International projectionist (Jan-Dec 1935)

Step-By-Step Analysis of Sound Reproducing Equipment By AARON NADELL XII. Western Electric 702-B Control Cabinet FIGURE 1 is a schematic diagram of the Western Electric 702 B Control Cabinet, one of the most widely used W. E. faders. It performs double duty, being both a switch and a volume control. As a switch, it operates to connect the system amplifier to either of the two projectors, and contains a special key switch for the use of a third projector, if one is installed. As a volume control, it acts by changing the nature of the transmission line between the projectors and the system amplifier without destroying impedance match at any volume setting whatever. This last is the most interesting function performed by this circuit. It must, as a volume control, alter the amount of current flowing through the transmission line to the amplifier, yet it must not change the impedance of that line. More detailed study of the wiring arrangements will show how this is possible. Figure 1 receives sound current from the projectors only; it does not discriminate between sound-on-film and soundon-disc. When it is used with a W. E. Universal Base, the switching between sound-on-film and sound-on-disc is done on the base itself; with the earlier style W. E. sound attachments a special switching cabinet designed for that purpose is mounted on the front wall of the projection room, directly above Fig. 1. The sound input is wired to the row of seven terminal posts at the top of the drawing. The sound output is taken from the two terminal posts at the bottom center of the drawing. The input binding posts are marked, from left to right: "G", for connection to the system ground; a pair designated as "Rep.", for connection to Reproducer (projector) No. 3, where there is one; a pair labelled "W" for connection to the "White" projector, which is normally the right-hand or No. 2 machine; and a pair marked "R" for connection to "Red" or left-hand or No. 1 projector. These seven terminals thus consist of three pairs and a ground, and it will be observed that four of the seven posts are connected together and grounded. Beginning at Terminal No. 6 a wire runs straight downward, past the common jumper, to the right-hand side of resistor 1-FK, still downward to pick up the ground connection to the cabinet frame (which is thus grounded both through its connecting conduit and through the system ground water-pipe connection to terminal "G"), and down another inch and a half to the central point of the long, horizontal resistor assembly. Thence at an angle left and down for about % inch, and straight down to the right-hand output terminal. Thus, one side of this transmission line between projectors and system amplifier runs solid through Fig. 1, and this is the side that is grounded. In other words, if we start at the system amplifier and trace this transmission line backward to the projectors, we find that one of its two wires runs unbroken through Fig. 1 to the input side, and there branches into four parallels, one of which goes to each of the three projectors, while the fourth is grounded. The first requisite here is to understand some of the physical arrangements of this fader. The long series of resistors that stretch horizontally across Fig. 1 are in physical fact arranged in a circle. Each of the two halves of the horizontal line of resistors, both the "Red" and the "White" halves, are therefore semi-circles. The arrowhead shown touching Terminal 5 of the Red resistance assembly is a movable contact, pivoted at the center of the circle. By turning a large knob on the outside of the cabinet the projectionist can swing that arrowhead through nearly a full circle, and cause it to make contact with any of the 31 resistor terminals shown in the drawing. In Fig. 1 the arrowhead is touching Terminal 5 of the Red side of the fader. Therefore, this fader (a switch as well as a volume control) is connecting the Red, or No. 1, Projector to the system amplifier, as can be seen by tracing the circuit backward from the fader output. We have already traced the grounded side of this transmission line back from the amplifier through Fader Output Terminal No. 16. Now tracing in through Output Terminal No. 15, our line runs up, left, up, and right to a point of junction. From this point one branch runs right another *4 inch, and up to a switch prong, which in this drawing is opencircuited and can be ignored for the moment. (The two heavy horizontal lines of that switch indicate not electrical connections but insulating bushings that [10] cause several prongs to move together as one unit). Returning, therefore, to the point of junction, we trace up, right and down to the long central prong of the set of three switch prongs shown just to the left of resistor 1-ED. That switch is closed in the drawing to the left-hand prong; therefore the circuit continues through that contact and up, left, down, right and up to the fourth prong from the left of the lower assembly of seven switch prongs. Thence through the switch contact to the third prong from the left, and down, left, down, right and up to the arrowhead. Through the arrowhead to Red contact point 5. Upward through the 190-ohm vertical resistor to the horizontal resistance. Left through the horizontal resistors to the extreme left end, then up through the 223.5 vertical resistor. Thence right and up to the extreme right-hand prong of the seven-prong switch assembly of Key switch 479-GS; thence to the second prong from the right of that assembly; thence right about 2 inches to the central prong of the left-hand, three-prong assembly of the 479-K key switch; thence to the right-hand prong of that assembly, a? lr,„e.nce up t0 the ungrounded side of "R" input. fT«£?,we see that the ungrounded side ol R input connects, through some switches to be examined later, and through the 223.5 ohm vertical resistor to the extreme left-hand end of the horizontal resistance assembly. And the grounded side of "R» input has already been traced to the central or u tap of the horizontal resistor line. Therefore the "Red" resistors of that line are connected directly across "R" input. The grounded side of this fader's outf'ftP? Terminal 15) also runs solid to the 0" or central tap of the long resistor line; while the ungrounded output (Output Terminal 15), has been traced through some switches to the arrowhead. Therefore: The "R" input looks into the Red horizontal resistors. The common output looks toward whatever portion of those resistors may be included at the moment between the "0" tap and the arrowhead. Fader Input Impedance # It is now possible to inquire into the impedance arrangements. Standing at u £of a,nd looking into the fader, the ZZ5.5 ohm vertical resistor might I