Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 ARMCHAIR CHATS ON SHORT-WAVE SUBJECTS 191 FIG. 3 thrill in fishing about among the short waves, uncertain of what one may land. TROUBLESHOOTING — IN ADVANCE IT MUST be repeated that there exists excellent * short-wave equipment. Roughly it divides into three classes which can be thought of as (A) complete receivers, (B) short-wave tuners designed for use in connection with the audio amplifier of a broadcast receiver, and (C) shortwave adapters designed to autodyne or heterodyne the short-wave into an existing r.f. or i.f. amplifier, which may then handle it in the usual way. Since good and bad examples of each exist, one arrives at rather bewildering conclusions, making it hard to choose correctly, especially as the price does not seem to have a direct relation to the efficiency of the device. Fortunately, it is possible to lay down some rules that will be of help. In types A and B we usually have a detector with a regeneration control. The coils may be of the plug-in type or a single coil may be used in the set. Before purchasing the set see if the regeneration control will cause the detector to oscillate over the entire tuning range of each coil. It is not serious if the oscillations fail below 15 meters or at the extreme ends (5 scale division or so) of other coils, provided there is enough overlap between coils so that this does not leave " blank" waves. Perhaps the best rule is to object if the set will not oscillate over the entire wavelength range without howling. The usual means of obtaining correct action should, of course, be tried. These include the use of a somewhat higher detector plate voltage and the use of several values of grid leaks between 1.5 and 8 megohms. If a means of loosening the antenna coupling is provided that also may be tried, and, if the tube is under suspicion, it may be changed. A reversed A battery will also cause trouble. If the set oscillates try the various coils in their sockets to make sure that a dependable contact exists. Finally, the tuning control and regeneration knob must operate smoothly, for, if there is any slipping or binding whatever, the tuner will be a constant aggravation. When the tuner is connected up it should be possible to operate both tuning and regeneration controls without a noticeable noise in the headset or loud speaker. One expects this from a standard-wave receiver as a matter of course, yet little attention seems to be paid to it by the makers of some of our short-wave jobs. Type C may be simply a detector oscillating feebly and tuned to shift the beat-frequency wave into some existing broadcast amplifying system of a normal receiver. It is somewhat harder to locate faulty action in such devices, since one cannot detect the action as easily by listening. Close observation will make it possible, however, since one can hear the usual "rushing" sound whenever the oscillator or autodyne is working properly into the broadcast receiver. The most frequent failure of these devices seems to be a tendency to squeal, occasioned by the over anxitty of the designer to insure oscillation on all wavelengths. Generally this can be cured by reducing the plate voltage of the oscillator or autodyne tube. Where it is not convenient to use a lower battery voltage one may connect into the lead from the battery a high-resistance rheostat (500,000-ohm) shunted by a 0.1-mfd paper condenser. TROUBLESHOOTING — AFTERWARD CUPPOSING that one already owns the set, ^ and finds that it must be made to work, several dodges are useful. First let us assume that there is difficulty in securing even oscillation— which means that one is also unable to secure smooth regeneration. If the coils are wound like Fig. 4A the difficulty probably lies right there, and rewinding the tickler with small wire, and bunching it as in 4B, will probably help materially. The number of turns can be determined by trial. Too many turns causes squealing, too few turns results in silence or feeble action. A fair rule is to start with § as many turns on the tickler as on the secondary. After a rough adjustment has been made try changing the grid leak and plate voltage in an attempt to secure smoother action, always making sure that each coil of the set continues to work. Generally it is the smallest coil that causes trouble. Having obtained fairly smooth action one may find that, when a signal is tuned in, the regeneration control cannot be adjusted without having to retune; this is often true in circuits of the type shown in diagrams c and d of Fig. 4. However, this tuning effect of the regeneration-control condenser, Q, can be reduced by moving the tickler coil, T, to the filament end of the grid coil, S, as in Fig. 4E, or if that is not convenient the control can be changed to the resistance type shown in Fig. 4F. In diagram f the condenser C2 can be set for approximately the correct action and thereafter R is used with little or no tuning effect. Unless one wishes to lose the calibration ("log S T ging") C2 must then be left alone. R must be noiseless in operation, and, so far as I have been able to determine, the Frost resistor is the only one which meets this requirement for any length of time. Wire-wound or step-by-step devices are hopeless, as are the compression types. The drive for C2 (if used) may be through a plain dial, and in no case should a high-ratio "vernier" dial be used. The drive for Ci should be the smoothest available. I have never found anything as beautiful in operation as the old-type National dial. Lest it be thought that undue emphasis is given the subject of dials one must consider that most short-wave tuners attempt to go from about 15 to 200 meters with perhaps 4 coils. In effect this means that one must cover about 20,000 kc. in going across the scale 4 times, as against some 1000 kc. for the scale of the usual broadcast receiver. With 5 times as cramped a scale, smooth operation is imperative. As regards the type C devices which heterodyne the signal into an existing i.f. system, the combinations here become so numerous that one can hardly lay down general rules. However, a hint may beof value. If theshort-waveheterodyne oscillator seems unwilling to work smoothly try converting the system into the circuit of 4F by connecting the primary of the first i.f. transformer to the points marked "output." The antenna may be connected to the grid end of the coil S (not to the grid but to the tuning condenser stator) through a small capacity of about 15 mmfd. (.000015 mfd.). A still better method might be to wind a single-turn primary around the lower end of the form on which S is wound. In either casethere may be "dead spots, and" this always suggests overdose antenna coupling or a bad choke coil. The former is remedied by reducing down the capacity of a "vernier" condenser in the antenna — even where the primary is used! The latter is a matter of cut and try. S I (F) 0.1 to 0.5 mfd RECEIVING CIRCUITS FIG. 4. SHORT-WAVE