The motion picture projectionist (Nov 1929-Oct 1930)

October, 1930 Motion Picture Projectionist Efficient Sound Reproduction By R. H. McCULLOUGH Supervisor of Projection, Fox West Coast Theatres COMPLAINTS and interruptions are very detrimental to any theatre. There is a reason for every interruption. There are many outstanding questions yet to be answered, as to the perfection of sound reproducing equipment. However, the secret of good reproduction with the present equipment is due to proper maintenance and careful operation. Inspect, examine, and test sound reproducing equipment before each operation. Make these inspections severe and be positively sure that everything is all right. Regardless of the size, every theatre requires good projection and good sound reproduction. There are many methods of correcting troubles, thus eliminating interruptions. The more every projectionist knows the easier it will be to locate trouble when it arises. There are many projectionists who are persistently studying and striving for perfection of operation of projection and sound reproducing equipment, which is steadfast allegiance to the exhibitor and the motion picture industry. 205-D Tubes A request has been received to explain the function of the four 205-D vacuum tubes in the W. E. A. C. motor control cabinet. Of these, one is used to supply rectified current for the field of the pilot alternator. Two of these tubes act as rectifiers supplying current to one winding of a special choke coil. This coil has a second winding placed in the circuit, which controls the motor brushes. When the current through the first winding is large, the choking action of this coil is not very pronounced, therefore, a relatively large rotor current can pass and the motor can speed up. As the current through the first winding is decreased, the choking action of the coil is increased and hence the motor speed begins to be limited. Therefore the motor speed can be controlled by regulating the output of the rectifier tubes. Their output depends on the bias supplied to the grids, and this, in turn, is controlled by the fourth vacuum tube which is operated as an amplifier by the tuned circuit. At speeds below 1,200 r.p.m. the grids -of the two rectifier tubes have very little negative bias and these tubes, therefore, pass a relatively large current through the first winding of the choke coil. Therefore, a large rotor current circulates, allowing the motor to speed up. At 1,200 r.p.m. the tuned circuit functions, causing the negative bias of the rectifier tubes to be increased. This decreases their output and causes the choke coil to cut down the rotor current so that no further speed increase can take place. If the speed tends to go above 1,200 r.p.m. this effect becomes still more pronounced. The A. C. motor control box functions in this manner when the regulating switch is in the "Reg." position. When it is in the "Var." position the tuned circuit is opened and does not function; the vacuum tubes continue in action, however, and the fourth, or amplifier tube, instead of being operated from the tuned circuit, is controlled by means of a variable resistance connected to the control knob, which therefore regulates the speed as desired when it is turned. Speed Control Motor The motor used with the A. C. control box is the repulsion type. Such motors have two windings : the stator winding, which is fixed and receives power from the main A. C. supply and the rotor winding which is on the revolving part of the motor and is not connected with the power supply. This winding is connected to a commutator which has two brushes. If these brushes be connected to a circuit so that current can flow from one to the other, then the speed of the motor will depend on the amount of this current; and by so regulating the latter one can regulate the speed of the motor. Under certain conditions the com S.M.P.E. on Wide Film A published report stated that the S.M.P.E. committee set up some months ago to inquire into standardization of wide film would do nothing further, and that the Society would leave this problem to be settled among the producers themselves. Many inquiries were received as the result of this published statement, and an appeal was made to President J. I. Crabtree for a statement as to the official stand of the Society on this matter. President Crabtree's answer follows: "In reply to your letter of recent date, I don't know where got its information which is very much in error. Our Society is very much concerned with the matter of wide film standards and the subject will receive its fair share of discussion at the Fall Meeting to be held at the Pennsylvania Hotel in New York, October 20-23 inclusive." mutator of a repulsion type motor will become shorted, which will cause the motor to operate at full speed. The commutator must always be kept free from dirt. Never allow the commutator to become rough. ■ Commutator Cleaner During my visit to one theatre, I was advised that one projector motor would intermittently speed-up and then slow down to normal. Upon investigation, I found that the projectionist had used emery cloth to clean the commutator, which shorted the commutator bars, thus causing the motor to speed-up beyond control. Never use emery cloth to clean a commutator. If the commutator requires polishing, use No. 00 sandpaper. However, in many cases, all the commutator needs is a good cleaning. It is advisable to secure a piece of canvas and dampen with carboni, which can be used to clean the commutator, when necessary. It is imperative to inspect the brushes of each motor weekly. See that they are cleaned so as to allow free action in the holders. Renew weak brush tension springs. Many troubles which are charged to brushes of motors and generators can be traced to the improper application and adjustment that show up in sparking at the commutator. The projectionist should therefore give due consideration to brush adjustment. Volume and Speed Control I have found many projectionists who raise the volume to the normal operating point, before the sound projector is up to a speed of 90 feet per minute, with the result that the quality of reproduction is spoiled. It is very important to see that the sound projector is up to the regular operating speed before raising the volume to the normal auditorium level. Under no circumstances should motion pictures with synchronized voice or music be run at a speed less or greater than 90 feet per minute. The recording speed for 35 mm. film is still 90 feet per minute, and therefore the projection speed should be maintained at this speed at all times. If the projection speed is varied, the quality of reproduction will be spoiled. General Notes The wide picture has not gone ahead as rapidly as predicted. No improvements have been noted in stereoscopic cinephotography, but it seems as though television continues to develop. However, exhibitions prove that it is still an experiment