Radio broadcast .. (1922-30)

When the Neighbor Calls Upon You to Overhaul a Recalcitrant Radio Receiver, Here Is the Way to Qo About It in a Systematic Manner By EDGAR H. FELIX EXPERT diagnosticians in analyzing radio troubles are rare. The position of a radio oracle is fraught with danger. Reputa- tions quickly fall by the wayside when friends and neighbors call upon "experts" to repair their sets. They expect that one look at an anemic receiver is sufficient to diagnose the trouble and that one telling flip of the screw driver is all that is necessary to repair it. But radio receivers are often mysterious in their ways. Even ser- vice men employed by radio dealers often find themselves stumped by the notorious stubborn- ness of radio sets out of order. As an aid to unfortunates who, with or without reason, have attained reputations as radio ex- perts in their circle of acquaintances, the sug- gestions which follow, for a systematic trouble search, have been prepared. Substituting this procedure for the haphazard hit-and-miss trouble hunt will enable you to maintain your reputation and dignity, and to go about the task of finding what is wrong with a radio receiver in a pro- fessional manner. Briefly, a receiver out of order can be classified in one if the following groups: 1. Totally dead. 2. Signals weak but clear. 3. Signals accompanied by whistling and uni- form distortion. 4. Reception accompanied by clicking, crack- ling noises. A receiver is often classified as dead when it is merely in a state of coma. When you are unctuously conducted to a receiver reputedly dead, behave as you would in the presence of the dead. Turn on the A-battery switch gently, displaying no emotion or expectation that the tubes will light. If they light, you have already determined that the A battery is functioning and that the tube filament circuit is complete. Be sure that all the tubes light to a normal brilliancy though, before presuming this. Continue your superficial inspection of the remains by pulling the loud speaker plug in and out. It there is a click as you do so, you may be sure that B battery cur- rent is flowing through the last tube circuit at least. If there are separate r. f., detector, and amplifier B battery leads, test the com- pleteness of each of these plate circuits by clicking the leads from the batteries to the binding posts. This should give you healthy clicks if all is well with the A and B battery connec- tions. Next proceed to test the grid connections by tapping with your moistened finger the grid binding post of each tube, beginning with the last and working for- ward. If you secure clicks all the way through, it is a sure indication that the grid-plate circuits are complete throughout. The nearest high-power broadcasting station should then be weakly audi- ble. The clicks are evidence that the slight change in grid circuit capacity caused by your touch is sufficient to affect the output of the receiver. These tests correspond to those of the physi- cian who first looks at your tongue and then feels your pulse. Sometimes this superficial examination leads to important evidence, con- fining your trouble hunt to one particular tube circuit. It is hardly necessary to explain what to do if the tubes do not light or if clicking one of the plate potential leads causes no sound in the loud speaker, for this definitely confines your trouble search to the power-supply circuit. If the superficial inspection yields no conclu- sive evidence, check up the voltage of the A, B, and C batteries with a meter, and note the polar- ity of their leads and inter-battery connections. If these prove to be connected correctly and deliver the requisite voltage to the set, but still no signal or receiver noise can be induced from the recalcitrant radio device, it is a good time to remember an important appointment else- where and leave the premises forthwith! The chances are that a broken wire or short-circuit has put the receiving set out of business. Pro- fessional service men, discovering this to be the case, usually subject the tubes to test and, if they find the tubes are good, take the receiver to the repair shop. Undoubtedly the set requires a tracing of its wiring, a tedious task not likely to be appreciated by an ordinary broadcast listener as a manifestation of expert technical knowledge. Receivers equipped with a headphone jack give one more point of test before this difficult process or flight must be undertaken. Listen-in with the headphones or, if they are not available, plug the loud speaker in the headphone jack. If good reception is secured through the head- phone jack but not through the loud speaker jack, trouble is definitely restricted to that part of the circuit which comes after the first jack. A burned-out transformer, a broken lead, or a defective coupling resistance in the audio ampli- TESTING EQUIPMENT A few of the pieces of apparatus which come in handy for testing a receiver for faulty operation. The battery and phones are largely employed to test continuity of circuits by the click method. The milliammeter shown at the extreme right, if connected in series with the battery and the circuit to be tested, will give a visual indication. The volt- meter is handy for checking voltages of batteries fication system are then likely causes of the re- ceiver's infirmity. If headphones in the loud speaker jack give a very loud signal on local reception, but the loud speaker does not make a sound in that position, the fault is, by this evi- dence, confined to the loud speaker, its leads, or the loud speaker plug. Examine the loud speaker wiring, including the flexible cord near the plug and the speaker itself. With sets giving excessive current output, loud speaker windings are sometimes burned out, although sets with moderate power rarely cause loud speaker burn- outs. With receivers of the neutrodyne or radio frequency type, it is often possible to localize the trouble in a defective r. f. stage to the particular tube circuit out of order. This is accomplished by disconnecting the antenna lead and applying the antenna input direct to the detector circuit. A convenient place to make this connection is at the plate socket terminal of the final radio fre- quency stage. If signals are now heard, though the receiver is silent with the antenna connected to its regular binding post, it is a sure indication that the trouble lies within the r. f. amplifier circuit. The antenna should next be connected to the plate post of the preceding r. f. tube. If signals are still heard, it is definitely established that the fault is in the ist r. f. circuit (in a cir- cuit with two r. f. stages). If no signals are heard with the antenna in the latter position (yet they are heard with the antenna on the second r. f. tube plate), the fault is necessarily in either the first or second r. f. amplifier circuits. The work of tracing the wiring is materially reduced by confining the trouble to a single-tube circuit as outlined above. If it is necessary to trace the set's wiring, dis- connect the set from its power source and an- tenna system and follow the wiring logically through from circuit to circuit. Begin with the antenna system, the first grid inductance, the plate circuit of the first tube, the input circuit of the second, and so on until the end. A shortr circuited grid inductance, for example, does not affect the continuity of any of the circuits (super- ficially dismissed by the click test) but may com- pletely eliminate signals. An open in the antenna circuit, with a receiver lo- cated far from any broad- casting stations, may mani- fest itself as a dead receiver so far as signals are con- cerned and yet show ade- quate A, B, and C power, and otherwise complete connections. Receivers of a hundred miles range or more reproduce the familiar at- mospheric noises even with- out antenna connected. Consequently, a receiver is not likely to be classed as "dead" when in working order by reason of antenna or lead-in breakage. RADIO BROADCAST Photograph