Radio Broadcast (May 1929-Apr 1930)

nected up again the 60-cycle hum evident in the loud speaker would be greater than its previous normal value, and in some cases the output voltage of the filter would be lower than normal — depending upon the position of the condenser in the circuit. The serviceman, still assuming that he had replaced the filter condenser with the proper value of capacity, would not know the reason for the increased hum or the lower voltage and might spend fruitless hours in endeavoring to ascertain the cause. On the other hand, if that serviceman were familiar with the two simple formulas for determining the total capacity of series and parallel arrangements, he would have connected the 2-mfd. capacities in parallel, thus obtaining the total capacity of 4 mfd. and when the equipment was connected up again for trial the hum would not be greater than it was before, and the voltages would be correct, assuming all other conditions to be normal. Assuming the same trouble as that described in the preceding paragraph, suppose that the only capacities available for replacing the 4-mfd. filter condenser were two capacities of 8 mfd. each, but the voltage rating of which was only half that required in the filter system at that point. The same serviceman would assume that the total capacity obtained by connecting those two capacities in series would be 16 mfd. Without even elementary knowledge of fundamental laws he might also assume it to be impossible to use those condensers because of their low voltage break-down rating. The serviceman who was familiar with the series and parallel capacity formulas and who also possessed a general knowledge of a.c. and d.c. phenomena would know, as illustrated in the preceding paragraph, that the sum of the two 8-mfd. capacities in series would be 4 mfd. and he would also know that the voltage drop across each of the two equal capacities would be approximately half of the total voltage across both of them (assuming the resistances of the two capacities to be approximately equal). The latter serviceman would insert those two condensers, knowing that they would meet the requirement, whereas the former serviceman, because of his lack of even elementary fundamental knowledge, would not use those two suitable replacement units and would keep the owner of the set waiting unnecessarily while he obtained a single 4-mfd. unit, and he would unnecessarily continue to carry in stock the two 8-mfd. units which might have been used up. These examples of the value of some knowledge of fundamentals are just a few picked at random for illustration, and enough similar illustrations of different cases where such knowledge is of very real value to the practicing serviceman could be given to fill many pages of this magazine. The money value of the time saved in actual practice by the serviceman possessing such knowledge is so much greater than the money value of the time necessary to acquire that knowledge that the returns on such an investment are well worth while. A.C. Knowledge Essential The questions under fundamentals in the examination referred to deal only with d. c. except for the last question on capacities. However, it is important and is becoming increasingly so because of the increasing use of a.c.-powered sets, for the serviceman to have a working knowledge of fundamental a.c. phenomena. In the examination we are now preparing with which to determine the knowledge of applicants who come to us this Fall and which will replace the one under discussion, there will be a number of questions devoted to a.c, under the heading of fundamentals. All signal currents, both radio frequency and audio frequency, are alternating. The filament currents employed to heat the tubes in modern receivers are alternating. It is alternating current which is rectified to direct current for supplying the plate currents used by the tubes. The only direct current used in A wetl-equipped service department plus servicemen carefully selected for their ability to create good will is one of the outstanding reasons for the success of Lancaster Radio Sales and Service, Philadelphia, Pa. impedance of approximately 4000 ohms at 1000 cycles, whereas the input impedance of the average dynamic speaker is of the order of about 10 ohms. Had that man been possessed of working knowledge of fundamental electrical laws he would have known that maximum transfer of energy is obtained when the impedance of the load matches the output impedance of the transformer or other supply to which the load is connected, and that the transfer of energy falls off badly as the load impedance is decreased. He would also know that there is a modification of that rule which applies to the transfer of energy in the audio circuits of radio receivers. The modification is that the greatest amount of undistorted energy is transferred when the load impedance is approximately twice that of the impedance out of which it works. The serviceman described who did not know those things spent a large amount of time trying to discover the cause of his trouble, with the sole result that he became thoroughly disgusted with the dynamic loud speaker and pronounced it to be defective, with no basis for that conclusion other than the fact that it did not work. Had he known those a.c. laws pertaining to the transfer of energy he would have procured, before he attempted to connect the dynamic loud speaker to his set, an output transformer with a sufficiently high stepdown turns ratio so that, while its primary impedance would still match the output impedance of the last tube, the impedance of its secondary would be equal to, or less than, the input impedance of the loud speaker at 1000 cycles. Higher Standards the modern radio receiver (other than those sets designed for operation from d.c. lighting circuits) is that which supplies power to the plate circuits of the tubes. All the rest is a.c. It would seem logical, even to a layman, that it would be worth while for a man whose vocation is the servicing of such receivers to know something of the fundamental phenomena of the currents with which he is constantly working. If a doctor whose business it is to work on a mechanism of arteries and veins carrying vital blood, which are comparable to the circuits of a radio receiver carrying vital electric currents, knew nothing of the fundamental laws governing the action of those vital currents in the human body, it is obvious that it would be impossible for him to succeed in his profession. Exactly the same conclusion may be drawn about the serviceman who knows nothing of the laws governing the action of the currents in a radio receiver. A very good example of the value of such knowledge was brought strongly home to us recently. A serviceman of our acquaintance purchased a dynamic loud speaker which did not have an input transformer with it. He connected it to the secondary terminals of an output transformer following the usual 17lA-type tube, fully expecting improved results over the cone loud speaker he had been using. Naturally the results were very poor, by virtue of the fact that the impedance of the secondary of the usual output transformer is designed to work into a loud speaker Standards of service are slowly but surely being raised. Service organizations discovered long ago that there is a very definite relationship between the extent of knowledge, theoretical as well as practical, possessed by a serviceman and the percentage of return calls with which it is necessary to follow that man's work. Dealers who realize the dollar and cents value of giving good service to their customers are discovering that the average servicemen who apply to them for positions in their service departments are not capable of giving their customers really satisfactory service. They are also discovering that the reason the average serviceman cannot perform efficient service is that he has not had adequate training for the work at which he professes to be an expert. A few dealers are learning gradually that there are, among the many servicemen who apply to them for work, a few here and there who have actually studied radio, and from their experience with the few men of that type whom they have been able to employ they are waking up to the fact that only such men are a profitable investment in service personnel. It behooves the servicemen who desire to continue servicing broadcast receivers as a means of livelihood and who have enough ambition to desire to increase their earnings by means of increasing their efficiency, to see the handwriting on the wall and realize that in order to learn thoroughly the subject of servicing radio receivers it must be studied with the same -diligence with which one would study any other highly specialized technical activity. • JULY 1 929 • • 147