International projectionist (Jan-Dec 1945)

a i '54 a 4/546 Qj jack plugged in — up and right to the left side of the lamp. Inside the dotted rectangle, just above the plug-in connection, there is a small jack seemingly permanently short circuited. Actually, when a plug is inserted in the sleeve of this jack (jack 12810), the tip is lifted leftward away from the short circuiting connection, and anything connected to the plug is then in series with the exciter lamp current. A rheostat can be used for remote control of volume since it will operate to vary exciter lamp current. The shielded lead plug-in arrangement below and left of jack 12810 was illustrated in the previous article describing this equipment. The picture referred to (page 9, Figure 7. June 1945 issue) showed that shielded lead being removed from the amplifier in order to take the amplifier out of the projector carrying case for servicing. Speech Input Circuits When the photocell, tube No. 1, is suitably illuminated through a moving soundtrack, it becomes a source of speech current (fluctuations, at speech frequency, in its plate current, which may be regarded as an a.c. component superimposed on the d.c. plate current). Anode and cathode of the cell are the source ter A02I7 minals of this a.c, and a load circuit can be traced down from cathode through 5.6 megohms, left, up, and right to anode through .05 mfd. The voltage drop of a one microampere fluctuation through 5.6 megohms would be 5.6 volts. Grid and cathode of tube No. 2 are connected across that speech voltage through A 56, .001 mfd to grid, and A 1541, 25 mfd, to cathode. The reason for using so large a value as 25 mfd in shunt to the cathode resistor of this input tube is not altogether clear, but like the 33,000 resistor A 213, in series with the grid, may be intended as extra assurance of stable operation in a tube of very high gain coupled to a very high impedance input. In the upper left-hand corner of the drawing there is a jack, A 1543, through which a high impedance microphone or phono pick-up may be substituted for the photocell. As drawn, the jack seems to be permanently short circuited, because the sleeve is wired directly to the ground bus (trace straight up) and the jack appears also to go to ground; trace down to A 1546, right, and up. However, the line running straight down from the tip of A 1543 to the sleeve of A 1546 is not an electrical connection. The fact that this line is numbered A 1549 shows that it is a part, not a wire; but this is one of those numerous cases in tracing schematics where doubt may have to be resolved by looking at the actual equipment. Part A 1549 is not a connection but a part that causes the two jacks, A 1543 and A 1546, to act in unison. When a plug is inserted in A 1543 the switch points A 1546 close. Reducing PEC Voltage The lower switch points of A 1546 then put a direct short circuit across the .24 ohm coil supplying the exciter lamp. The upper pair of contacts of A 1546. when closed, put a by-pass resistor across the photocell bias, reducing that bias to a very low value and thus eliminating any chance of PEC hiss while the microphone is being used. From the anode of the PEC trace down through A 916, right, and down through 5.600 ohms and left to a prong of A 1546. When that prong is closed to the line just above it continue to trace, a little to the right, and up to ground. The effect of bridging this line to ground through a resistance of such (comparatively) very low value at 5.600 ohms is to reduce the PEC voltage far below the 80 volts shown in the drawing. This is a point to be remembered in trouble shooting. The change can readily be calculated by Ohm's Law. Return to the right-hand side of the rectifier tube filament, where JULY 1945