Radio Broadcast (May 1928-Apr 1929)

"Our Readers Suggest 7:. : ii: ii:ii:!i:iii;mi:ili[llllll!ll[l!llllllll!llll!illlll A Simple Short-Wave Plug-in Coil AN OLD tube base makes an excellent wind/-\ ing form for short-wave coils. A set of « coils, covering the entire frequency gamut between 15 and 200 meters, is easily made, and may be plugged into the standard ux socket. Small holes are drilled into the sides of the tube base (after removing glass and cement) to permit the passage of the wires, which are soldered to the socket prongs. The wire may then be wound tightly in the conventional solenoid form or, if the tube base is grooved on a screw cutting lathe, a space wound coil may be made. If the threading tool is ground to a blunt point twice the diameter of the wire (generally No. 26 or 28), three wires can be wound in the same slot, as suggested in Fig. 1, with only a small increase in the distributed capacity of the coil. This is the familiar single bank winding. Everett Freeland, Dowagiac, Mich. STAFF COMMENT A THIRD or tickler coil can be wound, tied together with thread, dropped within the tube base, and secured with a bit of wax. All terminals can be brought out to the four prongs, in accordance with the circuit of Fig. 2 — a parallel feed tickler feedback arrangement. The following table gives the correct number of turns of wire for satisfactory tuning in the shortwave bands with a 0.00014-mfd. condenser: No. of Turns Primary Secondary Tickler 2 4 5 2 610 15 Wave Band 20 meters 40 meters 75 meters '5 A A High-Resistance Voltmeter HIGH-RESISTANCE voltmeter is essen OUR Readers Suggest" is a clearing house for short radio articles. There are many interesting ideas germane to the science of radio transmission and reception that can he made clear tn a concise exposition, and it is to these abbreviated notes that this department is dedicated. While many of these contributions are from the pens of professional writers and engineers, we particularly solicit short manuscripts from the average reader describing the various "kinks," radio short cuts, and economies that he necessarily runs across from time to time. A glance over this "Our Readers Suggest" will indicate the material that is acceptable. Possible ways of improving commercial apparatus is of interest to all readers. The addition of an extra stage of power amplification, is a good example of this sort of article. Economy "kinks," such as the home-made power unit, are most acceptable. And the Editor will always he glad to receive material designed to interest the experimental fan. Photographs are especially desirable and will be paid for. Material accepted will be paid for on publication at our usual rates with exira consideration for particularly meritorious ideas. — The Editor. put of a power supply unit. These instruments are generally rather expensive. The experimenter can occasionally save himself some money by converting a burned out thermo-galvanometer or ammeter into a high-resistance voltmeter. It is very unusual for the coil in a thermometer, such as the Weston No. 425, tO burn OUt. Gener Space /or Primary ally, it is the thermo-couple that is fused, rendering the instrument useless as far as its original purpose is concerned. However, by removing the burned out couple, and placing a high resistance in series with the coil, an excellent highresistance voltmeter will be had. It is necessary, of course, to recalibrate the scale to read in the correct number of volts. A resistance of 100,000 ohms connected in series with the coil of a Weston type 425 instru FIG. I Jim i " ^ FIG. 2 ment is about correct for full scale deflection at three hundred volts. Capt. R. B. Moran, Fort Monmouth, N. J. Testing Filter Condensers THE usual method of testing filter condensers by observing the size of the spark on discharge is only approximate and is often misleading. A condenser that is in poor condition and which leaks badly will usually give a spark on discharge, leading one to believe that the condenser is in good shape. Anyone havinga high-resistance voltmeter may use the following method £with reliable results. The condenser to be tested is charged from a d.c. source, either 90 volts of B battery or the 90-volt tap on a power unit. After waiting an arbitrary length of time — one minute, for instance — the condenser is discharged across the voltmeter terminals and the "kick" of the needle noted. It 'Twice Wire Dia. may be necessary to repeat this test, the fust operation giving the approximate scale reading; then, focusing the eye on this portion of the scale, the second reading may be obtained accurately. This will allow comparison between different condensers and will show up a poor one. Where a number of condensers are to be tested a clip connection will be found convenient. Care should be taken in handling the connecting wires to prevent the charge from leaking off through the resistance of the body before it is discharged across the voltmeter. When using a 90 volt charging source and a Weston voltmeter, 0-150 volts scale, havinga resistance of 13,500 ohms, it was found that high-grade condensers in good condition gave a needle swing on discharge of about 5 volts for each microfarad of rated capacity. D. C. Redgrave, Norfolk, Va. A Grid-Plate Detection Switching Arrangement DADIO BROADCAST has devoted considerable attention to the possibilities of plate or bias rectification in the detector circuit, stressing its advantages and disadvantages in comparison with the more conventional gridcondenser system. While the plate system reduces overloading on local stations, its lack of sensitivity does not recommend it for dx reception. A switching arrangement, permitting instantaneous change from one method of rectification to the other is obviously the ideal arrangement. The writer has evolved the simple circuit shown in Fig. 3. A double circuit jack is employed to effect the change. The jack is mounted on the operating panel close to the detector tube, and functions in a push-pull fashion by means of bakelite rod J" in diameter, fitting the jack like a phone plug. Two small cotter pins limit the motion of the pin. (A Yaxley type VM switch may be substituted for the improvised jack idea.) The insulation on the jack should be of the best, and no paste or acid flux should be employed in making the soldered lead connections. There is no difference in the tuning dial readings with either system of rectification. Anytype of detector tube can be used, the bias battery varying between 1.5 and 6 volts, depending 271