Radio broadcast .. (1922-30)

FEBRUARY, 1927 LOOKING FOR TROUBLE? 373 RADIO BROADCAST Photograph TESTING THE R. F. STAGES You can readily tell whether or not the radio frequency stages of your receiver are supplying notice- able gain over preceding stages by touching the antenna lead to the grids of the various r.f. tubes A pair of phones and a C battery are val- uable equipment for tracing circuits, especially in finding breaks in pigtail connectors and in testing the continuity of inductances. It is advisable not to undertake more than the superficial tests suggested in the set owner's home. Tracing all circuits with the aid of phones and battery, and detailed examination of the set, is a matter for the laboratory or repair shop. WEAK SIGNALS PROCEEDING to the receiver with weak * signals, the obvious and most frequent possi- bilities are run down plate power supply or worn out tubes. The only way to test the power sup- ply properly is with a voltmeter of adequate range. Tubes must be tested with a tube tester or else a complete set of borrowed tubes, known to be in good condition, substituted. If substi- tution remedies the set's weak volume, the cause of trouble has been determined as run down tubes. But there are other causes. Weak signals caused by a broken ground or antenna lead-in are always accompanied by exceedingly sharp tun- ing, both because of the removal of antenna cir- cuit resistance and the increased regenerative action therefrom. A break or disconnection in one of the grid circuits, a gang condenser slipped off tune, or a poor contact of a tube grid pin with its spring in the socket, are likely causes of weakened signals. WHISTLING WHEN reception is accompanied by a steady whistle, the nature of the whistle should be carefully analyzed before conclusions are reached. Determine first whether the whistle is entirely independent of tuning or whether it occurs only when the receiver is in resonance with an in- coming signal. If it occurs all over the dials, a likely cause is a defective grid leak or a reversed or run down C battery. On the other hand, if it occurs only with a certain station, the cause probably lies outside of the receiver. Heard only when in resonance with a station, especially at the lower end of the dial scale, it may be due to the fact that the receiver has become regener- ative. Radio frequency receivers of all types depend for absence of regeneration upon some method of balancing out or resisting the tendency toward regeneration. These systems may be upset by large changes in coupling of the plate potential source, whether of the B battery or line supply type. Since the B battery is common to all plate circuits, they are coupled through it. A run-down B battery, or a softened rectifier tube in a line power-supply device sometimes causes a shrill whistle. Sometimes a neutralized receiver becomes re- generative even though the plate potential source is in good shape. This may be due to changes in tube characteristics or in the adjustment of neu- tralizing condensers. By reducing the filament brilliancy gradually when tuned to a near-by high-power station, the signals may become clear although weak after a certain point in filament reduction. This indicates that the receiver needs re-neutralization. A dead C battery sometimes makes a terrifying and omnipresent whistle. A whistle which discontinues entirely when the detector tube is removed is very possibly caused by a defective grid leak. Moisture and temper- ature conditions sometimes cause grid leaks to deteriorate. Clicks when a receiver is touched or jarred slightly, indicate a loose wire or a defective tube. Observe whether the click has a ringing quality or whether it is fairly regular and sharp. The ringing quality generally indicates a microphonic tube while harsh clicks accompany broken wires or loose connections. When tracing connections examine first the battery leads and loud speaker cord, then remove and restore the tubes one at a time to confine the break to a single tube circuit. If you find the receiver silent when all the tubes are in except one, the chances are your break is in that particular tube circuit. An exception is the output tube of the receiver, trre removal of which silences clicking because there is no plate current through the loud speaker circuit. Its functioning can be checked through the phone jack. If reception is quiet and satis- factory through that jack, the break lies in the amplifier or its special power supply. Steady clicks experienced with mathematical evenness, unaffected by "jiggling" the receiver, are due, most frequently, to defective grid leaks. When looking for a broken wire, first check the power input connections, antenna lead-in, ground wire, and examine the leads to the rotors of the variable condensers, variometers, and couplers. A click heard at a certain position of the tuning dial is obviously caused by touching condenser plates or weakened rotor connections at that particular adjustment. The essential equipment of the trouble hunter includes a voltmeter covering the full range of A and B battery voltages, some means of testing tubes or a set of replacement tubes for compara- tive purposes, pliers, screw driver, a set of head- phones, and a flashlight for examination purposes and its use in supplying a convenient means of testing completeness of circuits with headphones in series with one of the cells. RADIO BROADCAST Photograph S TESTING THE PLATE CIRCUITS One way to determine whether the various plate circuits of the audio channel are complete is to unfasten one by one the B battery leads from their binding posts and touch them against the posts. If the circuit is complete a click will be heard each time in the loud speaker