Radio Broadcast (May 1929-Apr 1930)

Practical Talks to Service Workers— II ROUTINE TESTING OF FILAMENT CIRCUITS THE chart which is reproduced as part, of this article describes a method of testing the continuity of filament circuits in radio receivers. It does not apply to any particular make or model of receiver, but is intended only to apply to the three commonly used types of filament circuit. It does not pretend to contain all of the tests which may be necessary on a given receiver, but was designed to serve as a guide in developing an orderly, thorough, and rapid system of testing. There may be variations of the order given which will save more time. It is certain that precisely the same order cannot always be followed, because there is such wide variation of physical construction of receivers in use to-day. The important thing is that some carefully designed order of testing must be used consistently in order to gain at once the two-fold advantage of thoroughness and rapidity. Socket Trouble STARTING at the sockets, it is always well to ascertain visually the state of cleanliness, or dirtiness, of the socket prongs before inserting the diagnoser plug for testing. If they are not bright over the area in which contact is made with the tube prongs, they should always be cleaned before going farther. Many a no-charge call has been the result of failure to perform this simple operation, a fact especially true when servicing old sets with open, flat-prong sockets. Fortunately, it is less prevalent with the newer types of socket. In the old types, scraping the prongs with the end of a screw-driver is the easiest and quickest method of cleaning them, after which process it is usually advisable to bend them upward to insure proper tension. With the newer type of construction which does not readily permit visual inspection of the socket prongs, moving the top of the tube, in any socket, along the circumference of a small circle will produce noise from the loud speaker if the contacts are not clean and tight. That test may have to be deferred because of other troubles, but it should always be made. Such socket prongs may be cleaned from the top by the use of a piece of No. 14 solid antenna wire, bent over slightly at the end and sharpened, somewhat like an automobile carbon scraper. If the tension of the prongs has become insufficient, it usually can be remedied only from underneath the tube panel. However, the type of prong that is used in the Radiola 25 and other models may be tightened up from above with a special tool provided by RCA for that purpose. (Some servicemen always carry a crotchet needle in their kit as this may sometimes be used as a tool for the repair of sockets from above. Editor.) Egpi P jy. Mi^y 1 n II f^^' A i -/Pit On every service call the battery terminals should be carefully examined. By JOHN S. DUNHAM QRV Radio Service, Inc. Continuity Tests WITH the diagnoser plugged into a socket, if filament voltage is not obtained, the next place back along the fine of supply which can be tested readily is usually the terminal strip of the set. If filament voltage is obtained there, it establishes the continuity of the remainder of that supply circuit, and at the same time narrows down the One of the primary interests of radio dealers who write us about servicing seems to tie in determining the best methods of going about service problems in general. In the April Radio Broadcast, Mr. Dunham wrote the first of a series of articles on routine testing of radio sets. This was most favorably received and this article which deals with routine tests of the receiver in the customers home presents some interesting thoughts on routine tests for filament circuits. The table at the end of the article should be especially useful. — The Editor. search for trouble to that portion of the circuit which lies between the terminal strip and the socket prongs. Each of the two legs of that portion may then be analyzed separately. Speaking of a parallel d.c. circuit, for example, if voltage is obtained by testing from the negative strip terminal to the positive socket prong, the continuity of the positive leg of the circuit is established, thus further narrowing the search down to the negative leg. If the filament-control rheostat is in that leg, testing for voltage from the positive strip terminal to the rheostat will determine whether the trouble is there, between there and the strip, or between there and the socket. When the open has been located definitely in a small portion of a circuit by such an orderly process of elimination of other parts of the circuit, then in most cases it may be found quickly by visual examination. When the trouble is not a complete open, but is a partial break or a resistive contact, as indicated by low voltage, or fluctuation of voltage, at the socket, but with steady voltage of the proper value appearing at the terminal strip, the parts of the circuit in which the trouble does not exist may be eliminated by the same process. Then, if visual examination does not disclose the fault, it may be found by watching the voltage across the tube in the diagnoser socket, with the diagnoser plug in the set socket, while vigorously moving and pulling the particular length of wire to which the trouble has been traced. Rheostats are the most common source of noise and fluctuation of voltage in d.c. filament circuits, as are any continuously variable resistors in any receiver circuit. If turning a filament rheostat — whether it be the common control for all tubes, or simply used as a volume control affecting only a few tubes — produces noise from the loud speaker, that rheostat shoidd be thoroughly cleaned. Fine sandpaper is the best thing with which to clean the contact arm and the surface of the resistance wire on which it rides. If it cannot be reached easily to use sandpaper, in those sets which are designed without apparent thought to the difficulties of servicing them, a pipe-cleaner, dipped in alcohol, will sometimes suffice. If cleaning the end of the arm and the surface on which it makes sliding contact does not entirely eliminate the noise, then the rheostat should be dismantled and the sliding contact which connects the shaft to one terminal lug of the rheostat should be thoroughly cleaned, as well as the stationary part of that contact. Oil should never be used on the shaft bearings of a rheostat, for oil happens to be an insulator, a fact which seems to be ignored by a good many servicemen who ought to know it. There is one exception to the statement that oil should not be used on rheostat bearings. There is one manufacturer of rheostats in the United States (there may be a few others, but the author has never seen their product) who makes rheostats with a pigtail connection from the shaft to the terminal. Oil may be judiciously used on the bearing of that most excellent job. If you have not guessed it, the name of the wise manufacturer mentioned is the Yaxley Mfg. Co. Long may they prosper! Still using the battery-operated set as an example, if proper voltage is not obtained at the terminal strip, the next logical test point is the terminals of the A battery. If normal voltage appears there, it is obvious that the trouble exists between that point and the set terminals. If no voltage at all is obtained at the set terminals, one of the two filament leads in the cable must be broken at some point. The continuity of the positive one may be determined by a voltage test from its terminal strip end to the negative battery terminal, and that of the negative lead from its terminal strip end to the positive battery terminal. Corrosion on Terminals IF THE more usual trouble of slightly fluctuating voltage at the terminal strip is observed, the most probable cause is corrosion on the battery terminals, creating a varying resistive contact between them and the cable lugs or clips. On every service call, the storage-battery terminals should be examined carefully, and if the contact is not perfect they should be thoroughly cleaned and thickly coated with vaseline. Also, if battery clips are not being used, or if those used are small and have weak springs, then large clips with very strong springs shoidd always be put on. Failure to do those things is another prolific cause of no-charge calls and dissatisfied customers. Oil should never be used on the shaft bearings of a rheostat for oil happens to be an insulator. • may, 1929 . . . page 26 •