Journal of the Society of Motion Picture Engineers (1930-1949)

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12 S. L. CHERTOK Vol 43, No. l Again, in transport by Army truck or other means, equipment is not handled too gently. Consequently, amplifiers may be packed in their carrying cases and may be required to withstand drops from as high as 18 in. on a concrete floor, such as may occur when a man stumbles. A dozen or so drops is plenty tough on an amplifier, or anything else for that matter. There are also gun-shock considerations to be taken into account, such as may be experienced by equipment carried aboard ship. Permanently installed equipment is mounted on shock mounts which help reduce the intensity shock. Portable equipment lying on the floor may be put out of operation unless it is properly designed. The finishes and materials used in military amplifiers must be capable of withstanding exposure to salt-laden atmospheres, and liberal use is customarily made of stainless steel, plated brass, and other corrosion-resistant materials. Equipment specifications may call for metallic materials to withstand severe salt-spray tests under a 20 per cent salt-spray solution at 130 F for periods ranging from 50 to 200 hr. Quite often amplifier cases must be drip or splash proof. Where equipment must be capable of operating under these conditions, it means that parts such as volume controls, which protrude through the case, must have special gaskets to keep moisture from entering through the tiny clearance space between the shaft and the bearing. When splash-proof designs are called for, equipments must not leak water when exposed to a 1-in. stream of water under a head of 35 ft played directly on the equipment from a distance of 5 ft for a period of 5 min. Materials which are susceptible to attack by fungi, or are delectable morsels for tropical beetles, must be avoided. It is customary to spray completely assembled equipments with various fungicidal and insect-proof agents. The eating away of cotton wire insulation by tropical insects can be avoided by replacing with wire that has glass insulation. Fungi will also attack the paper or textile base cones of loud-speakers, and proper impregnants must be used for the speakers if they are to last a reasonable length of time. Raw edges of all laminated punchings must be sealed. Otherwise, the layers will soon separate under tropical conditions. So much for the general idea of what faces a military amplifier designer in his general choice of materials. As far as the design layout of parts is concerned, the layer-built,