Radio Broadcast (May 1928-Apr 1929)

THE RECEIVING EKSEMBLE At the left is the receiver itself; at the right is the amplifier -power unit designed for use with the receiver, and the other receiving accessories A Dual Control A* C. Receiver By Robert Burnhaiin SOME interesting features have been incorporated in this dual-control receiver employing jour stages of tuned radio-frequency amplification. The points of major interest are: Two of the radiofrequency transformers are arranged with adjustable primary windings so that the selectivity of the receiver may be adjusted to the conditions under which it is to be operated; an extra socket is placed in the receiver into which a phonograph pick-up may readily be placed; a combined power amplifier and B-supply unit has been constructed which is designed especially for use with the set and, although standard type cx-jo/A (ux-20/a) lubes are employed, the receiver may be made completely light socket-operated by using an A power unit in conjunction with this B-power unit. — The Editor THE problems associated with the construction of a radio receiver are many, and the final design is generally a compromise between various factors, such as the gain, type of tuning control, etc. The final design must be such as to produce an easily operated radio receiver with a reasonable amount of sensitivity and capable of giving good quality signals from any stations within a reasonable distance of the location in which the set is operated. This has been the mode of approach in the design of the receiver described in this article. The complete receiver contains three tuned stages of radio-frequency amplification followed by a non-regenerative detector and a two-stage audiofrequency amplifier, transformer coupled. The first two tuning condensers are ganged to one tuning dial and the third and fourth tuning condensers are ganged to a second dial. These are the two major tuning controls. Two vernier condensers are connected across the second and fourth tuning condensers so that accurate tuning may be obtained when a comparatively weak signal is being received. In the reception of local stations it is necessary to adjust only the two main tuning controls and the volume control. The receiver is quite selective. The only additional control on the panel is the filament rheostat, which is used in conjunction with the filament voltmeter to adjust the filament voltage applied to the tubes to the rated value of five volts. The ioo-volt voltmeter on the panel is also useful in obtaining and checking the proper operation of the receiver, but it is not absolutely essential and may be omitted if the constructor so desires. The receiver is shielded by means of the three aluminum shields. The first stage of audiofrequency amplification is located in the receiver proper and the second or power stage uses a 210 type tube and is a part of the combined power amplifier and B supply. A choke-condenser output circuit is used to keep the d.c. plate current of the 210 tube out of the loud speaker circuit. In the radio frequency, detector, and first audio sockets 21 OA type tubes are used and filament current for these tubes can be supplied from an A-power unit so that the entire installation is light socket operated. An additional socket is placed in the receiver so that a phonograph pick-up can be used. THE CIRCUIT OF THE RECEIVER A CAREFUL inspection of the circuit diagram in Fig. 4 will bring out some interesting points regarding the r.f. amplifier. In the first place, two of the r.f. transformers are arranged with primary windings whose coupling to the secondary may be readily varied. As the coupling is decreased, i.e., as the spacing between the primary and secondary is increased, the selectivity is increased; in this manner adjustments may be made to suit the conditions under which the receiver is to be operated. A second point of interest about the r. f. ampli i::'i'!il!l"i'ir!: "r;r II LISTS OF PARTS Karas 0.00037-mfd. variable condensers FOR THE RECEIVER £* iH ! R. F. transformers, Hammarlund Type V ' J"4 ! R. F. transformers, Hammarlund Type ' U 1 HQ-1 L9 — Hammarlund 85-millihenry r.f. choke Ci, C2, Cj — Carter 0.5-mfd. bypass condensers C4, Cs C6, C7 Cs, Ci — Hammarlund midget condensers, Type MC-5 C10— Muter 0.002-mfd. fixed condenser Cn — Muter 0.0005-mfd. fixed condenser C12— Muter 0.00025-mfd. fixed condenser C13 — X-L Variodenser, Type N Ri, R;— 1,500-Ohm Yaxley grid resistors, Type 71500 R2~1000-Ohm Yaxley grid resistor. Type 71000 Ri — Muter 6-ohm rheostat R5 — Electrad 2000-ohm variable resistor. Type F Rg — Muter i-ampere Tubestat R? — Muter 3-megohm grid leak Ti — Karas audio transformer. Type 28 Vi — Filament voltmeter, 0-8 volts, Jewell Type 135 V>— Plate voltmeter 0-100 volts, Jewell Type 135 3 Karas brackets 4 Alcoa shields Front panel 7" x 24" x TV' — Celoron Sub-panel 10" x 23" x fV' — Celoron 2 National illuminated vernier dials 1 Belden 7-wire fused cable ^Hl!:;':,::; :r;::,: 2 condenser shafts { inch diameter, 10 inches long Belden Colorubber hook-up wire 6 Benjamin base mounting sockets FOR THE POWER UNIT Ci, C2, Cs, C4, Co — Muter power condenser blocks. No. 598 Cs— Muter 0.5-mfd. condenser. No. 507 C? — Muter 2-mfd. power condenser, 600 Volt Ti — Power transformer ^ Both contained in I Thordarson Power Li, L2 — Filter choke coils ( Compact, Type 7 R-210 T2 — Karas audio transformer. Type 28 L3 — Muter audio choke, No. 3130 Ri — 1500-ohm Ward-Leonard resistor R2 — 150-Ohm Centralab variable power resistor Rj — Heavy-duty high-range Clarostat Rs, Rs— Muter 10,000-ohm resistors, Type 2910 8 Eby binding posts 2 Benjamin sockets Hook-up wire The following apparatus is also necessary to make the receiver operative: 5 CX-301A tubes 1 CX-310 power amplifier tube 1 CX-381 rectifier tube Loud speaker such as the Magnavox Dynamic Cone A-Power unit such as the Abox Frost 100-foot antenna wire, with ground lead and clamp 192