International projectionist (Jan 1943-Dec 1944)

undue violence. If the correct toggle switch replacement cannot be obtained, a knife switch or some other clumsy and undesirable, contrivance may have to be substituted. Plans dispensing with all such switches and controlling the circuit from some other point should be • made in advance; or if a given switch is indispensable, plans for replacing it with a knife switch or other device, in case of necessity, should be made in advance. Key switches of the telephone type present the most serious problem. These often are multi-blade, multi-contact affairs not always easy to obtain in a hurry even when supplies are plentiful; they go wrong in several ways and they can be damaged by well-intentioned acts of maintenance. Moreover, they generally carry weak sound voltages and currents, so that a very slight misfunctioning can cause serious trouble. They must be kept clean, because very little dirt or corrosion is needed to cause noisy sound. Sometimes their contact points must be filed or sandpapered to remove slight traces of corrosion. There is, however, a decided risk of creating greater trouble in the attempt to cure a lesser one, for careless working at the contacts may weaken or change the prong tension. The clearance in a switch of this type is a matter of a very small fraction of an inch. A slight change in prong tension, or any bending of the prongs, may prevent the switch from closing or prevent it from opening. Never try to straighten a bent prong or to improve prong tension. That can be done with a special tool made for the purpose, but in the absence of a suitable tool and experience in using it, any attempt to improve the action of these switches is apt to do more harm than good. In cleaning these switches, use cloth moistened with carbon tetrachloride and exert no pressure on the prongs in doing so. If it proves necessary to use fine sandpaper or a fine nail file on the contacts, exert the nearest thing to zero pressure on the prongs when this is done. And use neither file nor sandpaper until all efforts at simple cleaning have proved ineffective. Some of these key switches have more prongs and contacts than are actually used. That is to say, the manufacturer of the equipment found it less expensive to take a standard switch from stock than to build one specially for the particular switching job to be done. If one of these switches becomes faulty it is sometimes possible, after making a careful study of the circuits involved, to transfer some wires to unused prongs that are in good working order. Plans for doing this should be studied in ad vance so that trouble with critically located key switches during a show could be averted. Plans for replacing such switches, wherever possible, with simpler toggle or knife switches, should also be studied in advance. Jacks built to receive headphone plugs, or meter or other plugs, are often identical in construction to key switches, and everything said above in connection with key switches applies to such jacks also. Socket Contacts There are two general types of socket contacts — those that grip the side of the tube prong and those that press against the bottom of the prong. The latter are more likely to cause trouble. Both types need ordinary cleanliness. One way to clean the side-grip type is to insert and remove the tube several times (with current turned off, of course, to avoid arcing and possible damage). The tube prong may be moistened with carbon tetrachloride before it is inserted. A cloth moistened in carbon tetrachloride may be inserted with the help of a match or a toothpick; but this should be avoided unless it becomes essential, because the contact sometimes consists of a spiral spring into which the prong of the tube fits, and a match or toothpick used carelessly may damage or distort small springs of this kind. Some procedures for testing tubes and amplifiers involve removing and replacing the tube while current is turned on. Use these tests as sparingly as possible. Never use them unless they are essential ; they are rough on sockets. The type of socket contact that presses against the bottom of the tube prong is subject to loss of tension. If this happens, it no longer presses firmly against the bottom of the prong, and arcing-over may follow, particularly in the case of the filament or heater contacts of large tubes. Often the tension can be improved by removing and bending these contact blades; sometimes by removing and reversing them. Even in cases where they cannot be removed from the socket their tension can be bettered by careful and judicious application of a screwdriver or long-nosed pliers. Cleanliness is of course a routine essential. The area of electrical contact with the tube prong is fairly small, and very little dirt may cause arcing. Sometimes the tube prong is at fault in the sense that arcing which has occurred in the past has slightly melted and distorted the solder which forms the bottom of the prong. Be careful about trying to improve this condition by filing the solder smooth — that may make the prong in question so much shorter than the others that proper contact will become difficult or impossible. Also be careful about touching up the condition with a soldering iron. The solder at the bottom of tube prongs forms the connection with the wires that run up internally to the tube elements. In an emergency a fine file, fine sandpaper, or soldering iron may have to be used — with care. It is obviously better to inspect and clean the socket contacts often enough to prevent arcing and thus prevent distortion of the solder tip of the tube prong. Where arcing has occurred the metal of the socket contact may be burnt, and the coating of oxide thus formed may be a poor conductor, leading to more arcing. Any sign of burning at the point of contact should be removed. The eraser at the end of a pencil will usually do the job satisfactorily, although if the burn is deep it may be necessary to remove the contact from the socket in order to clean it thoroughly. Rheostats, Etc. Small rheostats are often used to control exciter lamp current. Small potentiometers are common as volume controls. Faults in these parts can produce a loud noise in the sound, or can stop the sound entirely. These devices are of two general types. In one the resistance element is a thin plate of composition resistance material, over which a sliding contact can be moved. The other type is the more familiar coil of resistance wire, shaped toa semi-circle, and also contacted by a sliding metal finger. The composition resistance plates have some tendency to crack. Moving the pointer past a crack produces a loud noise in the speakers. As cracks become more numerous the range of noiseless operation becomes smaller; and ultimately the cracks, responding to any vibration that may be present, produce noisy sound even when the contact is at rest. There is no remedy except replacement. When a rheostat or potentiometer of composition instruction is installed order a replacement at the first signs of noisy operation. There may be a long delay in obtaining it, but these devices hardly ever break down suddenly. If the replacement is ordered at the first symptoms of deterioration, it should be delivered before the condition becomes too serious. Composition types should not be opened for adjustment or cleaning. The resistance plate can't stand much handling. The wire-wound types can be adjusted with reference to contact tension. The {Continued on page 19) a INTERNATIONAL PROJECTIONIST