International projectionist (Oct 1931-Sept 1933)

8 INTERNATIONAL PROJECTIONIST November 1931 started at the same time, thus checking the volume of sound first on one and then on the other. Using Test Reels It is unnecessary to have two reels of film to accomplish this purpose. All that is necessary is to have two endless loops both with the same recording, thus threaded in each projector so as to give continuous operation. The correct method of making a volume balance test between two sound projectors with film and disc is to have four copies of the same subject; two on film and two on disc. The projectors are then threaded and started simultaneously with both disc reproducers in position; then, whichever way the fader is thrown and whichever way the transfer switch is pushed, the same sound should come from the stage horns. A second projectionist or service engineer usually assists in making this test. This observer usually signals the projectionist in the projection room when to change the fader from one projector to the other. Usually the projectionist removes the observation port glass adjacent to the master fader and inclines his head through this port, listens, and waits for the signal from the observer in the auditorium for switching the fader key or the fader arm from the left to the right projector, or vice versa. A person with an experienced ear can always detect any difference in volume, or in frequency response. It is important that the speed of each sound projector be maintained at 90 revolutions per minute. I have found that a very useful method for ascertaining if both projectors are running the same speed is to place an identical mark on each turntable and, having marks in similar positions, start both projectors simultaneously. If both projectors maintain the same speed, these marks should stay together in R. P. M. If they do not, one will creep ahead of the other, thus indicating that one projector is traveling faster than the other. The degree of similarity required in speed of sound projectors is extraordinarily high. When running two projectors simultaneously in testing for R. P. M., be positively sure that both projectors are running under the same conditions-:— that is, with or without films. I would suggest that this test be made while film is being run through each projector. Crackling Noises Crackling noises may be caused by the following, which are most common with film and disc reproductions: Dirty fader contacts Dirty gain control Dirty potentiometers R. H. McCULLOUGH whose articles will appear exclusively in International PttojEciioNisT Dirty rheostat in filament circuit of speech amplifier Fuse making poor contact Loose connection Dirty switch contacts Dirty variable resistance in horn receiver control cabinet Poor connection at receiver unit Poor connection between vacuum tube prongs and socket contacts Defective fuses Any piece of equipment, such as the fader, the gain control or potentiometer, rheostats or variable resistances and switches, require constant attention. They should be inspected and cleaned at regular intervals. These equipment troubles are inherent in all types of sound reproducing equipments. A dirty fader is the most common cause of all the aforementioned, which cause crackling noises. When fader contacts first become dirty, noise is first perceptible when moving the fader arm from one side of the fader to the other during the reproduction. Dirty fader contacts should be cleaned with tetrachloride: apply a very small amount of vaseline and polish contacts with a good grade of embossed paper. The first symptom of a dirty potentiometer or rheostat is a scratchy sound when the slider is rotated over the resistance wire. A dirty potentiometer or rheostat should be cleaned with tetrachloride and the movable contacts lubricated by rubbing a soft lead pencil over the wire and turning the knob in order to distribute the lead evenly. I recommend this as a lubricant because the lead in an ordinary pencil contains a certain amount of graphite which is a very good lubricant. I have found dirty fuse con tacts, on many occasions, which were responsible for crackling noises. This condition can be remedied by cleaning the fuse clips with fine sandpaper and then resetting fuse clips to give a tighter grip. Dirty switches are another common cause for crackling and popping noises. Switch contacts should be cleaned with tetrachloride and lubricated by applying a little vasaline. If the switch contacts are burned from arcing, it may be necessary to clean the contacts with very fine sandpaper. Small key switches require inspection at regular intervals. The small spring blades sometimes become bent and make a faulty contact. The contact points and blades of key switches should be cleaned with a small burnisher. Vacuum Tube Care When the vacuum tube prongs and the socket contacts have a film of corrosion over them, the grid and plate circuits are very likely to be afiFected, but the filament connections usually do not suffer from slight corrosion at these points as much as the grid and plate circuits. Dirt and corrosion on vacuum tube prongs can easily be cleaned off by means of a small nail file. The socket contacts should be given a similar treatment by holding a strip of very fine emery cloth over the ends of a spudger or any small flat stick, which permits one to clean the socket contacts without any trouble. Vacuum tube prongs and socket contacts should be cleaned frequently, to prevent corrosion. Vacuum tube prongs may be cleaned with a small typist's eraser. This type of an eraser consists of a small revolving piece of rubber erasing material at one end and a small brush at the other end. After cleaning the vacuum tube prongs with this type of an eraser the abrasions may be brushed away, thus assuring a perfectly clean contact. ELEMENTS OF THE EYE There are two parts in the perceptive elements of our eyes that function somewhat differently. The part concerned with direct vision is almost microscopic in size and is made up of minute terminal filaments something like cones. With these we get the details of the objects at which we look. It is this minute area that we use in reading or in getting the clearly defined outline of a distant object. If this is destroyed all accuracy of sight is lost. It does not function continuously. It perceives quickly but the impression fades with equal rapidity so that we actually see in a series of rapid flashes with intervals of about one-tenth of a second. The surrounding field of vision is that which perceives objects at which we are not directly looking, but which are at one or the other side of us.