International projectionist (Jan 1941-Dec 1942)

ous matter. The ionized molecules of gas will bombard the filament or cathode, destroying it or reducing its emissivity. Meanwhile, gas will impair sound quality. The warning symptom of gas is a bluish glow inside the tube. Where the lube contains gas it acts in a; way like a fluorescent lamp giving off a faint bluish light. This light flickers with changes in sound volume, that isl with changes in the current passing through the tube. Some tubes, however, are made with glass that gives off this bluish glow normally. It is important to observe the tube carefully to determine whether the glow is inside or only around the inner surface of the glass. The latter phenomenon is harmless. Lastly, a tube that seems gassy may really be exhibiting a different type of trouble. No vacuum is perfect: all tubes contain some residual gas. The larger types may become so hot that the gas is driven out of the crevices in which it had previously been hiding by adsorption. In such cases symptoms of gassiness do not necessarily indicate a faulty tube but one which, through its own or some other defect, is overheating. Tubes become noisy through imperfect mounting of their internal parts, a condition which may be induced by rough handling. Tapping the tube, and noting whether the noise produced in the speakers is abnormally loud or prolonged, will indicate such a defect. If so, the tube must be replaced, but not necessarily discarded. It may prove useful in some other socket where it is subject to less vibration or where its noise output is subject to less amplification. Checking on Resistors Resistors are of two general types, composition and wire-wound. This distinction applies also to variable resistors, such as rheostats, potentiometers and other controls. The composition-type is likely to contain conducting particles, usually carbon, embedded in a cementing material. The cementing materials used are partial conductors and their composition is varied according to the resistance desired; also, the proportion of particles to cementing material may be varied, or the particles omitted entirely. In this way any desired resistance can be obtained irrespective of the physical dimensions of the unit. Sometimes, especially when exposed to heat, the composition changes somewhat by drying. This may increase the resistance, causing the unit to work hot, which in turn produces further drying, and so on. The resistor is not as likely to burn out as to become noisy. There is no remedy except replacement. An ohmeter reading of a suspected resistor should show no departure from rating value of more than 10 per cent. The variable-type composition resistors commonly consist of a plate of resistance material over which the contact point slides. By drying out, by friction, or by rough handling, the thin plate of conducting resistance material, often a graphite composition . or pure graphite, may develop cracks.'' These will make the unit noisy when adjusted. They will ., also grow larger, making the unit still more noisy, and. finally unusable. • A unit of this type which is. noisy to adjust should be inspected internally. If the trouble is merely bad contact by the slider, or bad contact to the slider shaft, it can be adjusted. If the trouble is in the resistance element, the unit should be replaced without delay. Some of these units are so made they can't be opened easily for inspection and adjustment. In that case, discard the unit when it becomes noisy. They are not expensive. Wire-wound resistors are themselves of two types — bare wire and wire embedded in porcelain. The purpose of the porcelain is to carry away and radiate the heat produced by operation, thus making possible use of a smaller resistance unit. In either type prolonged overheating, or a temporary surge of high current, may burn out the resistor wire. Sometimes the gap caused by the burn-out can be bridged by a jumper, and the unit continued in use. Generally, it is best to replace it. In the bare-wire type the resistance wire may be wound on a composition core, and the core may give way through prolonged exposure to high temperature, cracking or crumbling, thus ceasing to offer a firm physical support to the wire coil. Where this is seen, even in small degree, the unit should be replaced at once. Otherwise continuation of the process will allow two adjacent turns of wire to touch, short-circuiting their resistance. An excessive current will then flow, with danger both to the resistance wire and to other parts of the same circuit. If, nevertheless, the unit is kept in use under the same conditions of high temperature, other portions of the composition core will crumble and many turns of wire will be touching their neighbors; or the whole coil may collapse, with disastrous consequences. Replace the unit at the first sign of this trouble. In wire-wound composition units the first sign of trouble may be roughness or noisiness in making adjustments. This happens when the core on which the wire is wound so shrinks or distorts, because of heat, that the coil no longer presents an even surface to the contactor. Although a wire-wound variable control which presents this condition on inspection can sometimes be kept in use for months longer, it is dangerous to do so; it should be discarded at once. Condensers are of two types, electrolytic, and paper or mica. The electrolytic type are subject to some degree of drying out [less now than in older models]. Where an electrolytic condenser is used in a filter circuit, drying out is signalized by a gradual increase in the background hum of the sound system. There is no cure except installation of new condensers. The trouble is progressive, and the identifying hum will slowly become more pronounced. The mica type of condenser, in which the insulation consists of a tarry substance, may show the effect of prolonged r— OUTPUT fif2 PMTE fe5V) +PE.CQ(85 V.) 6UD.O FIGURE Overheating of parts is a deceptive indication of trouble that can never be taken at face value. It .must be interpreted. In this diagram, short-circuit in C-7 would probably produce overheating in R-12; short-circuit in C-6 would likely overheat R-12 and R-9, etc. OGW. OA CHM6EOVEe PL. \FIL. 8 INTERNATIONAL PROJECTIONIST