Journal of the Society of Motion Picture and Television Engineers (1950-1954)

Record Details:

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pictures. The present form of this shield permits quick removal of the shield for head cleaning and degaussing. For severe hum fields, a box shield which slips over the magnetic head and is held by spring tension is provided. The magnetic pickup head is a Brush type BK1544-S1237, manufactured by the Brush Electronics Co. to reproduce film conforming to specifications supplied by Twentieth Century-Fox. The connections for the individual track heads and a ground are brought out to a Cannon type DA-15P plug mounted on the side of the head, and a mating plug and shielded harness carry the connections to a barrier terminal strip in the connection box on the front of the soundhead. Threading the soundhead is simple. The correct loop length is automatically obtained by threading tightly with the sprocket pad shoes open, since a pin on one pad shoe operating toggle limits the movement of one of the tension arms. Closing the pad shoe releases the tension arm and permits it to perform its filtering function. Preamplifier and Power-Supply Switcher Assemblies This equipment is mounted in wall mounting cases of two types, but identical external appearance (Figs. 4 and 5). The preamplifier case mounts four plug-in preamplifiers, and the powersupply switcher case mounts one powersupply and two "effects" switchers. The electronic assembly for two projectors consists of two preamplifier cases mounting four preamplifiers for each projector and one power-supply switcher case. For a three-projector installation, one preamplifier case and one power-supply case must be added. Under these conditions, the second power-supply case contains one power-supply and one effects switcher, and a removable (not plug-in) dummy load for the unused part of the power supply. Cannon type DPB twelve-pin plugs with twinax input connections are used for the preamplifiers, and I.P.C. type F-10A and M-10A connectors are used for the power-supply and effects switchers. Terminal blocks in the back of the mounting cases are connected by harnesses to the plug strips which are mounted on a channel strip. A powersupply case and a preamplifier case differ only in these plug-harness assemblies and the reversal of the mounting blocks for the plug channels. The preamplifier is of conventional design, employing a 5879 and a 12AT7 tube. The 5879 is connected as a pentode, and is followed by an RC equalizing circuit. The 12AT7 is connected as a cascaded triode amplifier, with feedback over the two stages. A potentiometer in the feedback network provides an approximately 6-db gain adjustment. The input transformer is a UTC HA100X, connected for a nominal 50-ohm input impedance, and the output transformer is a UTC A-25 with plate current in the primary, the output being connected for a 500-ohm output. For purposes of gain measurement, the head-source impedance is considered to be nominally 50 ohms. The dummy source which is actually used for measurement consists of a 6-ohm generator in series with a magnetic pickup head. For calibration, this source is terminated in 50 ohms and the power into this resistor is considered to be the signal delivered to a nominal 50-ohm load. With the gain control at maximum, the gain of the preamplifier at 1000 cycles/sec is 75 db. At maximum gain, input short-circuited, the output noise is —45 dbm. The output signal at 1000 cycles/sec, from a 50-mil track modulated to 3% distortion is -f-7 dbm and for a 29-mil track at 5% distortion, +1 dbm. The distortion at +8 dbm output is less than 0.75% above 300 cycles/sec. At low frequencies, the distortion rises to 1.2% at 50 cycles/sec. The recording pre 226 March 1954 Journal of the SMPTE Vol. 62