International projectionist (Jan-Dec 1945)

VOLUME XX JULY 1945 NUMBER 7 A Step-By-Step Analysis of the Filmosound V 16-mm Amplifier CIRCUIT arrangements that are moderately novel in sound service, and some unusual mechanical details, will be found in Figure 1. Some of these features will need not only careful study, but a little reference to the actual equipment before they can be clearly understood. These will be noted as they are encountered. Line power input is applied at the bottom, center. The primary of the input transformer is provided with three terminals which are wired to a three-point receptacle. A "polarity changer" or d.c. to a.c. converter is used with this amplifier where only d.c. line power is available. The three-terminal power input is intended for operation with that converter, the circuits of which may be examined in some future issue. Where the input is line a.c. only two terminals are used, in the ordinary way. The primary circuit completes its path through a switch and a 2-ampere fuse. The left-hand secondary heats the filament of a rectifier tube, tube No. 7; the center secondary supplies the plate of that tube; the right-hand secondary heats all the other cathodes of this amplifier (but not the exciter lamp filament). In accordance with common practice, to reduce strain on the cathode insulation and to reduce hum, the center tap of the right-hand secondary is grounded. Trace up, right to the first junction point, up, By AARON NADELL Herewith is presented a step-by-step analysis of a 16-mm schematic. Operation and maintenance of the projector of which this amplifier is a part were detailed in the June issue of I. P. and left as far as possible to ground. Tube No. 7 functions as a conventional full-wave rectifier. The two plates being connected to opposite ends of their secondary winding, one is always positive while the other is negative, and the electron circuit through the tube is closed to whichever is positive at the moment. That plate (and its associated end of the transformer secondary) is the most positive part of the circuit at the moment, and since it connects to cathode through the electron stream, cathode is the next most positive point ; and the center tap of the secondary is negative with respect to cathode. Cathode and center tap are, therefore, positive and negative terminals, respectively, of the d.c. output supplied by this rectifier. The Ground Bus The center tap of the plate secondary (like that of the right-hand secondary) is grounded. Trace to ground as before. The practical negative terminal of the d.c. plate supply, therefore, is ground, or chassis. The positive terminal is the junction point at the right of the rectifier tube filament. The ground terminal of the diagram is drawn, as may be noted, at the left-hand end of a line which runs almost all the way across the drawing from left to right. Also, from the ground terminal trace straight up as far as possible, and then right across the top of the drawing. These two lines leading across the drawing, together with the vertical line that connects them, may be regarded as the ground bus of the diagram. Plate and screen grid d.c. supplies to the tubes, and anode bias to the photocell, need, therefore, be followed only along the positive side of their circuits. For the negative side, it is sufficient to trace the cathode of tube or cell to this ground bus, since the true negative terminal, the center tap of the plate secondary of the power transformer, is also connected to that bus. Note further that the lower end of the output transformer secondary runs directly to the ground bus. It will be seen later that one leg of the inverse feedback circuit is provided by the common ground, and so only the ungrounded side of that circuit need be traced. It is always helpful, in dealing with any drawing, to try to identify any common ground (or common positive) which the draftsman may have put in, in such JULY 1945