Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 A NON-RADIATING SHORT WAVE TUNER 287 standard condensers, is quite noticeable at the short wavelengths. A final innovation in the condenser design, is a constant impedance pig-tail which eliminates variations in dial settings due to pig-tail inductance variations when the old style of "clock spring" contact is used. The r.f. choke coil, Gi in Fig. 2, used in the detector plate lead, is of the multi-section slotwound variety having very low distributed capacity over a wide band of frequencies. The other r.f. choke, or grid circuit impedance, G>, is one especially designed for the purpose and has an inductance of approximately 2 millihenries. Perhaps it would be well to caution at this time against the use of the wrong size filament resistor, Rj, for the ux-222 tube. This resistor should be of the 15-ohm size and not 22 ohms. Due to the type number of the tube, ux-222, and the practice of some resistor manufacturers of marking their 22-ohm units as type 22, many experimenters have assumed that a "type 22" resistor is the proper one to use with a ux-222 tube under any conditions. COIL DATA THE four coils used in this set have wavelength ranges as follows: Type A — 15.5 to 26.5 meters. Secondary, 4 turns of No. 14 enameled wire; tickler, 2 turns of No. 30 d.s.c. wire; primary, 3 turns of No. 28 enameled wire. Type B — 23.5 to 41 meters. Secondary, 7 turns of No. 14 enameled wire; tickler, 2 turns of No. 30 d.s.c. wire; primary, 6 turns of No. 28 enameled wire. Type C — 37.5 to 65 meters. Secondary^ 14 turns of No. 14 enameled wire; tickler, 3 turns No. 30 d.s.c. wire; primary, 14 turns No. 28 enameled wire. Type D — 64 to 115 meters. Secondary, 25 turns of No. 18 enameled wire; tickler, 4 turns No. 30 d.s.c. wire; primary, 25 turns No. 28 enameled wire. All of the coils are wound on 2 inch tubing. The secondaries of the first three types are spaced 8 turns to the inch; the secondary of D coil is spaced 14 turns to the inch. The tickler is wound in a slot f" below the filament end of the secondary and the primary is wound in the spaces between the secondary turns. CONSTRUCTIONAL NOTES BY CAREFULLY studying the illustrations, one will readily see how to mount all of the parts on the foundation unit. All holes are drilled and the work is very easy. As soon as the assembly is completed the wiring may be done. If the wiring diagram is carefully followed it is quite simple for anyone to obtain a very neat job. Needless to say, all connections should be carefully soldered. The two moulded mica condensers located under the subpanel, are fastened in place by soldering their terminals directly to the socket and coil clips between which they are connected. In order to make contact to the cap or control grid of the 222 tube, use a short length of small, flexible, rubber-covered wire, or very fine single silkcovered wire, running in a piece of small spaghetti and ending in a fuse clip or similar homemade clip, to snap on at the top of the cap. G' Output o Note: In the above diagram the lower lead from choke G2 should conned with the left filament lead of the 222 tube. +135 +45 + 67 B -A +A FIG. 2. THE CIRCUIT DIAGRAM AUDIO AMPLIFIERS FOR headphone reception, it is recommended that a single stage of transformer-coupled audio amplification be added to the output of the receiver as just described. While such an additional amplifier is not at all necessary, it will be found of considerable aid in receiving distant and weak signals. When loud speaker operation from such shortwave broadcasting stations as kdka, wabc, etc., is desired, then the use is recommended of a high grade two-stage transformer-coupled audio amplifier of either the straight or push-pull variety. LIST OF PARTS ^|"HE complete parts, together with the * foundation unit, may be obtained in kit form. However, all the parts, with the exception of the coils, are of standard des gn, and other equivalent parts may be substituted. The coil data is given in the text. A — 4 National short-wave transformer coils covering the range of 15 to 115 meters E — 1 National dial, type E, with type 28 illuminator OUTPUTS A A+ A B+135 B+67 FIG. 3. ABOVE THE SUB-PANEL F — 1 National condenser, short-wave type, 125 mmfd. Gi — 1 National r.f. choke, No. 90 G2 — 1 National, h.f. impedance, No. 10 Mi — 1 Aerovox molded mica condenser, 0.001 mfd. M2 — 1 Aerovox molded mica condenser, 0.00025 mfd. M3, M4 — 2 Aerovox bypass condensers, 0.5 mfd. M5 — 1 Aerovox bypass condenser, 1.0 mfd. Ri — 1 Electrad Royalty resistor, type L, 0-500, 000 ohms R2 — 1 Filament resistor, 2 ohms R3 — 1 Filament resistor, 15 ohms R4 — 1 grid leak, 6 megohms S — 1 Yaxley filament switch 8 Eby binding posts 1 Foundation Unit, including Westinghouse Micarta panels, sockets, gridleak and r.f. choke mounts, completely drilled, ready to assemble ux-222 and UX-201A tubes OPERATION OF THE RECEIVER AT THIS time, the writer feels that socket■<"»• power units are not suitable for use with a short-wave receiver. It is necessary for satisfactory results, therefore, to employ 135 volts of dry or storage B battery, in addition to the usual 6-volt storage A battery. If an audio amplifier of some kind is not to be employed, then a pair of phones should be connected to the "output" posts on the right-hand side of the subpanel. A good ground may be connected to minus A. In some instances, however, better results are obtained without the use of a ground. For an antenna it is recommended to use a single wire of from 35 to 100 feet in length and as high and free from surrounding objects as possible. By means of the variable resistance regeneration control (righthand knob) the detector tube may be made to oscillate, and then the R3 carrier of the station received. A slight readjustment of both con^2 trols should then bring in the station. The tuning of a short-wave receiver is a much more critical process than that of a standard broadcast receiver, and unless M4 care is Used, the novice is likely to pass right by a station. Station 5SW at Chelmsford, England, can generally be heard between 5 and 6 P.M. in Boston when using transformer B and with the dial set at about 3 1. ANT B+45