Radio Broadcast (May 1928-Apr 1929)

16 RADIO BROADCAST MAY, 1928 correspondingly increased amplification. (See "Complete Suppression of a Single Frequency by Means of Resonant Circuits and Regeneration," by J. A. Stratton. Journal Optical Society of America. Vol. 13, No. 1, July 1926.) Regeneration is accomplished by means of a small tickler, Lo, in the detector plate circuit of 25 turns wound directly on the plate coil of the second intermediate-frequency stage. The tickler can be cut in or out as desired by means of a switch, K, mounted on the front panel. The receiver was not designed for broadcast reception and with air condensers to tune the intermediate-frequency stages, the tuning is too sharp for quality reception. If, however, fixed condensers of the proper capacity are used to tune the intermediate stages, it is probable that the response curve would include a band 10 kc. wide. The amplification would probably be reduced accordingly. With a 50-mmfd. tuning condenser across the secondary of the short-wave detector out of voltage is then necessary as when receiving unmodulated signals. The oscillator is a "shunt feed" Hartley type the frequency of which is regulated by a variable condenser of 500-mmfd. capacity in shunt with a fixed condenser, C2, of 1000-mmfd. capacity for 30-kilocycle operation, or 750-mmfd. capacity for 50-kilocycle operation. The oscillator coil, L3, consists of 1500 turns of No. 28 wire wound on the same kind of spool as is used for the input transformer, T, the filament tap being made one third of the way from the grid end of the coil. Some difficulties were encountered in adding a stage of audio-frequency amplification as the separation between a 50-kilocycle radio-frequency signal and an audio-frequency signal of approximately 2 kilocycles is relatively small. To keep the 50-kc. component out of the audiofrequency circuit a single-section filter having a cut-off at approximately 5 kc. is used in the detector plate circuit of the detector. The filter is stage coupling through the B-battery leads. All battery binding posts, as well as the output posts, are mounted on a bakelite terminal strip at the rear of the set. The aluminum shields are 5" by 6" by 9" except the first which was cut down to 4" by 6" by 9". The first shield contains the input transformer, with its secondary fixed, and variable condensers. The second and third shields contain the first and second stages of intermediate-frequency respectively, along with their associated coils, condensers, etc. The 1 5-ohm fixed resistance in the filament circuit of the first screen-grid tube is within its aluminum shield while the fixed resistance in the filament circuit of the second screen-grid tube is mounted at the rear of the baseboard behind the shields. The input lead to the first stage is brought out through the top of the first shield and connects directly to the control grid cap of the first screengrid tube, which projects through a hole in the O 67»5 VL The circuit arrangement of the short-wave super-heterodyne unit. The output of the short-wave receiver which precedes the "super" connects to the two input posts on the extreme left. The first two tubes are the intermediate amplifiers, and they are followed by the second detector and audio stage. The lower tube is the beat frequency oscillator which this amplifier v/orks, the outfit is as selective as one dares make it. An a.c. note appears and disappears in one half a division on the 100 division scale of the condenser when working in the 3 to 4-megacycle (75-100 meter) band. In the ordinary short-wave receiver employing a simple oscillating tube as a detector of c.w. signals, the detector is made to oscillate at a frequency differing one or two kilocycles from the incoming signal and the resulting audible beat is heard in the telephone receivers. In tuning the detector when followed by the intermediate-frequency amplifier described here, the beat frequency is made an inaudible frequency of 50 kilocycles, instead of the usual one or two kilocycles. To reduce this to an audible frequency an oscillator whose frequency can be varied is made to beat with the 50-kilocycle intermediate frequency and give a difference frequency of one or two kilocycles as desired. The beating oscillator voltage is fed to the grid circuit of the second detector tube through the grid leak resistor. The audible beat frequency produced in the second detector is then amplified by the stage of audio amplification. For phone reception the oscillator tube is removed from its socket as no beating a simple "pi" section consisting of a 3-henry choke in the detector plate lead preceded and followed by 2000-mmfd. and 1000-mmfd. fixed condensers respectively. The choke is a Samson No. 3 which has a small open iron core. There is nothing unusual about the stage of audio amplification. The transformer is a Samson with a 6 to 1 ratio and there is provided a pair of output binding posts as well as the usual output jack. The B-battery voltage is 135 volts and the grid battery is 4.5 volts. The baseboard is 23^" by 10" by J". The front panel is of bakelite and is 24" by 7" by rVThe first three dials tune the intermediatefrequency stages while the fourth controls the frequency of the beating oscillator. All the dials are 4" National Velvet Vernier dials. At the right there is provided a Weston double-range voltmeter to read plate and filament voltages while below it are the 2-ohm master rheostat, filament switch, and output jack. The regeneration switch is between the third and fourth dials and is a Yaxley jack switch of the single-pole doublethrow type. The two input binding posts are at the extreme left of the panel, Plate-voltage leads are provided with Samson 3-henry chokes and i-mfd. bypass condensers to prevent inter top of the shield. The output lead of the first amplifier stage is brought out through a hole in the top of the second shield and connects with the grid cap of the second shield grid tube, which projects through a hole in the third shield. To prevent pick-up by these leads they are wrapped in copper foil which in turn is grounded. This aids in the prevention of oscillation in the stages and helps materially in securing stable operation. The tube sockets are raised on blocks of wood in order that the control grid may project as far as possible through the hole in the top of the shield. The grid terminal is insulated where it is likely to come into contact with the edge of the hole and the grid lead is wound into a spiral spring which makes a tight connection to the grid terminal. The last shield, placed at right angles to the first three, contains the audio beating oscillator. All the shields are connected together and brought to a binding post at the rear of the set marked " Ground." In this way effects of grounding the shields individually or collectively, the negative A alone, or shields and battery together, may be noted. Behind the oscillator shield there is the detector, filter, and the stage of audio amplification.