Radio Broadcast (May 1928-Apr 1929)

100 RADIO BROADCAST DECEMBER, 1928 connected directly in the tester, but it is a necessary piece of equipment which should be on hand at all times. One of the sockets called for in the list of parts is connected in the position indiacated by the tube in the diagram; the four wires marked "test plug" are connected to the terminals of the second socket. The notations on the drawing indicate many of the ways in which the tester may be used, but a few additional pointers may be of interest. In checking a battery-operated receiver with this set tester it is best to start by removing each tube of the set in turn and inserting it in the tube sockets of the tester, the test plug socket of the tester being used for the test cord which is also plugged into the empty socket of the receiver. Next, throw the double-pole switch to one side, but change it quickly to the other side if the d.c. voltmeter is incorrectly connected, i.e., if the needle tends to move in the wrong direction. With the tester connected in this manner the milliammeter will indicate the condition of the circuit under test; if the deflection of the meter is correct for the tube used, it is necessary to assume that the circuit and tube are operating properly, but a large deflection indicates a short circuit and zero deflection indicates an open circuit. After the above check has been completed it may be necessary to make further tests in order to locate the exact position of the opened or shorted circuit. In this connection it is interesting to note that tip jacks have been provided on the tester so that each of the meters may be used separately. When testing a.c. sets the portable a.c. voltmeter is used to measure the filament or heater potentials, and then the circuit is checked in the usual manner with the tester. It should be remembered that the d.c. voltmeter will not record the filament voltages of an a.c. set, and, therefore, the double-pole switch should be open when making the tests. The tools and replacement equipment which it is necessary for a service man to carry with him in the field vary in different territories. In districts where it is necessary to travel a considerable distance from the shop it is essential that the equipment be very complete, and it should consist of a large assortment of tools as well as a number of replacement parts, such as transformers, condensers, etc. On the other hand, when working in a city a large kit of tools and parts is not essential. However, it is always advisable to carry a complete assortment of tubes and a few minor parts such as grid leaks, fixed condensers, amperites, etc. For city work the tools which the average service man considers it necessary to carry are a pair of long-nose pliers, a screw driver and an electric soldering iron. AN EFFICIENT TOOL KIT "T^HE writer was once employed as field en' gineer for a well-known manufacturer, and, while acting in this capacity, he covered most of the southern states. In this work it was necessary for him to carry a very extensive kit of tools in order to be able to meet all conditions. In this particular case the tools were carried in a case with straps for each tool, thus making it difficult to lose equipment as each strap had to be filled before leaving the scene of work. A list of the tools included in this kit is as follows: flat-nose pliers, long-nose pliers round-nose pliers, diagonal pliers, slipjoint pliers, long-nose angle pliers, duckbill pliers, ratchet screw driver with three blades, screw driver with screw-holding attachment, set of Stevens Spintite wrenches with detachable handle, Yankee push drill, American Beauty soldering A HANDY A.C. VOLTMETER AND TEST PLUG iron, bits, reamers, and counter-sink bits. The miscellaneous equipment consisted of solder, soldering flux, tape, saddle tacks, lead-in strips, ground clamps, etc. SHORT-CIRCUIT TROUBLES TROUBLES caused by open circuits, which were discussed at length in last month's issue, are only one of the many types of difficulties experienced in the repair of radio receivers. Short circuits cause the service man nearly as much worry as the former, and they will be considered in the following paragraphs. Short circuits, it might be explained, occur in accessories as frequently as in the set itself, and they frequently are the result of carelessness or inexperience on the part of the person installing the receiver. Short circuits due to carelessness or inexperience frequently are found in the Radiola Super Eights and the Radiola 20's. In these receivers six dry cells are connected in series parallel, and in installing these batteries a short circuit is often caused by two negative terminals touching each other, thus shorting one or more batteries and causing the set to go "dead." Much time is lost and often another set of batteries is ruined before the real cause of the trouble is discovered. Another rather baffling short circuit, which is encountered frequently, is found in the older types of reflex receivers, such as the de Forest. Although there are few of these sets in city homes, many of them are still being used in outlying farm districts. This particular short manifests itself as an open circuit, and, when the set is tested with a set checker the results indicate a burned-out transformer. However, further testing will show that the transformer is perfect; the trouble being caused by a shorted condenser in shunt with the primary or secondary winding of the transformer. When the faulty condenser is removed the set will function but bad distortion will be experienced until the condenser is replaced. .' D.P D.T.S. To cut Voltmeter To reverse polarity from Circuit of Voltmeter jTo Filaments of t--' (Test Socket Plug FIG. I. DIAGRAM OF THE TEST SET An interesting short circuit of a peculiar nature was discovered recently in a Radiola 18. The receiver provided entirely satisfactory results for several months before the trouble developed, and then the owner reported that the only way it was possible to obtain reception was to remove the first r.f. tube. The service man who was sent on the job went prepared to replace the tube which he considered defective. However, it was found that fhis was not the cause of the trouble. Upon removing the set from the cabinet after the usual test failed to indicate the fault, it was discovered that when the tube was inserted in the socket, instead of fitting into the contact springs, one of the filament prongs pushed its contact spring against another lead, thus causing a dead short circuit through the first r.f. coil and making the set inoperative. A simple operation with a pair of pliers shifted the spring to its proper position and corrected the difficulty. One of the simplest receivers ever placed on the market developed one of the most unusual difficulties which has ever come to the attention of the writer. The set referred to was one of the old bread-board-type Atwater Kent receivers, and because of the simplicity of the receiver no complications were expected when the call for a service man was -received. The owner stated that the set performed perfectly on the previous evening, but that he was now unable to coax it into operating. When the service man arrived he discovered that the set would play satisfactorily when held in the hand, but as soon as it was placed on the table it was inoperative; this was indeed an unusual condition. After carefully checking the set it was found in perfect condition, and then the surroundings were examined. Finally the trouble was found to be caused by the table cover on which the set had been placed; the cover being made of metallic tinsel, it caused a number of short circuits in the exposed wiring of the receiver. Then a little judicious questioning disclosed that the lady of the house had placed the table cover under the set during the day, and this explained why the set performed on the previous evening. Table covers having metallic tinsel in their make-up have been the cause of more than one set being inoperative. A few days after the experience described above a service man was asked to repair a Radiola 18 which was found to have a short circuit caused in this way. In this particular case the location of the short circuit was never discovered, but removing the table cover corrected the difficulty. CARELESSNESS IN SOLDERING INNUMERABLE short circuits are caused by carelessness in soldering at the factory, and a recent example of this was found in a Radiola 30A. This set had been operating perfectly for a period of three months when it suddenly stopped, and all the usual tests failed to disclose the cause; even the continuity test of the manufacturer did not show anything wrong, still the set remained perfectly silent. Finally, it was discovered after a careful inspection, that a thread of solder, which was so fine that it was barely visible to the eye, was across the antenna and ground connections. This thread of solder probably was caused by the iron being slid from one connection to another when the set was wired and it did not cause a short circuit until the vibrations from the speaker caused it to sag. However, in the proper position, a thin piece of solder is as effective in stopping the operation of a receiver as a piece of No. 14 bus bar. ( Push Button to place high Scale ' t of Meter in Plate Circuit iConn.for external Bat. in series ' 1 with low side of Meter Conn.for separate use of Meter