Radio Broadcast (May 1928-Apr 1929)

A Serviceman' s Experiences TROUBLE SHOOTING IN THE POWER UNIT By B. B. ALCORN IN THE first three installments of this series of articles discussing the problems encountered in the servicing of radio receivers, the writer devoted considerable space in describing the symptoms, methods of detection, and rectification of troubles experienced with the usual types of factory and home-made sets, and also accessories such as tubes, batteries, cable connectors, etc. However, similar difficulties which are found in power-supply units as yet have not received their share of consideration. This month, therefore, the eccentricities of power devices are the chief subject of discussion. In the repair of the receiving set proper, the three most frequent causes of trouble, in the order of their importance, are open circuits, short circuits, and defective parts, respectively. On the other hand, the chief cause of breakdown in power-supply circuits are short circuits, although defective parts and open circuits are also major considerations. Concrete Examples IN ORDER to provide concrete examples of the problems encountered in the servicing of power units, a few of the writer's most recent experiences will be recounted. The first concerns an unusual short circuit which was found in the Radiola 104 loud speaker. In the power supply of this unit the usual trouble is shorted filter condensers, and this condition is manifest by the plates of the 216B-type rectifier tubes becoming red hot, due to the heating effect of the extra current which passes as a result of the short circuit. In the case under discussion, however, the plates of the rectifier tubes did not overheat, although the power unit was inoperative. In this instance, in performing the manufacturer's continuity test, it was found that sparks could be drawn from all parts of the chassis when the power was turned on. The trouble was located finally in the secondary winding of the power transformer which had become grounded to the chassis, thus shorting the rectifier tubes out of the circuit. The only possible repair was substituting a new transformer. Very often owners of Radiola 28 receivers and 104 loud speakers complain that one or all of the tubes of the set have burnt out. Fortunately, the serviceman usually finds that this is not true. With this combination of receiver and power unit the tubes of the set are heated with current obtained from the B-supply circuit of the power unit. Therefore, when the tubes of the set fail to fight it is often indicative of the fact that the emission of one or both of the 216B-type rectifier tubes has decreased. When this trouble develops it is necessary to replace the rectifier tube or tubes in order to bring the B current back to normal. A peculiar but not uncommon complaint was brought to our attention by the owner of a recent model Atwater Kent electric receiver. It was explained that the set functioned perfectly except for sharp snaps which were heard every once in a while and which did not seem to issue from the loud speaker. On his first visit the noise did not occur and the serviceman was forced to report that the set seemed to be in perfect condition. The second evening the noise was more in evidence, and, when the serviceman arrived in response to a call, he was able to satisfy himself that such a condition did exist, but it was necessary to remove the set to the shop in order to locate the trouble. In a 500-volt d.c. test it was found that the snaps were caused by temporary breakdowns in the dielectric of one of the filter condensers, thus permitting an occasional internal discharge. Sometimes it is possible to "service" a receiver via the telephone and an excellent illustration of this concerns a receiver operated with a Balkite A-power unit. It was explained that, although the set would operate satisfactorily when pulled away from the wall, the tubes would not light when the cabinet was in its usual position against the wall. We told the owner to add water to the cell of the A-power unit and then report on the operation of the set. In this case our guess was correct; the electrolyte in the rectifier cell was so low that it did not touch the electrode unless the front legs of the radio cabinet were raised by the rug when the cabinet was moved away from the wall. A Stromberg Carlson power unit which would go off and on alternately while the power was turned on proved to be another baffling problem for us to solve. In this case a small break in the filament of one of the 2-ampere Tungar rectifier tubes was the cause of the trouble. The expansion and contraction A typical serviceman's tool kit • february, 1929 . . . page 264 9 of the filament, which operates at a fairly high temperature, explained the peculiar performance of the power unit. Home-Made Repairs YXTHEN a receiver ceases to function an * * over-ambitious set-owner often endeavors to effect a repair before soliciting the aid of a serviceman. An amusing incident of this nature recently came to our attention, and, as usual, the repair attempted by the owner was unsuccessful. The serviceman, who was assigned to the job of eliminating the bad distortion which the set had developed, found the set in perfect condition and discovered that the distortion was caused by too much voltage on the plate of the detector tube. Therefore, the B-supply unit, which was a Philco socket power device, was examined for defects. As soon as the lid was removed the trouble became apparent; the cartridge resistor in the plate circuit of the detector tube was wrapped carefully with tin foil, thus preventing its operation as a voltage reducer. The owner explained that it had burnt out and he tried to repair the "fuse" by wrapping it with tin foil in a manner which he has found effective with automobile fuses. For some reason or other tin foil seems to be considered a universal remedy by set owners. Corrections TN NOVEMBER Radio Broadcast the J writer described an experience in servicing a Radiola 18 which may have been misleading; it was stated that an open circuit in an r.f . transformer manifests all the symptoms of a short. Frankly, we did not take time to ascertain a possible explanation for the seemingly impossible condition, but, after discussing the problem with others in the service field, it has been decided that a short must have been caused by the open ends of the offending r.f. transformer. In this case the repair of the open circuit would have remedied the short. An error also occurred in the December article of this series which we wish to correct. In a paragraph describing trouble encountered with a deForest reflex receiver, the statement was made that "this particular short manifests itself as an open circuit, and, when the set was tested with a set-checker the results indicated a burnt-out transformer. However, further tests showed that the transformer was perfect, the trouble being caused by shorted condenser in shunt with the winding of the transformer." This statement is incorrect as a check of the records indicates that the setchecker showed a "dead" short. We regret this error as it has confused a number of servicemen who have followed these articles.