International projectionist (Oct 1931-Sept 1933)

10 INTERNATIONAL PROJECTIONIST September 1933 unsteady voltage in direct current lines, and cqrrected by wiring condensers in parallel with such lines. Sound System Applications Condensers are used in sound systems wherever the unsteady direct voltage requires "filtering." This is the case with direct voltage drawn from a rectifier of any kind. It is very often the case with direct voltage drawn from a generator, because of the "ripple" caused by the action of the commutator. Battery D.C. is "pure" and needs no filtering (under normal circumstances). Sometimes, however, storage batteries are charged by means of a rectifier or motor generator which is used during show time. Often, in such cases, the charging D.C. will create hum in the sound that can be corrected by wiring condensers across the charging line. Condensers are used in amplifiers to "by-pass" alternating current. Often an amplifier will contain several stages of amplification. The tubes of all stages may be supplied with the power they need to do their work by means of a single source of A current and a single source of B current. But when this is done all stages are wired together at the sources of current supply, with consequent possibility of "feedback." Speech current of the last stage of amplification may be returned through the common source of power supply to an earlier stage. Feed-back of this kind will cause an amplifier to distort — it may distort so badly that it hums, howls or "motorboats". Condensers prevent this. The speech current of each stage (being A.C.) is short-circuited around the sources of power supply by means of condensers. The power supplies themselves (being D.C), are not influenced by the condensers, except in so far as the latter serve as filters to smooth out the supply current. Condensers are also used to influence the phase of an alternating circuit. In the condenser and in the wires connecting to it, current "leads" the voltage. A charge (voltage"), cannot be built up across a condenser until current has flowed into it; the current must lead — the voltage or potential difterence follows. Condenser Troubles The commonest trouble experienced with condensers is short-circuit of the insulating layer separating the plates. If the condenser is connected across a D.C. line to act as a filter, a "short" of this nature will also "short" the line, possibly damaging other apparatus. It is therefore unsafe practice to use filter condensers of doubtful quality, or to use any condenser of any quality without careful consideration of the voltage it will have to withstand and ample allowance for any possible rise in voltage. It is also necessary to remember that in an A.C. circuit voltage changes occur every moment, and the rated, or meter, voltage is not the maximum but only the average [more accurately, the root-mean-square] voltage. Thus a condenser wired across a 110-volt A.C. line must withstand a momentary peak voltage of about 155 volts during each alternation, — more than that, if the meter voltage fluctuates above 110, as often happens. If the manufacturer rates his condenser for 110 volts A.C, it may be used with safety — unless the line fluctuation is severe; but if only a D.C. rating is given, it is necessary to remember that the peak voltage of an alternating line in ordinary power service is nearly half again as high as the meter voltage. Short-circuit in a condenser is commonly tested with D.C, since some A.C. will "flow through" the condenser even if it is in perfect condition. A.C. tests, however, are often useful if conducted with the help of an ammeter. Short-circuit in a condenser is not always easy to find. Because of the nature of the insulation in common use (tar or wax), the condenser may be "self-healing" — that is, the tar or other insulating compound melts with the heat of the short-circuit, and when the line is opened, seeps about the point of rupture and hardens again. If the condenser is then tested with low voltage, the short-circuit may not be revealed ; in fact, the condenser may show no fault even at full operating potential. If the tar or wax is the only insulation, the condenser is literally self-healing, and may be kept in use after it has broken down and cured itself. But in most cases the soft British Initiative Reporting for the year 1932-33, the Guild of British Kinema Projectionists and Technicians lists the following lectures delivered to its members by qualified technicians: Projector Design, Carbons: Their Manufacture and Use; Television, Mercury Arc and Metal Rectifiers; Sound Systems; Lead and Alkaline Batteries, Optics (3 sessions), and other topics. American projectionists naturally do not need to study such subjects, knowing all there is to know about projection. Technical sessions for American projectionists are practically unknown. material only reinforces the paper, linen or mica used between the plates, and such a condenser cannot safely be used after it has once given way. Genuinely self-healing condensers are not common in sound equipment; in general no sound condenser that has short-circuited should be trusted. Suspected condensers, moreover, should always be tested at full operating voltage; other tests are valueless. Electrolytic condensers are self-healing, provided the internal short-circuit is not severe or prolonged. The electroplating action which takes place when the condenser is properly connected is capable of mending slight flaws in the insulating layer. But if this layer has been severely damaged,, it can only be repaired by a lengthy process, starting with a very low voltage and gradually increasing it over a period of days. External short-circuiting does not hurt a condenser: the device merely discharges. The heavy sparking sometimes observed when a condenser's terminals are short-circuited is not harmful. Warning of internal short-circuit is sometimes given in advance when the insulation consists of or is reinforced by tar or wax. This type of insulation may melt in the course of use and leak or force its way out of the condenser case. When this happens the effective insulation between the plates is reduced, with correspondingly increased danger of internal breakdown. A condenser showing this symptom should be replaced before it stops a show or, perhaps, burns out an amplifier. Open circuit within a condenser cannot be tested with ordinary D.C. methods, since the condenser will not conduct direct current in any case. A.C. may be used for this purpose ; or D.C. of suitable voltage may be used to attempt to charge the condenser and then draw a spark from it on discharge. Large banks of condensers, or electrolytic condensers such as are often used in sound systems, are dangerous. They hold their charge after the line has been disconnected. They remain alive after the power switch has been opened. It is unsafe to touch them or any circuit wired to them until after they have been discharged by shortcircuiting their terminals. A complete press book of advertising and publicity material on its High Fidelity sound equipment has been issued by the Photophone Division of RCA Victor Co. for distribution to all exhibitors who have contracted for new installations. This selling aid is a pip and should be used to the limit.