Radio Broadcast (May 1928-Apr 1929)

A Test Set for the Radio Service Man By B. B. ALCORN CT*HIS is the second article on practical ■*■ service problems by Mr. Alcorn. In his first story, the author discussed what seemed to him the ten leading service troubles, analyzing some of them in detail. This one deals with short circuits and the "case histories" represent actual and most interesting experiences which should be useful not only to those who have occasion to service or use the particular sets referred to, but also to those doing service work on almost any set. Surprisingly little really helpful data has appeared in print on service problems. It is our hope that these articles by Mr. Alcorn, and other similar articles which we may publish, will in a measure fill this real need. — The Editor. IN SERVICING radio receivers repair men and radio dealers are confronted constantly with mystifying problems which tax their ingenuity to the extreme. In some cases several hours of experimenting may be necessary before it is possible to locate the trouble and start work on the repair, whereas other times an unusually baffling condition may be cleared up accidentally in a few moments. The writer does not wish to infer that he has found the solution difficult in all, or even a majority, of the problems in radio servicing, for this is not the case. On the contrary, the owner's description of the receiver's performance often permits one to diagnose the trouble, and other times it is possible to analyze the difficulty by merely turning the tuning controls for a few moments. However, there are a sufficient number of unusual ailments which a receiver is apt to contract to make this line of work full of interest. In last month's article of this series the writer described several unusual experiences which he has had in locating troubles caused by open circuits. It is admitted that the difficulties cited were out of the ordinary, but they serve to point out the type of work a service man must be prepared to tackle. In all probability open circuits of the types described may never be experienced by the reader, but they are typical of the peculiar problems which present themselves every day. While on this subject there is one new case of open-circuit trouble which may be of interest. The writer was called upon to repair an Atwater Kent Model 35 receiver, and after testing the various circuits it was found that there were a great number of open circuits. Both audio transformers were burned out, the grid suppressors were open, the primary windings of the radiofrequency transformers were defective and, in addition, several by-pass condensers were blown out; in fact the set was almost a total wreck. However, after all of these parts had been repaired or replaced the set still refused to function, although the usual tests showed the continuity The second installment of a series of articles relating the experiences of a radio service man of all parts to be in order, except that the B potential did not reach the plates of the tubes. After considerable checking, the trouble was located in the battery cable, but not in the place where one would normally expect to find it. The A+ wire in the battery cable of this receiver is fitted with a lug for connection with the storage battery, and the B — wire is connected with the cable in this lug. In some way the B — wire had become loose, and an oxide had formed which insulated the wire from the cable. W hen this difficulty was repaired no further trouble was experienced with the receiver. GOOD EQUIPMENT NEEDED IN THE servicing of radio receivers a repair * man is lost without the proper tools and testing equipment. Both of these items are of equal importance and their selection deserves the most serious consideration. With a welldesigned portable test set it is possible to locate most all causes of trouble in a fraction of the time that would otherwise be required, and, if a complete set of tools is available, the receiver often may be repaired in the customer's home. However, when equipping himself with tools and test apparatus the service man should always consider the equipment from the viewpoint of portability and convenience. In addition to its utility value a service man's equipment accomplishes another very important result; namely, it creates a favorable impression in the mind of the client, and this is very important. Therefore, when building or buying portable equipment it is wise to consider its appearance as well as its usefulness. THE AUTHOR S TEST SET 99 There are a number of excellent test sets available on the market, and, if the service business will stand the strain on its pocketbook, one of these instruments is an excellent investment. Such apparatus is now being made by Hoyt, Jewell, Supreme Instrument, Weston, and other instrument manufacturers. The writer recently had an opportunity to experiment with the test set made by the Supreme Instrument Company, and, while it is rather expensive, it is as complete an outfit as could be desired. The set is designed especially for use in the field, but it is also excellent for work in the laboratory. It consists of an oscillator, a wavemeter, a calibrated variable condenser, and a powersupply circuit which operates direct from the a.c. line. The set is housed in a neat case which also provides space for carrying tools, tubes, and enough supplies to meet the usual requirements. It may be considered a portable laboratory. Unfortunately most service men cannot consider purchasing elaborate test equipment of the type referred to in the above paragraph, because of the high cost of such apparatus. However, it is possible to build a very satisfactory set-checking device at a considerable saving in expense. It is true that such a set tester may not be quite as versatile as a commercial product, but it may be constructed so that it is satisfactory for most purposes. The set checker designed by the writer will indicate short circuits, open circuits, and the general condition of tubes in both a.c. and d.c. receivers, and it is made up of meters that every service man should own. Of course, if high-grade meters are used the cost may be as high as $40, but this is considerably less than the average commercial outfit. PARTS NEEDED A LIST of the apparatus required for the construction of the set checker is as follows: One double-range panel-mounting highresistance d.c. voltmeter, 0-8 and 0-200 volts; One panel-mounting d.c. milliam meter, 0-100 milliamperes; One portable triple-range a.c. voltmeter, 0-4, 0-8 and 0-150 volts; Two Benjamin sockets, ux-type: Nine Carter tip jacks; Two single-pole, double-throw pushbutton switches; One double -pole double -throw switch; One double-pole single-throw switch; One wooden case (large enough to provide space for accessories) ; One bakelite panel. The accessories used with the test set follow: One set of test prods; One socket adapter, UX199 to UV199; One socket adapter, UV199 to UX199; One socket adapter, UX199 to uv201 a; One Jewell uy socket adapter, type521; One test cord (made by connecting two UX199 bases by six feet of four-wire battery cable). The complete schematic diagram of the set tester is given in Fig. 1 . The portable a.c. voltmeter is not