International projectionist (Jan-Dec 1935)

July 1935 INTERNATIONAL PROJECTIONIST 13 nections, and can be disregarded, as it does nothing. This switch is included for use in emergency, in case one of the resistors or circuits in the fader proper should develop a defect. Together with the 1-DD, the 1-ED and the 1-FK resistors, constituting a T-pad, it forms, in effect, a substitute fader. When it is thrown to Red, those resistors replace the red half of the fader proper; thrown to White, the white half. Volume control is no longer possible when this substitution is used, but it serves as a fixed volume setting equal to about Point 9 of the fader proper. If volume changes are necessary, they must be made at the system amplifier or with the exciting light, until fader repairs can be effected. Suppose the 479GS key to be moved leftward, to Red. All the grounded lines remain solid, input to output, as always, since there are no switches in the grounded side of this transmission line. Tracing the ungrounded White input shows no change; however, the arrowhead is open-circuited. But tracing the Red input line when the 479GS key is thrown to Red, shows the Red input opened and resistors 1-DD and 1-FK substituted for it as follows: From the ungrounded Red input (Input Terminal No. 1) down, right and through the left-hand long prong of the 479K key, thence left to the sixth prong from the left of the seven-prong assembly of the 479GS key. This is no longer closed to the seventh prong of that assembly and thence to the high side of the Red resistance line, but, instead, to the fifth prong of the seven-prong assembly, and thence right, and up through the 1-DD resistor, and right through the 1-FK resistor to the other side of the input line. These two resistors total 502 ohms, connected across the red projector input. The voltage drop developed in the 1-FK resistor is connected across the fader output as follows: one side of that resistor goes to the grounded side of the line, and then to Output Terminal No. 16. The other side runs through the 1-ED resistor (total Fader output impedance, 205 ohms), and thence up, left, down and right to the extreme left-hand prong of the seven-prong assembly. This is now closed to the second prong from the left, through which the circuit continues down, left, down and right to the ungrounded output terminal. Thus a substitute T-pad has been switched in in place of the defective Red side of the fader. If the White side of the fader still operates normally, the 479GS key can be thrown to neutral for projection from the White machine, and the white side of the fader operated in the usual manner. Or, the GS key can be moved rightward to "White" position, and 1-ED, 1-FK and 1-DD used in place of the usual White resistors. In that case the circuit runs from the ungrounded White input, No. 3, down and right to the right-hand prong assembly; from the central prong of that assembly left to the central prong of the lower right three-prong assembly of the 479GS key (which is now closed to the right instead of to the left as the drawing shows it) ; thence down and upward through 1-DD and back to the grounded side of the input, through 1-FK. The potential difference developed across 1-FK connects to the ungrounded output through 1-ED and the upper threeprong assembly of the GS key, which is now closed to the right. From the central prong of that assembly left, down, left, down and right to the ungrounded output terminal. This arrangement works precisely the same way if No. 3 projector has been switched in, through key 479K, in place of either the Red or White projector, since, as has already been traced, in that case No. 3 merely connects to one of the two central prongs of the K key in place of the projector for which it substitutes; and it is from those central prongs that the GS emergency key derives sound current. The W. E. 705-A Control Cabinet By AARON NADELL Articles in this series (Step-By-Step Circuit Analysis) requiring close attention and careful checking, have been limited to one in each issue. It is believed, however, that the best interests of readers will be served by simultaneous presentation of two of the three articles relating to control cabinets, the final installment of which group will appear next month. — Editor FIGURE 1 is a schematic diagram of a combination volume control and switching circuit, or fader. It contains one feature that is often overlooked in these days of sound-on-film operation, namely, the switching arrangements for the little-used disc reproducers. These witches and their wiring remain installed and "alive" in a great many theatres and consequently are capable of causing trouble and of complicating the work of tracing troubles that arise elsewhere. The input-output terminal block is shown at the left of the drawing. It consists of the upright rectangle and carries eleven terminal posts. The four lower posts are labelled "disc" and are numbered in pairs — Pair No. 1 and Pair No. 2 — referring to the projectors to which they are wired. At the top of the rectangle are five similar terminals. Four of them are similarly numbered, but are labelled "film." The fifth, or top, terminal connects to the cabinet cover and thence through the connecting conduit to the ground. Apart from the ground terminal, there are eight input binding posts, providing for four input circuits. At the center of the rectangle are two more binding posts, not numbered, but merely labelled "output." Since the drawing shows no other posts for external connections, obviously Figure 1 must function to select any of the four input circuits, as desired, for contact with the output line. In addition, as will be seen by tracing the details, it controls the sound volume from the input source thus chosen. The Ground Circuit Figure 1 shows three wires connecting to the topmost terminal. One is a heavy line leading downward to the upper of the two output posts, serving only to ground that side of the output line. The second leads from the ground terminal upward and right to the line of dashes representing the metal cabinet cover. A third, labelled "blk" (black) leads right to a loop surrounding two wires. These are therefore shielded wires, and the loop indicates that the shielding has been soldered to the ground line. Figure 1 of this article appears on the next page. Tracing the ground wire right and then downward, we see that the shielding of five separate pairs of wires is grounded. That is, all four input circuits and the output circuit run through this cabinet in shielded cable, and all the cable shields are connected to the ground. The black wire leading to the ground terminal is labelled "G" — the letter can be seen on the terminal block just to the right of the "m" in "film." An external wire, running to a water pipe, or other suitable ground, is wired to this terminal. The Input Circuits The No. 2 input pair may be traced directly right from their terminals immediately below the ground terminal. They run to the second and fourth switchprongs of the lower right-hand switch assembly of the 479-DA key switch. Directly below these the No. 1 film connections run right, up, right and down to the upper right-hand switch assembly of the same key. Ignoring the output line for the moment, and continuing down along the terminal block to the No. 2 disc input, we may trace this right, up and right to the lower left prong as