATING COMPANY GRAND RAPIDS, MICHIGAN .1 Aisle standrd dccorat n s are 1 notional. J Full depth i back panel protects seat j .1 cushions, J ii\^