Radio Broadcast (May 1929-Apr 1930)

. RADIO BROADCAST. knows that the voltage supplied by the storage battery is 6 volts, that to be supplied at the tubes is 5 volts, and he knows, therefore, that the minimum drop which must be caused across the rheostat by the current taken by those tubes must be one volt. But he knows also that it would be advisable to be able to reduce the voltage at the tubes to about 2 volts so that the maximum drop which will be necessary across the rheostat is 4 volts. Knowing the current through the rheostat and the maximum drop required he can substitute those values in the simple formula and very quickly determine the resistance value of the needed rheostat. Knowing that value and the value of current which the rheostat must carry safely, he can telephone an order to his distributor for a new one and thereby save himself and his organization the time which he would otherwise waste in hunting for a rheostat to match the old one. If the type of replacement which he orders is rated in watts instead of in current carrying capacity, he can calculate the wattage required by substituting the voltage and current values in one of the two simple Ohm's law formulas, W = EI, or W = 12R. The time required for those computations should not exceed five minutes, but the time required in searching for a physical mate of the damaged rheostat may very easily extend into hours. Exactly the same considerations apply in most instances when the replacement of any fixed or variable resistor is necessary in sets for which no parts catalogues are in existence, or, if in existence, are difficult to obtain. The experienced serviceman knows that there are literally hundreds of models of receivers in use to-day for which such parts lists cannot be obtained. He also knows that in most cases neither the resistance, current, nor wattage values are marked on the parts which need replacement. For that reason alone the few hours that it would take the serviceman to learn the fundamental formulas of Ohm's law would, generally speaking, save him at least an equal number of hours monthly, and often weekly. Measuring Other Resistors There are a good many times when it is impossible to match resistors by their physical appearance. It is usually possible to do so with variable resistors of low value, such as filament rheostats and wirewound potentiometers of less than 1000 ohms, but it is rarely possible to do so with any other types of resistors, either variable or fixed. Fixed resistors such as those used in voltage-divider systems are usually covered iwith protective enamel so that the wire itself cannot even be seen. Resistors of higher orders of value than are generally employed in divider systems, such as grid-leaks which run up into the hundreds of thousands and millions of ohms, are usually not wire wound. Even if they were wire wound and the wire exposed, the size of wire used would be so small that it would be impossible to compare its resistance with that of another resistor simply by visual inspection. In any such case the only method by which a new resistor of the proper value can be selected in the absence of a parts list is by arithmetical calculation of the value needed in that particular part of the circuit. The serviceman who is not able to perform the necessary computation can be more hopelessly lost than a child separated from its parents at Coney Island on a hot Sunday, for there may be no kindly policeman around to guide him. Let us take another example of a service problem that arises frequently with sets supplied from either alternating or directcurrent lighting circuits. In both of those types we have voltage dividers usually consisting of wire-wound resistors upon the value of which and the current through them depend the voltages applied to the plates of the tubes. If the value of one of those resistors changes in use the voltage applied to the tubes will no longer be the correct value. When servicing a set, if the EJ at the tube or group of tubes with common plate supply is incorrect, but the Ip is normal, the voltage supplied to the divider itself is normal, and there are no incorrect conditions elsewhere in the set, it is highly probable that the value of the resistance providing the drop for that tube or group of tubes has changed. There are two ways in which the correctness of that supposition may be determined. One is by substituting a new resistor of known value for the suspected resistor and then determining by test whether the EP has returned to normal. That method can be used only if the serviceman happens to have with him the proper replacement unit for that particular divider system, which he is unlikely to FREQUENTLY USED FORMULAS The formulas given below are those which the serviceman may use daily in his work. Each of these should be memorized so that they become a practical part of his working knowledge of electricity. Ohm's Law (Three Versions) Volts (E) = Amperes (I) X Ohms (R), 1=5 11 Power in Wails (Three Formulas) E2 R W = I X E W W = I2 R Condensers in Parallel C total = Ci + Cj Condensers in Series C,XC, 1 C, + C2 1_ 1 Ci C2 Resistors in Parallel Ri x R2 1 Ri + R2 1 + 1 Ri Ri Resistors in Scries R TOTAL = I*! + R; have in most cases because the values and physical sizes of such resistors vary so widely that it would be necessary for a man who was performing service on all makes of sets to carry an entirely impractical number of such units with him. The other method is to determine the exact value of the suspected resistor without removing it from the circuit, by the use of Ohm's law. Measuring the current through the resistor by inserting a milliammeter in series with it and measuring the voltage across it and the milliammeter at the same time, substituting the values obtained in the Ohm's law formula (R = E/I), and solving for the value of R, is an operation which can be accomplished in less time than would be required to remove the resistor and connect a new one in its place. That method not only saves time but it accurately determines the amount of variation from the correct value, and it also may be performed readily without equipment other than the usual good set analyzer. The man equipped with Ohm's law in that case saves time and he obtains definite knowledge which the man who is not so equipped could not obtain. That ability to calculate the value of a resistance is even more important when servicing some of the older socket-powered receivers than it is with the usual modern set, because some manufacturers were guilty of employing voltage-divider resistors which were not wire-wound (of the grid-leak type) which often do not have a sufficiently high wattage rating, with the result that their values are subject to wide changes. Series and Parallel Resistors Suppose in the shop it is found that the value of a divider resistor in a power pack has risen above its correct value to an extent which requires that it be replaced. Suppose also that no new resistor of the proper value is at hand, but there are in the shop a miscellaneous assortment of resistors of suitable wattage rating, which might be used for replacement. If the serviceman does not know how to calculate the value of resistance obtained by connecting resistors of known values in series or in parallel he will be unable to obtain the value he desires to use except by a lengthy and haphazard process of trial. If, on the other hand, he knows the two simple formulas for calculating the resultant value of both series and parallel arrangements, he can quickly, readily, and accurately pick out from his miscellaneous assortment a combination of resistors which not only will give him the desired value for replacement, but will also employ for that purpose the smallest number of those resistors which is necessary. The process of determining whether the miscellaneous resistors in the shop will be suitable and of designing a combination to give the proper value is one which may be performed in a very short time, whereas the trial method without the use of formulas can consume a very long time. Capacity Determinations The same considerations apply to the value of knowing the formulas for series and parallel capacities. For example, suppose that the familiar ailment of a broken-down filter condenser has occurred and the only capacities which are available for replacement of that condenser are of different values than the one required. Assume that the value of the condenser which has broken down is 4 mfd. and the serviceman has available in his shop, or with him on the job, two capacities of 2 mfd., each of which would be suitable for replacement in the filter in so far as their voltage rating is concerned. If the serviceman does not know the effect on total capacity of either series or parallel connection of capacities, and if he happens to know that connecting two resistances in series increases the total resistance in that circuit — which he might know without being familiar with the formula — it would be logical to assume that he would connect those two capacities in series in the belief that the total capacity obtained would be the sum of their separate capacities, or 4 mfd. Actually, of course, the total capacity obtained would be 1 mfd. and in most cases when the set was con 146 • • JULY 1 929 •