Radio Broadcast (May 1928-Apr 1929)

^ 25 Watt Lamp v# or Ammeter Condenser Block -ipn 110 VAC. I J 2 m. Fig. 2 — Method of determining approximate capacities of various units in a condenser block. cuit which could not be shown by 250-volt test. — Editor.} — W. C. Roemer, New Haven, Connecticut Testing Condensers: Several days ago in building up a power amplifier, I tore down an old power unit in order to salvage the condensers. I lost track of the various leads on the condenser bank and was faced with the problem of finding a quick and easy method of deterrninining the common ground and the various capacity leads. I finally hit upon this scheme: The two leads from the 110volt a.c. main are connected in series with a 15 or 25watt lamp and the condenser to be tested. See Fig. 2. Since a.c. is employed, some current will flow. The amount which flows depends on the capacity of the condenser. The glow of each section was compared with the glow using a standard 2-nifd. condenser. In this way I could determine not only the ground lead or common lead, but also the approximate capacity of each section. If an a.c. ammeter is available, a much more accurate check can be made. For an ordinary condenser bank, however, the lamp method is quite satisfactory. — Karl F. Oerlein, Philadelphia, Pa. Common A.C. Receiver Troubles: The simple chart below lists some of the most frequently encountered troubles in a.c. receivers. The chart is not exhaustive by any means, but may suggest to others ways of organizing service information which they have or may gather in the course of their work. The use of this chart in conjunction with a good set tester, such as Jewell, Weston, Supreme, will enable the serviceman to locate quickly the defective part or condition. Symptoms: For causes refer below to: No Filament Voltage No. 1 No Grid Voltage No. 2 No Plate Voltage No. 3, 4, 5, 6, 7 Voltage with little or no current in Plate Circuit No. 8, 9 Excessive Plate Current No. 10 Causes: 1 — Loose terminals, poor soldering, open or shorted power transformer winding. 2 — Open grid suppressor, open divider resistor, open a.f. or r.f. transformer secondary. 3 — Open divider resistor. 4 — Open r.f. primary. 5 — Open a.f. primary. 6 — Shorted by-pass or filter condensers. 7 — High-resistance connections, open leads, or loose terminals. 8 — Low-emission tube. 9 — Defective socket, dirty contacts. 10 — Open a.f. or r.f. grid circuit, no bias, excessive plate voltage, defective tubes. Carlton W. Croteau, Mount Carmel, Conn. Shooting Trouble on Atwater Kent No. 20: Having made many service calls on Atwater Kent model 20 sets, two main sources of trouble stand out: blown out by-pass condensers, and blown resistors. To test properly for these troubles, a regular set tester is desirable, but if none is available, a high-resistance voltmeter (1000 ohms per volt) should be used. Test the plate voltage on each of the r.f. sockets. If there is no plate voltage, remove the set from its cabinet and make a circuit tester from a high-resistance voltmeter. See. Fig. 3. If the condenser passes a steady voltage, it should be replaced. To test the resistors, the set must be removed from the cabinet and tested with the meter arrangement suggested. My experience has shown that in nine out of every ten service calls on this set these two tests made in this way will locate the trouble. — F. D. Mitchell, Collingdale, Pennsylvania. Hum in Moving-Coil Loud Speakers: "I have had two a.c. Peerless dynamic speakers which hummed badly, due apparently to feedback on the 8-volt a.c. links, " writes L. A. Moss of Los Angeles, California. "A 4000-mfd. dry "A" condenser stopped it, so that there is now no hum at all. Fig. 4 shows the method of connection. [The suggestion is excellent, but the reason for the hum is not correct. Hum in a.c.-operated moving-coil Fig. 3 — A high-resistance voltmeter and battery provide an efficient circuit tester speakers is due to the fact that the rectifiers employed do not supply pure d.c, but a pulsating direct current. Placing a condenser across the field cuts out much of the ripple and the field is therefore supplied with current more nearly pure d.c. — Editor.] Items of Interest 1X/TANY servicemen are called upon to -L* J remedy interference due to oil burners, mechanical refrigerators, leaky power lines, door bells or the neighbor's pet dog. We have answered many requests from men in the field as to what printed matter is available on the solution of interference problems. These are the references: "Suppressing Radio Interference," by A. T. Lawton Radio Broadcast, September, November, 1927; January and March, 1928. 35 cents each. Circuits and methods for solving every kind of interference problem. Order copies from subscription department, Doubleday Doran & Co., Inc., Garden City, New York. Radio Interference Problems, a bulletin. National Electric Light Ass n., 420 Lexington avenue, New York City, 60 cents each. Radio Interference, Casey. Distributed by Radio Manufacturers Ass'n, 32 West Randolph Street, Chicago, Price 25 cents. The information from each of these sources does not differ greatly. In our opinion, the Radio Rroadcast articles by Lawton are the most complete, with the National Electric Light Association pamphlet a close second. The RMA pamphlet is also good. {If The bound volume of Radio Rroadcast's Laboratory Data Sheets contains a great deal of useful informat ion for radio servicemen. Many have written us ordering extra copies for use in the field, and, even though the book has been on sale for little more than a month, thousands of orders have been filled from servicemen. If your newsdealer does not have the Sheets orders can be sent directly to the Circulation Department, Doubleday Doran & Co., Inc., Garden City. Price, one dollar. {If In a recent article in this magazine by R. R. Alcorn, the use of test prods was discussed. While made-up prods can be had, Mr. Alcorn advises that those he used were made in his shop. He bought fibre tubing with an inside diameter about the size of the average pin jack. The Weston Electric Instrument Company, Newark, N. J., supply with some of their meters a long-pointed prong. Extra prongs can be had from Weston. These were slipped inside the tubing and flexible leads soldered to the far end. And so you have perfectly satisfactory prods which are really invaluable for service work in the field or on the shop bench. {]f The Tobe Deutschmann Company, Canton, Massachusetts, have been supplying for some time devices for interference reduction and elimination. Mr. Deutschmann writes that in addition to the apparatus they have for sale, the engineering department of the company is glad to lend its aid in helping to solve immediate interference problems which present themselves. Inquiries should be sent direct to Mr. Deutschmann at Canton. Tobe Deutschmann now make the following devices for interference work: radio interference filter No. 1 (large capacity condensers connecting across the supply fine), filterette No. 22 (metal box containing fuses, condensers, etc.,), filterette No. 31 (for sign flashers). {]J A dealer in Manhattan, Kansas, the Kolster Radio Company informs us, got good radio reception where it seemed almost impossible in a downtown building. A motion-picture theatre wanted to receive a particular broadcast and reproduce it in the auditorium through Kolster movingcoil reproducers. A special telephone line was secured, connecting the theatre and the home of G. W. Livingston, the local Kolster representative. A Kolster K-20 set was installed and its output transmitted over the telephone line to the theatre where the reproducers were connected. This bit of initiative brought credit to the dealer, the set, the theatre, and unquestionably made some set sales. Fig. 4 — Hum in a.c.-operated dynamic loud speakers may be reduced by use of a 4000-mfd "A" condenser. • february, 1929 page 256 •