Projection engineering (Jan 1932-Mar 1933)

JANUARY, 1932 Page 15 similar to that of the pilot switch on the standard switchboard. That is, one throw of the pilot switch connects the circuit directly to a hot-bus connection so that the circuit may be controlled independent of any of the master setups. A second position of this switch connects the circuit through a color master control so that it is possible to control an entire color by means of a single color master. The middle position of the switch is the "off" position. The pilot lamp is wired in connection with the pilot switch so as to indicate when the circuit is hot, that is, when the pilot switch is thrown directly to the hot-bus position or when the pilot switch is thrown to the color master position and the color master is energized. The selector switch is used for transferring the control from the individual potentiometer to the preset potentiometer. In one position of this switch the grid lead from the tube circuit is connected to the moving arm of the individual potentiometer. In this position, the output of the tube circuit may be controlled by manipulating the individual potentiometer and it is not affected by any changes made in the preset potentiometer. In the second position of the selector switch the grid lead is connected through the fader to the preset potentiometer. In this position the circuit, is controlled through the preset control by means of which the intensities of the circuit may be set up for five scenes in advance. Preset Regarding the operation of the preset potentiometer, let us first go back to the operation of the tube unit. As has been previously stated, the output of the tube unit and consequently the intensity of the lighting circuit is changed by means of changing the grid potential on the control tube. This grid potential is obtained through a system of potentiometers from the direct-current control source as indicated in Fig. 1. Thus to preset the intensities of the lighting circuit for a number of scenes in advance, it is necessary to be able to preset the potential applied to the grid circuit of the tube unit. This is accomplished as follows : for the Los Angeles job, the five-scene preset type of switchboard was supplied. In order to obtain these five presets, five small slider type of potentiometers were supplied, one for each preset. In addition to this a flashing arrangement was supplied whereby it was possible for the operator to immediately flash from one scene to another by simply operating a push-button, which operates relays 1, 2, 3, 4 or 5 as indicated. Relay (1) has its contact closed whenever all of the other five relays are in the open posi tion. The normal position of the relays for operating the scene master fader is to have relays 1 to 5 open and relay (1) closed. Now to set up circuit No. 1 for full intensity the slider of preset potentiometer No. 1 is moved to the positive end of the potentiometer. To set up circuit No. 2 for black-out the slider is moved to the negative end. In order to obtain intermediate intensities on the other scenes, the sliders are moved to intermediate positions corresponding to the intensities desired. Now assuming that the switchboard is operating on scene No. 1, the pointer of the dimming fader will be connected to Point No. 1 on this piece of apparatus. In order to transfer to scene No. 2, the dimming fader, which consists of a unit for each circuit to be controlled on a common drive, is moved either by a hand-wheel or by means of a motor drive to position No. 2. It can be seen therefore that since point No. 1 of the dimming fader is connected to the slider of preset potentiometer No. 1, and is therefore at the same potential as its preset for this scene, and since point No. 2 is connected to the slider of preset potentiometer No. 2, there is a gradual transmission from the potential set for preset No. 1 to that for preset No. 2. This gives a gradual change in the potential impressed on the grid circuit of the tube unit and, therefore, a proportional change in the lighting circuit. A similar operation is also performed to transfer from scene 2 to scene 3. It is furthermore possible with this type of equipment to set up the scenes not in use for additional effects without affecting the scene in progress. Scene Flashing The purpose of the scene flashing equipment is to allow the operator to transfer immediately from the effect that may be set up from one scene to the effect set up for any other scene and have all the lighting circuits come to the desired preset intensity. This is accomplished by means of the relays 1, 2, 3, 4 and 5, Fig. 1, which operate to disconnect the grid lead from the scene fader and connect it to the preset potentiometer associated with the relay selected. Thus the potential which has already been set up on this potentiometer is applied to the grid lead of the tube unit and a corresponding intensity of the lighting circuit results. When any other relay is operated, the relay previously closed is automatically disconnected by means of the switch keys which are interlocked. In order to obtain color master operation of any of the circuits as has been previously described, the pilot switch is thrown to the color master position. This transfers the lead to the positive end of the control potentiometer from the positive bus and connects it to the sliding arm of the color master potentiometer. Thus it may be seen that the voltage on all of the potentiometers connected to this particular color master is varied by moving its sliding arm. Consequently, a proportional change in the voltage impressed on the sliding arm of the individual potentiometers is obtained which results in a proportional change in the lighting intensity of the circuits connected to these controls. Thus if one of the circuits connected to this SCHEMATIC DIA&RAM OF SCENE FLASH/KG CONTROL THERMIONIC Type THEATRE SWITCHBOARD PRESET POTENTIO METERS s scenes inoiviDuAL potentiometer ^gHil' r v v\ o £ risk* circuit V ■SSKSP C,RCU,T'Z" 1 SW . ! SH . ! 3* . . I J* . . I *<v. \neri \Keri \Ktrj Ucr« \kiy s H h H h h 0000© OTHER CIRCUITS SAME AS c/R~/i2 MUL Tl CON TACT R£L ATS Fig. 1. Control switchboard for theatie lighting.