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THEORY AND WIRING OF AMPLIFIERS 527
the schematic by tracing the wiring. Since this takes a great deal of time, it should of course be' done in advance. In other words, if the projectionist is responsible for sound results, and if he and his manager cannot obtain schematic drawings of the system installed, one of the first things to do is to make those drawings. Until they are at hand, shooting trouble may prove mostly a blindfold game of stab and miss, with no great hope of ever getting anywhere in particular.
However, if the reader should be caught with trouble and no schematic, he should note that a knowledge of, or schematic of, the entire amplifier may not be necessary. The trouble symptoms may or may not be such as to localize the fault to one part of the circuit, in which case that part only needs attention. For example, in Fig. 201 meter tests might show low or zero plate voltage at VT-1 only. Obviously the thing to do is to check the plate lead to that tube, for which purpose a schematic is not needed, or at most only a very abbreviated schematic, dealing with a few connections only. On the other hand, suppose Fig. 201 suddenly delivers distorted sound. With no knowledge of the circuit and no diagram of it, any man may find himself very nearly helpless. In those circumstances, the best trouble-shooter can do just two things. He can stumble through the wiring more or less blindly until a fragmentary, imperfect picture of Fig. 201 slowly builds itself in his mind, hoping for luck meanwhile; or he can proceed to draw Fig. 201 or such portions of it as seem necessary. The second method usually proves far the faster.
A further point in connection with schematic drawings obtained from the manufacturer is the need for checking them with the actual apparatus wherever there is any room for doubt as to the two corresponding. The apparatus may have been modified in some way since it was installed — a very common occurrence. Or the drawing, if obtained after the original installation, may incorporate improvements not actually present in the projection room. The manufacturer may be queried on this last point when the drawings are requested, but in every