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MAINTENANCE OF SOUND EQUIPMENT 661
former may be regarded as an instrument made up of two or more choke coils, each of which is tested separately. When there are many terminals the transformer casing, or the wiring diagram of the apparatus in which it was used, will show the internal connections.
Warning: Never test a speech transformer with more current than is necessary to operate a high-resistance voltmeter.
Testing Condensers
(33) Some combination circuit-testers-tube-testers contain provisions for measuring the capacitance of condensers, but this is not usually a matter for projection room test. Condensers are tested for internal short circuit with d. c. and d. c. ammeter or milliammeter, or a trouble lamp. Only short circuit of the plates will permit d. c. to pass through a condenser (except for a momentary charging current). Internal open-circuit is tested with a. c, which will not flow through the condenser if an open exists.
In testing electrolytic condensers with d. c. be careful not to reverse their polarity. In testing electrolytic condensers with a. c. be careful to apply the current for only a second or less.
Testing Tubes
(34) Many tube testers check amplification factor and mutual conductance (these are units in which the quality of tubes is measured). Normally, however, projection room tests do not go beyond reading plate current, which procedure is explained in our discussion of amplifier circuits.
Purchasing
The projectionist sometimes does his own buying of parts and spares, but in general practice this is attended to by the manager. The projectionist, in turn, tells the manager what he needs, and often advises on matters of quality and brand. Advising the manager intelligently in such matters is a proper and very important part of the maintenance duties of the projectionist.