Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 RADIO BROADCAST 277 No. 7. September, 1928 [ Radio' Broadcast's Service Data Sheets on Manufactured Receivers THE Stromberg-Carlson receivers Nos. 635 and 636 are self-contained, completely light-socketoperated radio receivers, the receiving apparatus and the complete power supply being mounted on one metal base and contained in a single cabinet. The No. 635 receiver is housed in a table mounting cabinet ; No. 636 is] a console model. The same apparatus and circuit is employed in both. Type 227 tubes are used throughout the receiver with the exception of the audio output tube, which is a type 171 tube, and the rectifier which is a full-wave type 280. The tuning system uses four stages, consisting of the tuned antenna stage followed by three stages of tuned and neutralized radio-frequency amplification. The coils are enclosed in aluminum shields and the variable condensers are all ganged to the single tuning control. The detector is the C-battery type, and the audio amplifier is a two-stage transformer coupled affair. The volume control consists of two variable resistors mounted on the same shaft and operated from a common control, one of the resistors controlling the amount of signal energy entering the r.f. amplifier and the other controlling the amount of energy admitted to the detector. The Nos. 635-A and 636-A receivers are designed to operate efficienUy on 105-125 volts, (50-60 cycle a.c; the Nos. 635-B and 636-B receivers on 105125 volts, 25-40 cycle a.c; and the Nos. 635-C and 636-C receivers on 210-250 volts, 40-60 cycle a.c. The receiver is equipped with a switch to compensate low and high line voltages. With voltages lower than those recommended for efficient operation of the receiver, the receiver may opera e satisfactorily, but the amplification will be decreased and the tone quality impaired; and with voltages greater than those recommended, the life of the tubes will be somewhat shortened. TECHNICAL DISCUSSION 1. The Tuning System. The antenna stage of the receiver is tuned, which makes the set more sensitive than receivers of the same type employing an untuned antenna system. One control operates all of the tuning condensers, C, simultaneously, accurate electrical alignment of the various stages being obtained by the use of small midget condensers, Ci, in each stage connecting across the main tuning condensers; these latter condensers are adjusted at the factory. 2. Detector and Audio System. Plate rectification or "grid bias" method of detection is used, followed by two stages of highquality transformer coupled audio amplification, the ratio of the first transformer being 4:1, and that 5 pBfew — 1 MODEL of the second being 2.8:1. The secondary of each audio transformer is shunted with a 1.0 megohm resistor to obtain the desirable audio-frequency characteristic. The output of the audio system is coupled to the loud speaker by means of a 60henry choke and a 2.0-mfd. condenser A correctly designed "high-frequency cut-off" audio filter is included in the audio output system. A phonograph pick-up jack is provided in the front panel of the receiver, which, upon insertion of the phonograph pick-up plug, connects the output of the pick-up to the audio system in place of the detector output of the receiver. 3. Volume Control. The volume control consists of two separate units operated simultaneously by the same control knob. The primary of the antenna coil has a 10,000ohm potentiometer, Ri, shunted across it, with the variable contact grounded. This controls the amount of signal admitted to the radio-frequency amplifiers. The second unit is a 10,000-ohm variable resistor, R2, shunted across the primary of the third radio-frequency transformer. This controls the amount of signal admitted to the detector. 4. Filament Circuits. The heaters of the three radio-frequency and first audio tubes are connected in parallel, but with separate twisted pair connections to each tube from the power transformer secondary, which supplies approximately 2.3 volts to each of these tubes. The power transformer secondary which supplies these tubes is provided with a grounded center tap for hum balance. A separate secondary of the power transformer supplies approximately 2.3 volts to the detector tube heater; this results in a better condition for suppressing hum. A 10ohm potentiometer, R3, with its variable contact grounded for hum balance, is shunted across the current supply to this tube. The filaments of the audio output tube and the dial light are connected in parallel and are supplied with approximately 4.5 volts from the power transformer. A 20-ohm potentiometer, R4, with its variable contact grounded for hum balance, is shunted across this current supply. 5. Plate Circuits. The plates of the radio-frequency and first audio tubes are supplied with approximately 110 volts d.c. from the power equipment. The detector plate is supplied with approximately 36 volts d. c. and the audio output tube is supplied with approximately 180 volts d. c. from the power equipment. The radiofrequency and first audio plate supply is "bypassed" to ground by a 3-mfd. condenser, C2, which is contained in the power equipment, and the plate supply to the radio-frequency tubes is "bypassed" to the cathode at each radiofrequency tube by a 0.5-mfd. capacitor. The detector plate supply is "bypassed" to ground by a 3-mfd. capacitor, C3, which is contained in the power equipment. 6. Grid Circuits The grids of the radio-frequency and first audio tubes are biased negatively approximately 5 volts with respect to the cathodes, by means of a 1500ohm resistor connected between each cathode and ground. The detector grid is biased negatively approximately 3.5 volts with respect to the cathode, by means of a 10,000-ohm resistor connected between the cathode and ground. These biasing resistors are "bypassed" by 0.5-mfd. capacitors in the radio-frequency stages, and by a 1-mfd. capacitor in the detector stage. This prevents the resistors from having any effect on the radio frequency current and insures stability. The power equipment supplies approximately 40.5 volts negative bias to the grid of the audio output tube. 7. The Power Supply The power transformer consists of a primary winding and secondary windings which supply the radio-frequency and first audio tube heaters, the detector tube heater, the audio output tube and dial light filaments, the rectifier tube filament, and the high voltage which is rectified and filtered to supply the plate voltage for the receiving tubes. The grid bias voltage for the audio output tube is also supplied from this source. The primary circuit of the power transformer is provided with a "HiLo" switch, which compensates for high or low line voltages. Full-wave rectification is accomplished by one ux-280 Radiotron or cx-380 Cunningham rectifier tube, and the filter system consists of a 5°-henry choke, a 3-mfd. condenser and a 10-mfd. condenser. A voltage divider connected across the output of the rectifier and filter system supplies the correct voltages to the radio-frequency, detector, and audio tube plates and to the grid of the audio output tube. ;770ohm 1 Diai ' Lamp THE RECEIVER CIRCUIT