Radio Broadcast (May 1928-Apr 1929)

No. 18. Radio Broadcast's Service Data Sheets February, 1929. Browning-Drake Receiver Models 34, 36, and 38 THE Browning-Drake Model 34 receiver is a completely self-contained table-model a.c. set designed for 105-120 volt 50-60 cycle current. In a consolette cabinet with a dynamic or airchrome speaker the receiver is known as Model 36D or 36A, and in a highboy console it is listed as Model 38. A noteworthy feature is the symmetrical mechanical construction, all power equipment forming an integral part of the chassis, which is of aluminum and which is perfectly balanced in all dimensions. The seven tubes and rectifier are all mounted along the rear of the chassis, adjacent to the shielding partitions, and form the basis for the description " Eight-in-Line" applied to this receiver in advertising. The amplifier tubes are all of the 226 type, with the exception of the last stage, which is a 171a. The detector is of the 227 type. A 280-type fullwave rectifier is used. The coils are at right angles to each other and are mounted on a bakelite strip beneath the base. The variable condensers are ganged and operated by a knob controlling a large drum. A special mechanism prevents slack, the two driving cylinders being joined by a phosphor bronze spring and connected to the drum by a beaded chain. Even illumination is provided by a miniature lamp in back of the scale, which rotates behind a recessed escutcheon plate. The cabinet work of the 1929 models is confined almost entirely to walnut with Duco finish. Technical Discussion 1. THE R.F. TUNING SYSTEM The antenna stage of the receiver is of the untuned type with a 1000-ohm variable resistor, Ri, connected between the grid and filament of the first tube. This unit is controlled fr< m the front panel and constitutes the volume control. The second and third radio-frequency stages and detector are tuned through a single dial. Perfect alignment is secured through the use of small compensating condensers, C3, C4, and d, across the main tuning condensers, Ci, C2, and C3. The compensating condensers are accurately set at the factory and are not accessible in the cabinet models. 2. THE DETECTOR AND AUDIO SYSTEM The audio amplifier employs a three-stage resistance-coupled circuit. Type 226 tubes in the first, two stages and a type 171a tube is employed in the output stage. Grid detection is used in the Model 34 receiver. The leak, R2, has a resistance of 8 megohms and the condenser, Cs, has a capacity of 0.00005 mfd. In the audio amplifier, plate resistors, R3, of 100,000 ohms are used, while the grid resistors are 500,000 ohms, R4, for the first stage and 100.000 ohms, R5, for the second and third stages. The coupling condensers, C7, in the amplifier have a capacity of 0.1 mfd. 3. FILAMENT CIRCUITS The filament supply system consists of five separate windings on the power transformer, T. The supply, Si, for the 280-type rectifier is center tapped on the winding. The 1.5-volt winding, S2, for the first two audio amplifier tubes, the 5.0-volt supply, S5, for the power amplifier, the 2.3-volt filament winding. Si, for the detector, and the 1.5volt r.f. filament supply, Se, are shunted with resistors to get the center point. Through the use of separate windings, for the r.f. and a.f. tubes, tendency toward hum is reduced greatly. Each of the windings. Si, S2, S5, and Si;, is provided with a 60-ohm center-tapped resistor, Rs, accurate to less than 0.5 per cent . Inasmuch as leads are short and the windings are separate, the use of such accurate center-tap connections helps greatly in the elimination of hum. The 1.5-volt radio-frequency supply is bypassed at. the sockets by one. 0.1 mfd. condenser. C», on each side of the f lament. This prevents common coupling in the center-tapped resistor and aids in the elimination cf 60 or 120-cycle modulation of the incoming signal. 4. PLATE CIRCUITS The plates of the radio-frequency tubes are supplied with 140 volts d.c. from the power supply and draw a plate current approximately from ten to twelve milliamperes. The detector plate is supplied with 20 volts and the current drawn in this circuit is one milliampere. The audio amplifier tube plates are furnished from the 200-volt source in the power supply. The voltage, drop in their respective plate resistors reduces the plate voltage on the first audio tube to 60 volts and that on the second amplifier to approximately 75 volts. The plate current in each case is between 1.5 and 2.0 milliamperes depending upon the tube. The full voltage of the supply, 220 volts, is impressed on the power amplifier plate. The actual voltage across the tube is less than this value, however, by the amount of the grid-bias voltage as is indicated later. The plate current in the power amplfier tube is 20 milliamperes. 5. GRID CIRCUITS The radio-frequency amplifier grids are connected to ground or to the chassis frame through their respective r.f. transformer secondaries. The r.f. bias is applied between the filaments and ground and is secured by the IR drop through a 1000-ohm resistor, R7. The negative bias thus applied to the r.f. grids is from ten to twelve volts. The detector grid is operated at cathode potential, no biasing being required for grid detection. The audio amplifier tube grids carry a negative bias of from 4 to 5 volts furnished by the IR drop across the 500-ohm resistor Rs. The power amplifier grid is biased in the same manner, the plate current to the tube passing through a 2000ohm resistor, R9, in the filament circuit. This resistor is a part of the voltage divider and is bypassed with a 1.0-mfd. condenser, C9. The value of this bias voltage is approximately 40.5 volts. 6. THE POWER SUPPLY The power supply contains a power transformer, T, a 30-henry filter choke, L, a triple-section Mershon filter condenser, C10, and a voltage divider, Rio. The power transformer has a single primary winding, a high-voltage secondary, S3, having 300 volts on each side of the center tap, a 5-volt center-tapped windng, Si, for the rectifier, two separate 1.5-volt windings, S2 and S6. for the a.f. and r.f. amplifiers, a 5-volt winding, Ss, for the power amplifier, and a 2.3-volt winding. Si. for the detector. The 2.3-volt winding supplies the dial light as well as the detector. Full-wave rectification with a 280-type tube is employed. The output of the rectifier is passed through the 30-henry choke with 8 mfds. on each side. This filtered d.c. is then carried to the voltage divider where taps are taken off for the 140-volt supply to the radio-frequency plates and 45 volts for the detector heater. The 140-volt tap is bypassed with the remaining 8-mfd. section of, the Mershon condenser. The power transformer choke, and filter condenser are all operated well under their normal rating so that excellent filtering and freedom from trouble are assured. CX-371A UX-171-A Circuit Diagram of Receiver and Poirer Unit. • february, 1929 . page 266 •