International projectionist (Jan-Dec 1946)

diode detectors to fairly strong receivers. The superhet is of this type due to the large amount of amplification in the IF amplifier. Figure 5 shows a special type of diode detecior circuit that is commonly used in present-day receivers. A tube of unusual design used in this circuit is known as a duo-diode, high-mu triode. combining in one envelope the functions of a diode detector and an audio voltage amplifier. The tube types usually used in this type of circuit are the 6SQ7 and the 12SQ7. The diode plates are connected together and go to the secondary of the second if transformer. The other lead of this winding goes to the ground through a 50,000-ohm resistor in series with a 500.000-ohm volume control. The diode plates conduct on the positive half-cycles of the if carrier. The detected current flows through the volume control to ground. The voltage developed across the volume control will contain the audio variations which were originally fed into the microphone at the transmitter. The 50.000-ohm resistor and the two .0001-mfd. condensers connected across it form a "pie" type of filter which removes the if carrier from the audio output, thus keeping it out of the audio amplifier circuits where it could cause distortion. The audio output is connected to the control grid of the audio amplifier section of the combination detector-amplifier tube through a .005-mfd. coupling condenser. The 10-megohm resistor which is connected to the control grid and ground provides grid bias for the audio amplifier section of the tube and is known as a contact bias resistor. The plate circuit of this tube is provided with a positive voltage through the plate load resistor of 250,000 ohms, and the amplified audio from this circuit is fed into one or more stages of power amplification through the .05-mfd. coupling condenser. Figure 5 shows that a 1.25-megohm resistor is connected to the junction of the 50,000-ohm and the 500,000-ohm resistors. The other end of this resistor Presenting: Mike Berkowitz Career Encompasses Entire Motion Picture History THE "presentation" of Mike Berkowitz in the limited space available herein is almost a mockery of the career of a man whose experience as designer, expert machinist and projectionist encompasses the entire history of the motion picture business. Strange it is that a man of Mike's stature in the film world should have received so little recognition, even at the hands of those who are familiar with his many outstanding contributions to the art. The Berkowitz saga began in Odessa, Russia, where he was born in 1874. The alacrity with which Mike subscribes to this date lends color to the suspicion of is connected to ground through a .05-mfd. condenser. The circuit consisting of the diode return circuit and the resistor and condenser is known as the avc circuit, its purpose being to provide a means for automatically controlling the volume of the receiver during fading distant reception and to prevent blasting when tuning in a strong local station. The direction of the arrows indicates that the 1.25-megohm resistor is connected to a point which has a negative potential with respect to ground when the current due to detection flows in the FIGURE 5. Diode detector, 1st audio and AVC. 2nd DETECTOR sf AUDIO AVC. .05 TO POWEE. AMPLIFIER 250K his friends that he is indulging in a bit of self-cheating for, say, a year — or two — or three. . . . Anyhow, Mike vows that he was 12 years old when he landed in America in 1886. Almost immediately he got a job in a brass factory, wherein he soon became an expert finisher. One nippy late-Fall day in 1890 Mike, with obvious acquisitiveness, was peering into a Bowery pawnbroker's window at a long-yearned-for watch when, responding to a tap on the shoulder, he turned to be confronted by a man, obviously a "foreigner," who articulated in a meaningless jumble of French and English. Mike finally caught the phrase "moving pictures," which he understood as an offer of a job of toting pictures on his back. Finally, however, the Frenchman overcame Mike's outraged sense of mechanical craftsmanship by conveying the idea, illustrated by fluent hand-designs on the store window, that he wished him to operate a machine that showed pictures. The salary was $12 weekly — very big money in 1890 — and Mike promptly forgot about the brass factory. Mike had a really tough time getting started on his new job because both he and the Frenchman knew only how to murder the English language. But he soon learned how to operate the then strange mechanism. Shows were given in a Bowery place known as the Gaiety (Continued on page 26) circuit. The amplitude of the negative voltage depends upon the strength of the incoming signal. When the incoming signal is strong, a large negative voltage will be developed at the point where the 1.25-megohm resistor is connected, because a large detected current will flow through the detector circuit. When the signal is weak the negative voltage will be small. The voltage developed by the avc circuit is used as a grid bias in the if amplifier circuit and is sometimes connected to the RF amplifier grid return circuit. When the negative bias is increased on the grid of an if or RF amplifier, the gain of the stage will decrease. This means that the gain of the if or RF amplifier will increase when the signal is weak and will decrease when the signal is strong, thus giving an automatic control of volume. -r250V SEPTEMBER QUESTIONS 1. Why is a superhet better thai* a T. R. F.? 2. Why is AVC used? The answers to these questions will appear in the next issue. 22 INTERNATIONAL PROJECTIONIST • September, 1946