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

April 12, 1930 Exhibitors Herald-World 71 with everything on but without film. If pushing in a slip of paper which stops the light beam stops the noise, this factor is contributing. In severe cases the exciter lamp mount is usually at fault. With the 32 watt exciter lamp a three dimensional adjustment is necessary and this increases the possibility of play in any direction. If the mount has any play it should be doctored. If it has no appreciable play careful focusing of the exciter lamp so it is half way between the two cut off or blue fringe positions in the vertical direction will minimize it. Where new equipment is to be designed, either the 50 or 75 watt 10 volt exciter lamps are recommended. They have heavy, short, rigid filaments which both prevent vibration and permit a prefocused socket without all the elaborate three adjustment features to be used; and have the further advantage of operating at about 200 and 400 degrees, respectively, above the 32 watt type. This increased light with a suitable optical system makes possible a much greater p.e. cell output which permits a reduction in the overall amplification and in the stray or “crap” to sound level. Little can be said of group (c) or strays due to obstructions in the aperture except that many “in distress” calls for the service man have been due to it. Cases have come to my attention in which no sound came through at all due to the accumulation of grease in the aperture. If there are abrupt changes in the sound level accompanied by “plunks,” the aperture may be nearly full. Sharper clicks may be due to foreign material which is rapidly deposited and then carried off or more likely to group (d). The strays in group (d) include those due to the film itself and one type of trouble due to the machine itself. They have been classified in one group however because they are strays which occur only when a film is running through the machine. All strays in this group will not be heard when the machine is running and everything is turned on but will be heard when a film is threaded up and the machine is run. We will first consider those due to the machine. The first group are due to film weave and produce what is analogous to a waver or tremolo in the disc attachment. It may be due to a number of causes but should be remedied by the manufacturer of the machine. Where a service man is not available or for the benefit of unexperienced service men, the following principal causes will be listed. Everything which will cause a regular or periodic change in the film speed past the aperture will cause a “wow” or waver. Hold a pencil rigidly against the side of the sound head frame and the point within a thousandth of an inch, or less, of the outside of the sprocket “blank” near the teeth, that is on the part of the sprocket on which the film rides. If the sprocket strikes once each revolution it may be off center (due possibly to a faulty fixture used in holding it on the milling machine or to its being reamed too large) or the shaft may be sprung. The latter can be detected by holding the pencil point near the center of the shaft which can usually be found by looking for the marks of the cut off tool. The service man should carry a spare sound sprocket that he knows to have carefully cut teeth and one that is as nearly perfect as posible for test work. Wows may be also be due to one or more of the teeth being improperly cut (when for example the dividing head is set improperly or there is a slight amount of play in the fixture) but this can only be definitely demonstrated by having a good sprocket to substitute. Probably the most common source of waver is due to changes in the speed (angular velocity) of the sound sprocket. If there is no mechanical filter on the sound sprocket shaft, it may be due to insufficient clearance between the gear on this shaft and the driving gear; or to some irregularity farther up the gear train being transmitted to it. Where the lower take up is driven from the sound sprocket shaft (a practice that should be discouraged) it may oe due to the take up clutch being rough. A rough take up will also jerk the film at the aperture unless a hold back sprocket is used. Where a mechanical filter is used, particularly when the ability of the designer is questionable, it should come in for its full share of attention. All filters have one or more frequencies at which they tend, at least, to be resonant or oscillate. A rough qualitative check on this may be made where a filter is used by displacing the flywheel on the sound sprocket as far as it will go and then releasing it. If it oscillates two or three times one can be reasonably certain that it requires some dampening or friction in the system. In the spring flywheel type this is usually introduced by wrapping the springs lightly with felt and/or stuffing them with cotton, and by placing a little tension on the outside of the felt. Troubles of this type should be referred to the designer of the equipment where possible. In no event let a glib salesman persuade you that the waver is “in the print.” If possible, to settle the argument, and to check the system have the service man bring or borrow a constant frequency film, either one with the whole musical scale on it to check the audio response of the system as well or one having a single frequency (usually about 1000 cycles where the ear is most sensitive) for a “wow” test. The note should sound very pure or mellow like a flute note and have little or no suggestion or harshness and no waver. 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