International projectionist (Jan-Dec 1945)

a way that it is easily recognizable. Thereafter, individual circuits can be traced to that bus, instead of all the way back to their source, which is simpler, saves time, and helps avoid confusion. Plate Power Supply From the right-hand side of the rectifier tube filament trace right, up to the first junction point, right, up as far as possible, right, and through the two halves of the transformer primary to the two plates of the output tubes. From the rectifier tube filament as before trace right, up, right, up to the first connection • point, left, through A 1254 and left to the screen grids of the two output tubes. Following exactly the same path, continue left after passing through A 1254, left through A 330, and up to the two plates of tube No. 3. Still following the same path, continue left after passing through A 330, then down, left through A 337, up at the next junction point then left to the plate of tube No. 2. Again on the same path, follow left through A 337, up to the first connection point, left through A 1285, and up to the screen grid of tube No. 2. The oscillator tube and the PEC are on a slightly different line. From the righthand side of the rectifier tube filament trace right, up, right, then down through A 1700, left, and down to the next junction point. From that point trace left through A 1360 to the screen grid. From the same point trace down a trifle, and left through the 36 ohm plate coil to the plate. Note that resistor A 1361 is marked 39,000 (39 M) ohms, 5%. The 5% means allowable tolerance; namely, if the resistor is found to have any value between 37,050 and 40,950 ohms it may be considered in good condition. Incidentally, it should be noted that some small tolerance must be applied by the projectionist in comparing the voltage readings of this diagram to those he may take in the actual equipment. Since his voltmeter and the one the manufacturer used probably will not be of the same ohms-per-volt rating, some discrepancies are to be expected, and if of a minor nature should not be considered a sign of trouble. For the PEC anode bias, trace from the right-hand side of the rectifier tube filament right, up to the first junction, left, up through A 1556, left as far as possible and up through A 916. These plate supply fines are bridged to ground at various points by capacitors which serve as filters in association with the series resistors of the line. The same capacitors and resistors in some cases do double duty as recoupling devices; the resistors tending to keep speech current out of the power supply while the capacitors provide speech current with a more direct path to ground. Furthermore, the series resistors serve to reduce the line voltage from the 317 volts applied to the output tube plates to the 220 volts and 86 volts of tube No. 3 and finally to the 61 volts at the plate of the input tube. In this diagram those condensers that are drawn as parallel rectangles, instead of parallel lines, are located in condenser banks. A small symbol, a triangle or a cross within a circle, placed near the condenser, identifies the bank in which it is to be found. There is only one suppressor grid in Figure 1 — in tube No. 2. It is tied directly to cathode and, therefore, at cathode potential. The control grid of tube No. 2 may be traced to ground through A 213. right through A 63, and up to the bus through A 1553. No d.c. can flow through these resistors since they are isolated by the negative grid and by condensers; there is rio d.c. voltage drop across them — no voltage drop between grid and ground. Grid is at ground potential. Cathode of the same tube returns to ground through the 4,700 ohm cathode resistor A 1260 and straight down to the ground bus. Since plate current completes its course to ground through that resistor, a d.c. voltage drop does exist across it, and the diagram notes that this drop is 1.7 volts. (Query: then what is the value of plate current?) Cathode is 1.7 volts more positive than ground, or. to say the same thing another way, grid is 1.7 volts negative with respect to cathode. No external bias is applied to the oscillator tube, No. 6. From grid trace left, down through A 1554, right through A 1698. up. and right to cathode. The left-hand cathode of tube No. 3 goes to ground through A 119. and the diagram notes that the voltage drop through that resistor is 1.3 volts. The left-hand grid of that tube goes to ground through A 1550, in which no d.c. flows and which, therefore, has no d.c. voltage drop. Grid consequently is 1.3 volts negative with respect to cathode. The right-hand cathode of that tube goes to ground through 103.300 ohms, and is 92 volts positive with respect to ground. Grid, however, is not 92 volts negative with respect to cathode, because grid does not go directly to ground. It can be traced right, up through A 349, and left to the negative side of the 3,300 ohm resistor. It is negative with respect to cathode by the extent of the voltage drop through that resistor, which is roughly l/30th of the total drop of 92 volts, or about 3 volts. The combined plate currents of the two output tubes return from their cathodes to ground right through A 1701, 250 ohms, and up to the bus. The two grids of those two tubes return to ground through their respective grid resistors, from the grid of tube No. 4 trace left and up to the bus through A 295; from the grid of tube No. 5 trace left and down through A 295 (the same part number but not the same part, obviously) to the bus. Consequently both these grids are negative with respect to cathode by the extent of the voltage drop through the cathode resistor, A 1701. The bias is shown in the diagram at the point where the cathodes join — 19.5 volts. The Oscillator Circuit The oscillator tube in the lower lefthand corner, tube No. 6. has a coil in series with its plate, and that coil is "tuned" by a .001 mfd condenser connected across it. The control grid of the same tube has a coil in series with it, and that coil is tuned by a series condenser, A 1551, of .0001 mfd and resistors A 1554 and A 210. As drawn, plate and grid coils are in inductive relation to each other, and the tube is of the screen grid type, wired as a screen grid tube. This is a Hartley oscillating circuit, with feedback between plate and grid secured by the inductive relationship of the two coils. It generates a.c. for the same reason a sound system "sings" when the microphone is placed in front of the loudspeaker — direct feedback. The frequency generated depends on the constants of the circuit, particularly the inductance and capacitance of the tuned circuit, and in this case, according to the manufacturer, it is 60,000 cycles. Since that frequency is far beyond the audible range, it can be applied to the filament of the exciting lamp without creating any a.c. disturbance in the sound. The exciting lamp, however, requires low voltage and relatively high current, and the a.c. generated is rather of high voltage and low current. That is obviously a job for a stepdown transformer, and the coil in the extreme left-hand corner, of .24 ohms, is the secondary of that transformer. Examination of the apparatus will show whether it is the grid or the plate coil, or both, that act as primaries. Secondary output is 60,000 cycles at 3.5-3.75 volts, 0.75 amperes. Assume that the little jack shown inside the dotted rectangle is closed in such way that the plug is inserted in contact with the shielded lead. Then the .24 ohm coil is a source of 60,000 cycle a.c, and the exciter filament the load connected across that source. From the right side of the coil trace straight up, right, down and left to the right side of the lamp. From the left side of the coil trace up through the shielded lead (the ungrounded side of the circuit) and right — with the little h INTERNATIONAL PROJECTI ON 1ST