Radio Broadcast (Nov 1926-Apr 1927)

562 RADIO BROADCAST APRIL, 1927 outside ends and using this connecting point as the center tap of the loop. At the original center tap the continuity of the turns of the loop is broken thus providing two leads which may be fastened to the terminals of the plug. Fig. 7 illustrates this point. SHIELDING A SET may gain a great deal by partially shielding it. It is probable that even sticking a metal plate between the coils, or condensers, of a set does some good. At least the inclusion of the plate must distort the field between the apparatus on its two sides in such a manner that some interapparatus coupling is eliminated, and therefore it may be argued that the metal plate does some good. It is certain that surrounding the coil, condenser, and tube of the radio-frequency amplifier of the "Lab" circuit with a tightly fitted metal box isolates that circuit from the detector much better than any trick method of placing coils, and that the resultant freedom from unwanted coupling between amplifier and detector is worth every minute of the time involved. In the Laboratory, such shielding nas been made of aluminum, copper, or brass. On one occasion a copper box was made from a single piece of metal and every crack was soldered, so that the amplifier was as totally shielded as possible. Wires came into it through very small holes, and there was provision for grounding the shield, or letting it hang "high and dry," assuming any desired potential with respect to earth. In every case the shielded receiver had a marked freedom from hand capacity, a much greater freedom from unwanted pickup from strong local stations, and, when, equipped with plate voltage filters consisting of condensers and chokes as shown in Fig. 8, was remarkably stable and easy to operate. With a loop input, the detector unit should be shielded as well as the r. f. amplifier if the best possible results are desired. In this case there will be a minimum of feedback from the detector coil to the loop. A complete metal panel will be much better than a metal plate behind only the antenna tuning condenser. TROUBLE SHOOTING THE following paragraphs are not for those unfortunate members of the home constructors society who, like Voltaire's Doctor Pangloss, believe this to be the best of all possible worlds — all because they, personally, never make a mistake. Rather, these few words are for ordinary mortals like ourselves who are occasionally faced with a receiver that is surprisingly silent. In the first place, one should take courage for, as Edgar Felix recently pointed out in Radio Broadcast, a receiver that seems dead may actually be in a state of coma only; like the Sleeping Princess, it may await the wakening hand. Let us suppose the four-tube receiver has been connected up — properly, of course — and that we have gone over the connections several times, but that no sounds emanate from the loud speaker. The first thingtodo is to test out the audioamplifier. It is a two-stage affair using Amertran DeLuxe transformers, and should have an overall voltage amplification of about 300. To test it, T i_ LOOP Original center tap broken apart To plug Center tap to pin jack plug FIG. 7 gently tap the detector tube when a resounding "bong" should come from the loud speaker, or from a pair of phones across the output. If there is no "bong," start from the loud speaker end and make certain that each of the two tubes gets plate voltage. A sharp click will be heard when the plate voltage is taken from either of the tubes. If the tubes get plate voltage, and if the connec tions are correct, shorting the input of the amplifier should also produce a click — which may require a pair of receivers to hear. Speaking into a pair of receivers connected across the input to the amplifier will make it possible to test the amplifier as a whole, the receivers acting as the pick-up device. If the loud speaker indicates that the amplifier reproduces the owner's voice, it proves that the amplifier is working properly. Then the receiver-pickup may be placed in series with the detector tube B-battery lead and again spoken into. This will indicate whether or not the detector gets plate voltage. If the amplifier works, and the detector has plate current, the next step is to attach the antenna on the plate terminal of the radiofrequency amplifier with that tube out of its socket. The receiver will then be a single-circuit blooper and carrier waves at least should be picked up. If signals are heard it proves that the final three tubes in the set are working properly. The antenna should then be placed on the inductance side of the neutralizing condenser. Signals should now be weak — but if they have the same intensity as before it proves that the neutralizing condenser is shorted, a fact that may not be apparent unless tested in this way. Shorting takes place occasionally in a compression type of neutralizing condenser and causes considerable trouble. The next step is to place the r.f. tube in its socket and to connect the antenna to the grid of that tube. If signals are now received the antenna may be connected to its proper terminal. With local stations it often happens that signals will be heard even though the r. f. tube is not getting plate voltage. Under these conditions, adjusting the neutralizing condenser will seem to have little effect and signals will be weak since there is no gain in the amplifier tube. It is even possible to receive signals if the detector gets no B battery voltage. If there is difficulty in neutralizing the amplifier tube, plate voltage filtering will aid. The lower part of the plate inductance should be bypassed back to the amplifier negative filament lead and the B-battery voltage fed through another radio-frequency choke. It is also well to filter the detector plate voltage, thereby making the radio-frequency circuits short and direct, and keeping the r.f. currents where they belong. Fig. 8 shows where these bypass condensers are located. They may be anything from 0.0 1 to 1 mfd. in capacity, and probably o. 1 mfd, will doall that we can expect. The choke in the 90-volt lead may be a Samson or similar 85 mh. inductance. If the audio amplifier "rasps" on medium or loud signals of rather high frequency, it is regenerating. This may be due to high-resistance B batteries or plate voltage supply, or to a direct feedback between the input plate lead and the output leads. If the audio amplifier is distinct from the twotube tuner unit, the input to the amplifier should not be run in cable from the tuner unit. FIG. 8