Radio Broadcast (May 1928-Apr 1929)

122 RADIO BROADCAST DECEMBER, 1928 No. 14. December, 1928. St'« The Fada 50, 70, 7/, 72 A. C. Electric Radio Receivers THE Fada receivers, models t>0, 70. 71 and 72, all use the same chassis so that in operation and in technical characteristics they are similar. The model 50 is a table model, the 70 is housed in a low-boy console, the 71 in a high-boy console, and the 72 is contained in a console which also contains a phonograph turntable and pick-up unit . The receivers are of the single-control type, the station finder being calibrated in wavelengths and also in degrees. The sets may be operated on either a loop or antenna. The models 70, 71, and 72 are equipped with a new Fada model-14 dynamic speaker. TECHNICAL DISCUSSION 1. Tuning System The tuning system used in these receivers comprises four r.f. transformers and tuning condensers and include of LiO, L2C2, L3C3 and L.1C4. The four tuning condensers are ganged to one control and are operated by a single drum dial. Between the tirst and second r.f. tubes is placed a special untuned radio-frequency transformer designed with a gain-frequency characteristic essentially opposite to that of an ordinary tuned radio-frequency transformer; the result of using such a transformer is that the gain-frequency characteristic of the entire receiver is made much more uniform than it would otherwise be. All of the stages are neutralized by the Hazeltine method. The neutralizing condensers are C*. Ce, C7 and Cs. Each radio-frequency transformer is enclosed in a shield in order to prevent interstage coupling. The tuning condensers, C>, C3 and Ci, are shunted by small midget condensers which are adjusted at the factory to bring each stage into exact resonance. 2. Detector and Audio System A grid-leak-condenser-type detector is used, Cs being the grid condenser and Ri the grid leak. The output of the detector is bypassed to ground by the fixed condenser Cm, and, in order to keep the radio-frequency currents out of the audio system, the r.f. choke coil, L5, is placed in the detector plate lead. The audio amplifier is a two-stage transformercoupled affair with a phonograph jack placed across the primary terminals of the first audio transformer. An interesting point about the audio amplifier is that the d.c. plate current of the first audio tube is kept out of the primary of the input push-pull transformer, T2, by means of the fixed condenser C9 and the audio-frequency choke coil, Le. The removal of the direct current from the transformer winding eliminates the possi Loop MODEL 72 bility of saturation in the core which would lower the inductance and cause a loss in amplification at low frequencies. 3. Volume Control The volume control, R2, is a variable high resistor connected across the secondary of the untuned radio-frequency transformer. In this position it serves to control the amount of energy fed into the remainder of the r.f. amplifier and detector system. 4. Filament Circuits Since 227-type tubes are used in all the sockets of this receiver with the exception of the power stage only two filament windings are necessary on the power transformer. One of these windings supplies approximately 2.5 volts to the heaters of all the 227-type tubes and the other winding supplies current to the power tubes in the push-pull amplifier. To prevent the r.f. currents in the amplifier circuits from circulating around the various cir cuits, choke coils, L?, Ls Ls, L10, are placed in the cathode leads to each of the four r.f. tubes. 5. Plate Circuits Filters are used in the plate circuits of all the r.f. tubes so that the r.f. currents are compelled to return directly to the cathodes of the 227-type tubes. If the filter systems were not used these currents would pass into the power system where common coupling would result. The filter systems in the r.f. plate circuits consist in each case of Ri, and C11. The filter system in the plate circuit of the detector tube consists simply of C12. 6. Grid Circuits C bias for the various grid circuits is obtained by connecting fixed resistors in series with the cathode leads in the case of the 227-type tubes and in series with the center-tap connection of the filamenttransformer winding in the case of the power tube. For bias on all the r.f. tubes a common resistor, Rj, is used. It is bypassed by C15. Bias for the first audio tube is obtained from R5 and bias for the power tubes is obtained from the resistor, Re, located in the power-supply device. 7. The Power Supply Two power-supply units are available for use with this series of receivers. The type E-420 is for use with 210-type power tubes and the type E-180 is for use with 17lA-type power tubes. The latter power unit is illustrated in the circuit below. This power unit consists essentially of the power transformer, T4, which supplies plate and filament voltages for operation of a 280-type full-wave rectifier tube and filament voltages for the receiver. The output of the full -wave rectifier leads into the filter system, consisting of the two filter choke coils, Ln and L12, and the filter condensers C13. The condenser, Cm, connects from side of the 110-volt line to the ground circuit and this condenser serves to bypass to ground any line noises which might otherwise be audible in the output of the receiver. Plate voltage for the power tube is obtained at the junction between the two filter choke coils, and plate voltages for all the other tubes in the receiver are obtained by connecting suitable resistors across the output of the filter system. The primary of the power transformer is arranged with two taps, one for use with line potentials of 110 to 130 volts and the other for use on line potentials of 90 to 110 volts. Power to the entire receiver is controlled by the switch, SW. The power units E-420 and E-180 are both available in two models so that the receiver may be operated from power lines with a frequency of 50-60 cycles or 25-49 cycles. C(Common)7 Amp. (6) Pwr(4) ' Fil.(3> THE RECEIVER AND POWER CIRCUITS