International projectionist (Oct 1931-Sept 1933)

June 1933 INTERNATIONAL PROJECTIONIST 21 proximating the conditions of a neat and trouble-proof repair. Another restriction is also important: the emergency repair should be one that requires the least disturbance in the position of wires and of other parts. In some cases, this restriction is vital : in dealing with filters, for example, displacement or lengthening of wires will in many cases result in the introduction of serious hum into the sound. Again, it is always necessary to maintain at least approximate impedance match in speech circuits, for which reason it may occa.sionally be necessary, when emergency substitutions are made, to introduce added resistance in some form across a line that has been unbalanced by the attempted repair. (A headset furnishes a handy source of additional impedance. One receiver alone commonly has an impedance of about 1,000 ohms, giving 500 ohms for the two in parallel and 2,000 ohms for the two in series). The circuit drawing of the apparatus that has been disturbed by the attempted repair will give the best indication of the manner in which the impedance balance may have been upset, and the amount and circuit of the impedance needed to restore proper operating conditions. Justifying Emergency Repairs Last, but most important of all, any repair "that reduces the resistance of a circuit may result in excessive current flow that will damage or destroy some other part of the apparatus. Any emergency step that involves possible reduction in the resistance of any circuit is dangerous unless adecjuate steps have been taken to insure that the attempted repair will not result in the creation of additional damage. In this connection it is important to remember that any inductive winding offers greater resistance to the passage of alternating current than to direct current and requires especial precaution against excessive flow of A. C. whenever any such winding is changed. Because of these restrictions upon the freedom with which emergency methods can be applied such methods should, as far as possible, be worked out in advance, with the advice and help, where obtainable, of the manufacturer of the equipment. But before entering into that angle, it may be more advisable to inquire, at this point, when and under what circumstances emergency methods are to be used at all. There are two sets of circumstances that justify application of well-advi.sed but unusual emergency practices: 1. Whenever a serious trouble has been traced down as far as some one amplifier, rectifier, filter circuit or other limited portion of the equipment. Under such circumstances the show can be restored in two ways : by tracing further until the exact nature of the trouble within that amplifier, filter circuit or whatever it is has been run down and repaired in the customary manner. On the other hand, time and refunds may be saved if some rapid substitution can be made that will eliminate the guilty portion altogether, leaving more detailed investigation and more thorough repairs to wait until the end of the show. Consider, for example, the case of that type of fader commonly used. Suppose the trouble has been traced to that fader. It may be opened up and the circuits traced until the exact trouble is found. On the other hand a double-pole, double-throw switch can be substituted for it, a pair of headphones connected across the switch blades to preserve a rough impedance match, and the show will go on, volume being controlled at the amplifier or by means of the exciting lamp rheostats. 2. Emergency repair methods are justified when a serious trouble has been definitely traced to some part (such as the plate power transformer mentioned above), for which no replacement can be secured within reasonable time. It is apparent, therefore, that the question of emergency repairs ties in very closely with the matter of maintaining on hand .i suitable supply of spare parts, and chat advance planning of emergency methods is useful not only to assure the success of such methods, v\hen they are necessary, but has an additional value in governing the intelligent preparation of a list of those spares which must be kept available at all times. When repairs are carefully planned in advance of an emergency, the theatre's investment in spare parts can be economically limited to such equipment for which, as leisurely consideration shows, no practical or safe Film PROX * 1 emergency substitute is easily available. Advance planning for emergency procedure naturally involves two sets of plans, based upon the two classifications just given. In the first set will be contained a list of methods available for eliminating, temporarily, any portion of the equipment to which, but not withm which, trouble may have been traced. Such plans are best drawn up with the help of a "block schematic" — the simple drawing, shown in Figure 1, described in an earlier article of this series. Consulting his schematic, the projectionist asks himself: "Now, what could be done, if this block went wrong?" The commonly used type of fader, mentioned above, would constitute one block of such a drawing. On the question of procedure in emergencies he may consult the manufacturer of his equipment or any source of advice available to him, and if he is not entirely satisfied with the steps he has planned for himself he may submit them for criticism to the manufacturer or to any other advisor. Or, if he is at least certain that they cannot do any serious harm — thoroughly certain — he may try them out for himself, after show time. Even those plans which have secured approval from competent advisors should be tried out, as a matter of rehearsal, to make sure they can be applied quickly and effectively amid the confusion of an emergency. Replacement Requisites In the second list of advance plans will be included methods of replacing any single part — power transformer, condenser, choke coil, resistor or what not — that is at all likely to cause trouble and for which no suitable substitute can be obtained immediately. This second list will be longer than the first, but when it is complete it can advantageously be compared with the list of spare parts kept on hand at all times, and the spare parts in Monrtor Loud -Speaker Disc PROJ. * 2 Disc