Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

70 Better Theatres Section April 12, 1930 Lo eating the Causes of Faulty Reproduction STRAY NOISES WHICH HAY BE BLAMED ON VARIATION IN LIGHT INTENSITY In this article, the fourth of a series, discussion of "strays" in sound-on-film attachments is continued By HUGH S. KNOWLES THE fourth principal source of strays in p.e. cell attachments, namely those due to variations in the intensity of the light reaching the p.e. cell are very common and fortunately fairly easy to find. The first subdivision (see last article for classification) of this group is (a) strays due to actual changes in the luminosity of the exciter lamp filament. This trouble is infrequent in a well designed system but should be carefully watched in any system which does not use a storage battery supply. These strays result from sudden changes in current through the exciter lamp filament which changes its temperature. The current change may be due to a faulty connection at the storage battery, at the socket, or in the wiring. It may also be due to current changes at the source where rectifiers and filters are used as battery substitutes. Irregular changes in the intensity of the filament can be detected by watching some surface illuminated by the exciter lamp (not the filament itself without smoked glasses). If these are coincident with a click or stray the whole supply system should be checked. See also paragraph below on socket inspection. A very common stray is a “plunk” which is heard in the horns when the exciter lamp of the second machine is switched on for a change-over. This stray is due to a change in the exciter lamp filament current and also to a field generated by this current change which produces a stray by induction (group 2). The change in voltage across or the current through the filament may be due to either the use of a common pair of wires from the storage battery or to a drop in the voltage of the battery, battery “eliminator,” or generator resulting from the increased load (i.e. caused by the regulation of the supply). In many installations a common pair of wires is run from the battery or the power supply panel to a pull or junction box where two pairs of wires are spliced on; one for either projector. In this case the load of the second lamp which is initially very high increases the voltage drop along the common pair of leads resulting in a sudden dimming of the lamp which is in use. This changes the light reaching the p.e. cell and contributes to the “plunk.” The initial load is very high due to the fact that the resistance of tungsten like most substances increases with temperature. Its cold resistance is therefore very low and it momentarily represents almost a short circuit. There is also a change in the voltage delivered by the storage batery or substitute supply with different loads due to the voltage drop in their “internal resistance.” Normally this may be neglected in a battery but it may be a factor when it is low — one of the many reasons for keeping the batteries well charged and in good condition. T HE trouble from this particular stray may be minimized by running separate leads from the battery to each exciter lamp (or by using heavy wire in the common pair). These should be run in armored cable or otherwise shielded from the rest of the wiring. The use of a rheostat to bring up the filament current gradually will eliminate this source even when a common supply line is used. In most cases the rheostat used to regulate the current in each lamp may be set either in the minimum current or “off” position if there is one and then brought up gradually just before the changeover. If a flickering of the exciter lamp is accompanied by a crackling noise in the horns when the clips on the storage battery are tight and well cleaned, the exciter lamp socket should be carefully inspected. Moulded composition or “mud” bayonet type sockets are frequently used and particularly in poorly ventilated exciter lamp housings the temperature of the socket may exceed the softening temperature of the composition. In other cases, the spring which maintains the pressure on the contact is so designed that it carries the full lamp current and with the high current types used in sound attachments the spring may overheat and lose its temper. If necessary, the socket should be replaced by an all metal type with bakelite insulation in which the positive lead is connected directly to the contact pin. Group (b), or strays resulting from vibration of the exciter lamp filament with respect to the optical systems which produce changes in the light reaching the p.e. cell, are quite common in systems using the 8 volt 4 ampere exciter lamp. This lamp has a long narrow filament the image of which is focused on a slit about one thousandth of an inch wide inside the lens barrel. Assuming the lamp is in focus in the back and forth and front and back directions the only remaining adjustment is in the vertical direction. With the white card used for focusing in position (usually at the p.e. cell housing opening with the aperture shoe assembly removed or in the p.e. cell housing) slowly change the vertical adjustment. A bright position will be found above and below which one edge of the spot will begin to have a blue and yellow fringe (due to chromatic dispersion at the knife edge aperture) . It is evident that if the lamp is left in either end adjustment where the edge of the filament image begins to move out of the aperture in the lens barrel the slightest vibration of the filament will make it alternately come in and go out resulting in a large change in light reaching the p.e. cell and a loud stray. This may sound like a “gear noise” due to the vibration of the filament being in synchronism with the disturbing vibration. This type of trouble can easily be detected by turning the fader to the full on position and operating the machine