International projectionist (Oct 1931-Sept 1933)

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July-August 1933 INTERNATIONAL PROJECTIONIST some equipment; but many smaller theatres will find the cost of a complete test kit rather high. Such theatres might buy a minimum of the testing equipment they should have, trusting to be able to borrow the rest at the radio store, when needed. The usual screwdriver, pliers and socket wrenches need no mention here ; special tools are sometimes in order when working on a sound attachment; in most such cases those tools are furnished by the manufacturer of the equipment. In ordinary work on amplifiers there are only two electrical tools of unusual design — the "diagonal" or cutting pliers, and long-nose pliers. Jack switches are sometimes adjusted with the help of a special jig made for the purpose, but this is not strictly necessary. The soldering iron, however, deserves a word. It should be made for the purpose, neither too light to carry enough heat nor too large to reach into crowded, difficult comers. Sound parts a!re generally somewhat heavier than radio parts; look at the radio man's soldering iron and then order one about one size larger. Indicating Devices Among meters, the volt-ohmmeter is perhaps the most useful, but a trifle expensive, and can be replaced by the volt-ammeter, used in conjunction with a "C" battery. By application of Ohm's Law to the reading of such a meter, resistance can be determined. The volt-ammeter, or the volt-ohmmeter, or almost any meter intended for use with a sound system, should have more than one range of readings. A meter restricted to a single scale will not suffice : a number of such instruments will be needed, and they are more economcially replaced than a single, multi-range device. For the same reason, the volt-ammeter and the volt-ohmmeter are less expensive than separate meters for volts, amperes and ohms. A little pocket device called "Testo-lite" which distinguishes between A.C. and D.C. is useful and very inexpensive, although it does not register potentials much lower than 100 volts. The projection room should have at least one A.C. instrument. The most useful instrument of this type is probably the copper-oxide-rectifier A.C. voltmeter with multi-range scale. Although moderately expensive, this instrument possess the additional advantage of being useful as a volume indicator. It measures volts rather than decibels, but on the other hand it costs only half as much as the more ordinary type of output meter. The out put meter calibrated in decibels, and obtainable for any desired line impedance, is useful chiefly in conjunction with test film or records of suitable frequency. These are not readily obtainable, but film exchanges have been known to sell them. Headphones are very useful for test purposes of all kinds, and for rough tests will substitute satisfactorily for meters. Good quality phones, of about 2,000 ohms impedance, are customarily used. They will not detect a weak photo-cell current, but for testing most portions of the sound system in a quick and moderately accurate way they are invaluable. Test buzzers should not be used in a sound system, as they pass too much current and may have an undesirable magnetic effect upon the cores of some of the transformers or other windings. For the same reasons, test lamps should be used only with extreme caution; it is best to avoid their use altogether if meters or headphones are available. Ohmmeters, and ammeters used as ohmmeters with the help of a dry battery-, should always be set for minimum current> flow. Tube testers are commonly composed of more than one meter. Singlemeter testers commonly read plate current only. Most sound systems, amplifiers or other panels in which tubes are used, are equipped with meters that read plate current, so there is no advantage in buying a tube tester for that purpose. Moreover, plate current alone offers only a very limited indication of the condition of the tube. Testers that give more complete information will commonly be found too costly for a single theatre, although chains use them to considerable advantage. Circuit testers are commonly used by radio repair men in tracing trouble. They have not yet found their way, to any great extent, into theatre work, but in the opinion of this writer, they should. They afford a handy and extremely rapid way of finding trouble, when used by a man who understands the tester and understands the circuits he is testing. To test an amplifier circuit with this device a tube is removed from its socket and plugged into an appropriate socket mounted on the tester. A plug, like a tube base, connected with the tester, is then plugged into the amplifier in place of the tube that was removed. The amplifier then works in the usual way, except that one of its tubes is located in the tester instead of in the usual socket, being connected to the socket by means of a cable and the tester plug. The effect is to put all the meters of the tester into the circuit of that tube. Grid, filament and plate current and voltages can be read, and the approximate nature and location of any trouble is found at once. Some of these circuit testers make excellent tube testers, also. One disadvantage of their use in theatres is that commonly they are not equipped to measure the higher voltages and currents used in some theatre amplifiers. In smaller theatres they will serve almost every test purpose ; in larger houses, however, most such testers will have to be modified for use with the power amplifiers. Permanent Repairs the Goal Repair equipment (and knowledge) are just one more part of the complicated question of permanent repairs. No trouble, as has been said, is permanently fixed until every reasonable effort has been made to prevent its recurrence. But some troubles — quite a few — can never be wholly prevented from coming back. In that case, the repair is not a complete job until every reasonable precaution has been taken to find and fix that trouble, when it does return, with the least possible loss of time. If an hour was needed to find the trouble last time, only ten minutes should be needed next time ; and the repair job is not complete until the knowledge and equipment and spare parts and whatever else may be needed, to keep the time of the next repair down to ten minutes or less, have been made available. It will be seen that the standard for first-class repair work runs rather high. Few if any theatres or projectionists will care to go to the full extent of meeting that standard at every point. But, probably it will be a long time before the industry as a whole tries to do as much. Nevertheless, here is a standard to aim at; one can only try to come as close to it as his circumstances will permit : 1. No repair is complete until every part of the equipment, whether affected by the trouble itself or the work of testing or repair, has been restored to the best possible condition. 2. No repair is complete until every possible precaution against its repetition has been taken: (a) by modifying equipment so as to make future appearance of the same difficulty impossible; (b) by in.stalling an inspection routine that will detect symptoms of trouble, and (c) by providing repair facilities and instituting repair routines. Tools, meters and spare parts that will help to locate and repair the trouble with a minimum of delay should be available.