Radio Broadcast (May 1929-Apr 1930)

~z_ .RADIO BROADCAST. .X" discussion of the subject, by Herbert M. Isaacson, is scheduled to appear soon in Radio Broadcast. Suffice it to say here that the conditions which cause trouble in iron-core transformers are so different in air-core transformers as to practically exempt them from that sort of trouble, and consequently they have an indefinitely long life. Iron-core transformers, on the other hand, have a relatively short life. In its relation to practical servicing, the trouble in iron-core transformers is such that, when a transformer starts to become noisy, we know definitely that it will become progressively noisier and will finally open. Sometimes, when the noise has just started, it will be very intermittent and may occur only at intervals of hours or even days. Sometimes fairly high voltage impressed directly across the terminals of such a transformer will not open the winding, and will sometimes apparently clear up the trouble. But because of the nature of the cause of that trouble, it will always recur, and transformers which have shown any evidence of that condition always ought to be replaced immediately. In socket-powered receivers, other than those operated from external B-power units (sets originally designed for battery operation) and those having series filaments, the grid-bias resistor of a particular tube, or a group of tubes whose plate supply is common, is in the plate circuit. It is not part of the supply system, but is part of the load on the supply, as it is between the most negative end of the supply and the filament. The filament is positive with respect to the negative end of the supply by the amount of drop across that resistor. If the resistor is open, a test at the socket from plate to filament will not show any voltage, but a test from grid to filament will show the voltage that is the IR drop across the meter itself. The meter is then acting as a substitute for the bias resistor. On the other hand, if there is an open at any other point in the plate circuit, no voltage will be obtained testing from the filament to either plate or grid, because there can be no drop across the C-bias resistor unless plate current is flowing. All that makes it important to remember three things when servicing that predominant type of a.c.-operated set. First, that normal C-bias voltage can be obtained only when a good tube is in the socket, and that no grid voltage at all will be registered if there is no tube in the socket. Second, that if no voltage is obtained from plate to filament, but voltage is secured from grid to filament, the trouble can only be an open grid-bias resistor. Third, that if voltage is absent across both plate to filament and grid to filament paths, the trouble is not in the grid resistor, but is elsewhere in the plate circuit, except in extremely rare cases when opens might appear in two parts of the circuit simultaneously. If any of the foregoing is not quite clear to any serviceman who has read it, the way to make it clear and to make it a useful part of his working knowledge is to draw a diagram of the circuits involved,' represent the conditions graphically, and study it until he is thoroughly familiar with the possibilities described. Then he will know a great deal more about it than he would if such a diagram were reproduced with this article. That is an observation that holds true for the study of any problem in any circuit. When one doesn't clearly SYMBOLS IN COMMON USE The following is a list of radio abbreviations most frequently used as a kind of technical shorthand. It should be posted for convenient reference in every servicemkin's shop. SYMBOL MEANING EB B supply voltage EP Voltage at plate of tube Ec or E g Grid-bias voltage Ef Filament terminal voltage Ip Plate current Ie Grid current If Filament current R, Plate resistance of tube Ml) (n) Amplification constant of tube Gm Mutual conductance of tube RL Load resistance in plate circuit of tube DB Transmission unit IR Current times resistance understand a set of conditions that are found in a receiver, draw a diagram of the circuits involved and study the possibilities as they are governed by the circuit arrangement and the effects which have been observed. Drawing diagrams for the purpose of studying a particular problem, or as an exercise in learning the circuits of a receiver, is a very profitable pursuit. RADIO SET ANALYSIS 2 is'zi TUBC NO. ORDCft TYPE OF TUBC POSITION TUBC l«T.«.F..OCT..CTC. TUBC in TESTER VOLTS B VOLTB VOLTB a VOLTS c VOLT6 PLATE VolA lut -K.fr S.Z es s.o 90 O ii. i VOi-A S.r 95 S.o o O.Z 0.3 3 loi-A S. 2. 9S s.o SO o r. 2 10-t 4 S. (. ?s S. 0 ts o /•7 ss S. 4 o S. 0 O 4.S o o nt S. 2 s.o is a ■vi.o S.O • s , ... , , s tfrf&t'i. After servicing a receiver the wise serviceman fills out a chart of the type illustrated above. This saves considerable time on the next call. This form is supplied by Jewell. Servicing B-Power Units rpHE problem of finding opens in a BX supply unit of any kind is not a tremendously difficult one. It means simply going from one point to the next, eliminating sections of the circuit until the right one has been located. In the voltage divider itself, one must have some knowledge of how the circuit works. For example, in the commonly used parallel type of divider, if detector voltage is not obtained, but the other voltages are normal or high, so that we know the trouble is not due to the additional load caused by the breaking down of the by-pass condenser connected from the detector tap to minus B, the open must be between the detector tap and the next higher voltage tap, for the detector voltage does not depend on a drop between its tap and minus B, but on the difference between the drop from that tap to maximum plus B and the drop across the whole supply from minus to maximum plus B. Most dividers have a section from the detector tap to minus B, but for a purpose other than that of furnishing drop for the detector voltage. If voltage is not obtained from minus B to the tap which is next to the detector tap, towards the plus end, and we know from other evidence that the voltage at the next higher tap (usually maximum B plus) is high, then the open must be in the section between the intermediate tap and the maximum. If the open does exist there, then both the intermediate and detector taps will show open. That particular open is another trouble which may be determined definitely, in the vast majority of cases, by thoughtful testing at the sockets. The section of the divider between the detector tap and minus B is added so that there will be a continuous path for current through the divider from plus to minus, thus providing a slight load on the supply system even when the set itself is not drawing any current, as would be the case with all the tubes removed, or if the set were entirely disconnected. If there is no load across the supply system, no current is being drawn from the system and the IR drops across the rectifier tube and transformer secondary are negligible, with the result that the peaks of the a.c. voltage at the terminals of the secondary of the supply transformer are impressed across the filter condensers. The peaks of sine wave a.c. voltage have an amplitude of roughly 1.4 times the average or r.m.s. value. If the no-load terminal voltage of the transformer secondary is, for example, -100 volts to supply 180 volts from plate to filament of a 171 after the drops in the rectifier tube, chokes, output transformer. C-bias resistor, and the transformer itself have been subtracted, the peak voltage impressed across the filter condensers when no load exists across the filter output will be approximately 400 times 1.4, or 560 volts. If even a light load of the order of 5 or 10 milliamperes is across the output of the filter system, the IR drops across the tube and transformer secondary increase, the voltage impressed on the first filter condenser will be much less than the maximum peak voltage, and that on the succeeding condenser or condensers will be still less by the amount of drop across the chokes preceding them, which means that with a minimum load provided the condensers need not be constructed to withstand as high a voltage as they would wore a no-load condition possible. 98 • • JUNE • 1929 •