Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 THE ISOTONE SCREEN-GRID "SUPER" 175 fier section is in the output circuit of the phonograph pick-up unit; it has a turns ratio of 1 to 1. When the set is being used as a radio receiver, this transformer and its associate tube is switched entirely out of the circuit by the automatic control switch, S. By referring to the schematic diagram, it will be seen that the plate voltage to the phonograph tube, and also to the second-detector tube, is supplied through the resistor, Ri, connected between B-plus terminal of thefirst audio-frequency transformer and the 135-volt supply lead. This resistor is bypassed by a i-mfd. condenser. The audio-frequency transformer, T6, has a ratio of 5 to 1, and the input transformer, T7, to the push-pull circuit has a ratio of 2 to 1. The schematic diagram shows that the output of the loud speaker is taken from opposite ends of the high-impedance choke, Li. THE TUNING UNIT THE long unit immediately in front of the two amplifiers contains the tuning circuits. This unit consists of the antenna-tuning stage at the left, the oscillator at the right, and in the center are located the control resistors and the automatic ballasting switch. The antenna-tuning circuit is equipped with detachable leads to the coil, L> — a highly desirable feature inasmuch as it permits the operation of the set on either a loop or an outside antenna. Ordinarily the instrument is set up for loop operation, but the operator may employ an outside antenna by simply plugging the three flexible connections from the coil into three tip jacks in the antenna compartment. The antenna may then be connected directly to the antenna binding post. This circuit is tuned by the condenser, C3, having a capacity of •0.000475 mfd., and the inductance of the coil Lo is such that the dial reading of C3 will coincide with those of the oscillator tuning dial, C4, when the two dials are properly matched by means of a small midget condenser, C5, in the oscillator circuit. These two dials may then be operated with readings almost exactly the same over 85 per cent, of the dial. The oscillator circuit is tuned by a condenser, Q, having a capacity of 0.00025 mfd. The r.f. heterodyne voltage is transferred from circuit to that of the antenna circuit through the pick-up coil, L3, which is connected to the centertap terminal of the loop antenna (or L2 if an antenna is used). The controlling devices for the receiver are located in the small metal compartment situated between the two drum dials in the center of the VIEW OF COMPLETED RECEIVER This picture of the Isotone shows the set with shields in place. It may be noted that the receiver consists of five units which are mounted on a steel chassis front tuning unit. . This metal compartment houses a special wire-wound potentiometer, R2, having a value of 25,000 ohms and serving as a voltage divider in the screen-grid circuits. This control allows any potential from o to 675 volts to be placed upon the screen grids of the tubes. The other variable control is a 500,000-ohm PICTURE WIRING DIAGRAM The design of the Isotone is such that its construction may be accomplished in less than one hour. The simplicity of wiring is indicated in this diagram which shows all connections which it is necessary for the set-builder to make in order to complete the circuits of the five individually wired units this volume-control potentiometer, R5, which is connected across the secondary of the first audiofrequency transformer. The switch, S, in the center handles several operations. In one position it automatically connects all of the circuits required to make the Isotone operate as a radio receiver. With this adjustment, the phonograph-amplifier tube is VIEW OF SET WITH SHIELDS REMOVED The arrangement of apparatus on the chassis of this receiver is shown clearly in the above picture. The symbols used for the purpose of identifying parts correspond with those used in the text, list of parts, and schematic diagram disconnected from the circuit. In the reverse position the Isotone operates as a phonograph amplifier; the switch connects only the last four tubes of the instrument, or the audio amplifier, and at the same time disconnects the remaining six tubes in the radio section of the set. One of the interesting features of the controlling system is the 6.6-ohm resistor, R4, which is automatically connected across the filament-supply circuit when the audio amplifier is being used for phonograph work. The extreme desirability of this arrangement is appreciated when it is realized that this ballast resistor has a load characteristic which corresponds with that of the six tubes which are disconnected when the set is being used for phonograph reproduction. This permits the use of an A-power unit and the voltage supplied to the tube remains steady at all times, regardless of the position of the control switch. If it were not for this ballast resistor, the filament voltage would jump suddenly upward when the six tubes were disconnected, and the remaining four tubes in the audio amplifier would be subjected to a filament potential considerably above their rating. ASSEMBLY AND WIRING THE assembly of an Isotone receiver is such a simple procedure that even a novice setbuilder would not experience difficulty with the task: The main steel base plate is supplied with all necessary holes punched in the proper positions. The assembly operation is started by simply placing the three main Units down on the base plate and securing them in position by means of a few nuts and bolts. Slots have been cut in the base plate and the terminal strips of the individual units protrude down through the slots. The positions of the terminal strips are such that when the three units are mounted in their respective positions, practically all of the connections can be made by means of metal connecting strips which are furnished with the kit Therefore, it is only necessary for the constructor to slip the thirteen metal connecting strips down over their respective bolts and tighten them on by means of nuts. The wiring is then completed by running the power connections from the cable receptacle to the proper points on the terminal strips. There are ten wires to be connected in all. The above describes all of the connections which it is necessary for the assembly of the receiver. There is \" of space between the bottoms of the individual unit base pans and the top of the