Radio Broadcast (May 1928-Apr 1929)

180 RADIO BROADCAST JANUARY, 1929 constructed by a man who had had considerable radio experience, but it refused to function. No error was found in the wiring diagram which was studied carefully, and an electrical test failed to disclose the trouble. However, it was discovered that with the antenna connected to the grid of the detector tube, the set would pick-up signals, but with the shields in place and the antenna connected with the antenna post the set was "dead." After considerable checking it was found that a 0.0002 5-mfd. by-pass condenser had accidentally been connected in shunt with the secondary of the detector r.f. transformer, thus causing the detector circuit to tune to a much higher wavelength than the preceeding r.f. stage which was tuned by a condenser on the same shaft as the detector condenser. As soon as this condenser was removed from the circuit the receiver provided excellent results. The troubles located in homereconstructed commercial receivers usually are as foolish as those found in other home-made sets. We were recently called upon to repair an old four-tube Garod reflex receiver, this set having performed satisfactorily until the owner decided to improve the quality of reproduction by substituting new a.f. transformers. However, after the new transformers had been installed the volume was much lower than it had been originally. An examination of wiring disclosed the fact that the first transformer was connected backwards, i.e., the secondary winding was connected in the plate circuit. The next incident, which concerns a Radiola 1 7, is more amusing than instructive. The set had an open grid suppressor and it was brought to the shop for repair, but when the cabinet was opened it was found that around each tube was a piece of friction tape which held in place a large square of tin foil. After the grid suppressor had been replaced the set, minus the tin foil decorations, was returned to the owner, and out of curiosity we asked his reason for placing the tin foil on the tubes. He explained that when the set started to lose volume he thought that the tubes were at fault. After noting that the "silver plating" on the inside of some tubes was heavier than on others, he decided to repair the "weak" tubes with tin foil. The novice experimenter who builds receivers is not the only radio fan who causes trouble for the service man. On the other hand there is the ambitious radio fan who adds accessories and other "gadgets" to his factory-built receiver, as in many cases this is also the cause of poor results. This fact calls to mind a short of a peculiar nature occurring in an Atwater Kent model 20 receiver, and in this particular instance the trouble was very difficult to locate. The owner of the receiver purchased a Philco B-supply unit and for several days he was very much pleased with the results. Then, one evening he returned the power unit to the dealer and stated that it would not deliver current, and, as no amount of argument would convince him that the power unit was in perfect condition, he was given a Bosch power pack in exchange. The next morning the Bosch unit was returned and the customer requested that a service man examine his receiver. The man who tackled the job found a very unusual condition; both power units would work satisfactorily if, when changing the connections from the batteries to the power unit, the tubes were not turned off, but if the tubes were turned off and on the power unit would not operate the receiver. Further examination disclosed a short circuit in the B + 135-volt lead of the set, and it was discovered that a by-pass condenser had blown out. When new condenser was installed in the set perfect results were obtained. The Atwater Kent model 20 receiver is not the only set in which by-pass condensers are apt to become shorted upon the addition of a B-power unit, as the trouble is experienced frequently with many old-type receivers. In this connection the writer would suggest replacing the by-pass condensers of old sets with new condensers, which are capable of withstanding a higher voltage, before installing a B-supply device. While on the subject of B-supply devices a 0 SERVICE man — and bis number is legion — can bope to succeed completely unless be bas the best of radio backgrounds. He must know, almost instinctively why things go wrong and where to look for the trouble. The service man can learn only little if he attempts to remember — merely — bow a certain trouble was cured; be must go back to fundamentals. These articles by Mr. Alcorn, himself a practising service man, are designed not so much to tell service men how to cure specific troubles, but more to discuss common troubles and their remedies. This, we bope, will help to show those who are eager to learn something about what that knowledge must he and bow they must apply it. — The Editor. operating properly except that signals cannot be picked up. In the repair of the receiver the condenser may be omitted from the circuit if another is not available, as little or no difference will be noted in the results. Short circuits in accessories, such as loud speakers, lightning arrestors, extension cords, etc., should receive some mention at this time as devices of this type are the cause of considerable trouble and it seldom occurs to the service man to look for shorts in these parts of the circuit This is especially true of the loud speaker, and for this reason it is advisable to connect a pair of phones to the output of the set before making further tests. Often it will be found that the windings of the loud speaker have become shorted or burnt out, or the cords have become defective. TWO INTERESTING OPEN CIRCUITS -TH 1 at HERE recently have come to our short of a peculiar nature which was encountered in the power circuit of a Freshman Equaphase a.c. receiver might be of interest. In this particular case the receiver behaved in a very strange manner; in the middle of a musical selection the volume would increase to terrific proportions and then die away to a whisper the next moment. After considerable checking it was found that the trouble was caused by an uninsulated wire which short circuited the detector resistance of the voltage-divider strip. However, it was discovered that, as a result of the short circuit, considerable current passed through the wire, thus producing sufficient heat to cause the wire to expand and open the shorted resistor. Of course, when the wire cooled it contracted again and shorted the resistor, and then the cycle was repeated. This short circuit proved very difficult to locate as the wire in question was fastened to the cover of the power unit and nothing out of the ordinary could be noted when the apparatus was examined. However, the set tester described last month proved its values as it detected the variations in plate voltage. MANIFESTATIONS OF SHORTS IN B-supply devices and the power packs of receivers shorted filter condensers and choke coils make their presence noted by an increased a.c. hum in the loud speaker. Incidentally the only remedy for such trouble is to replace the defective parts. Also, the writer wishes to state that all radio service men would appreciate it if less insulating material were used in the manufacture of these units, as the time employed in digging out the defective parts certainly could be employed more profitably. Another shorted condenser which develops quite frequently in Radiola models 17 and 18 receivers occurs in a part of the circuit where one would not be apt to look for trouble. The circuit diagram of Radiola 17 — it was incorrectly labelled as the 18 — was given in the first article of this series (Page 26 November Radio Broadcast). The small condensers designated by the letters G are the ones which have been found defective in a number of instances, G3 being the unit which generally is found at fault. This short is very difficult to locate the first time it is encountered, as the set seems to be which are considered of particular interest, because in each case the set-checking device failed to detect the defect. The first instance concerns a Radiola 17 receiver, and when the complaint was received the cause was diagnosed as an opened grid suppressor. However, an electrical test showed the circuit to be in good condition, even though the set lacked volume on all stations, including powerful local broadcasters. In this case the problem was solved by the old method of attaching the antenna to the grid of each of the various r.f. tubes. As soon as the antenna wire was touched to the grid of the second tube signals weTe received: with greatly .improved vofume, thus indicating that the trouble must be located in the first r.f. circuit. Finally it was discovered that an open circuit existed in the volume control which is in the antenna circuit of the receiver. The second open-circuit difficulty was experienced with a Crosley Bandbox receiver, and it was suspected that the trouble would be found in the external power-supply unit. The set did not provide sufficient volume, but the setchecking device indicated that the receiving circuit was satisfactory, and the output voltages of the power-supply unit were found to be correct with a d.c. voltmeter. As a last resort the batteryvoltmeter continuity test was given the set and an open was found in one of the r.f. choke coils. We have never been able to determine why this defect did not show up in the test with the setchecking device, but it did not and we were forced to spend more time in looking for the trouble than was profitable. In addition, it was necessary to rewind the choke coil as none of this type was available at the time. Another thing which should be remembered in connection with Crosley Bandbox receivers is that the external power unit is not of the dry type; it employs a Merschon filter condenser which is of electrolytic construction. Because of this fact the power unit should never be installed on its side, as the electrolyte is apt to seep through the cork and cause corrosion. The writer knows of several cases where this mistake has been made and in one instance a beautiful mahogany bookcase was badly discolored. Incidentally, it is never good policy to install power units on their side when a filament-type tube is used as a rectifier. In the instructions which are supplied with each tube the manufacturer recommends that the tube be operated in a vertical position if maximum life is desired. When the tubes are operated in a horizontal position the filament is apt to sag and cause trouble.