Radio Broadcast (May 1928-Apr 1929)

THE SERVICEMAN'S CORNER ~T~\\DIO Interference from House Plumbing: /| Two extraordinary but similar cases of radio interference have come to my attention. Although both conditions arose in the operation of short-wave receivers, the trouble may be affecting ordinary receivers, and telling of the experiences here may enable others to clear up an obscure source of trouble. In the first instance, a microphonic trouble developed in the receiver. The noise was terrible. The effect was suggestive of a loose connection. Sometimes a violent rapping on the set produced little effect, while placing an object on a distant table produced an explosion. Every connection from antenna to ground on a cold-water pipe was thoroughly overhauled. The trouble would reappear after each discovery and removal of its supposed cause. Matters reached a desperate state. As a final test, the set was put in operation and a 40-foot extension cord attached to the head set. The set was in the dining room and the floors of the dining room and kitchen were explored by rapping with a stick from point to point, like a blind man and the resulting static was carefully studied. A lone point on the floor, remote from the set, was found to be the most "sensitive." The extension was then carried through the floor to the cellar and the exploring transferred to the pipes hung from the floor overhead. Some were quite sensitive but they were not coupled with the ground connection. The trouble was quite by chance traced to the metal stopper of a laundry tub. This stopper fitted loosely in the drain outlet and was attached by a brass chain to the cold water faucet. This proved to be a most sensitive microphone affected by vibrations conveyed to it from pipes hung overhead. It acted as a variable short circuit in the pipe ground system, changing the electrical constants of the system. Pulling out the plug and hanging it over the side entirely cleared this vexing interference. Later an annoying but less overwhelming noise was traced to a variable contact between two pipes in the cellar that crossed each other. A little wedge of wood placed between them remedied this trouble. Probably many similar cases of interference exist which have not been traced down. This is particularly likely to be true of those who are now acquiring short-wave receivers. If trouble from loose connections cannot be found in wires in the attic, it is time to be suspicious of pipes in the cellar! — C. A. Briggs, Washington, D. C. A simple method of determining if a noisy receiver is suffering from the trouble described by Mr. Briggs is to run fifty feet of wire almost anywhere, and use this as a counterpoise in place of the ground. Also, a simple cure might be a permanent counterpoise or six feet of iron pipe driven into the earth. With this issue of Radio Broadcast, " The Serviceman's Corner" stretches into its natural stride. The purpose of this department is to publish everything and anything of genuine interest to the radio serviceman that can he briefly and thoroughly covered. Subjects justifying longer treatment will be covered in complete articles elsewhere in Radio Broadcast. Contributions, payable at our usual rate, will be welcome from engineers, manufacturers, servicemen, and dealers who have been intimately associated with any of these problems. It is requested that the contributor write us on his professional stationery, enclosing with his letter copies of his business cards and business literature if any. — The Editor. Servicing Magnavox Receivers: William K. Aughenbaugh, of Altoona, Pa., has run across several Magnavox receivers that would not function when the original tubes were replaced with R. C. A. or Cunningham tubes. The difficulty, he points out, can be remedied by short circuiting the coil of wire that will be found under the cardboard at the bottom of the set — near the front panel. Also the pin on the volume-control rheostat should be removed or bent so that the rheostat can be adjusted to the full " on " position if necessary. Finding tube-locations: I was recently called on to install a new a.c. set. Not finding any installation instructions or data on proper location of tubes, I hit upon a useful method of locating the proper socket for the proper tube. Fig. 1 — Portable radio receiving apparatus that suggests a useful adjunct to the serviceman s equipment for determining general receiving conditions No mistake can be made aoout the 280 or the y-227, especially since the latter has five prongs. The set in question required four 226's, one 227, two 171's and one 280. Putting a 226 in a 171 socket won't do the 226 any good. I took a 171 and put it into the first socket next to the 227. I was sure about the location of the 227. Not seeing the filament light, I assumed it to be a 226 socket. In this way, by trying all the other sockets, I found which were the 226 sockets and which the 171. Fred Berkley, Astoria, Long Island. Polarity Incorrectly Stamped: I just serviced a Radiola No. 20. The owner of this set was using a 22.5-volt B battery as a C battery, connected correctly. I tested the set as usual. It would receive only locals, and these not at all well. Closer inspection, with a voltmeter, showed that the C battery was incorrectly stamped, the stamping being reversed for positive and negative. This is the second time in my eight years of servicing sets that this same thing has come to my attention. George A. Hartmann, Howell, Indiana. Terminal reversal has also happened within the experience of the editor. A check of the socket and tube connections with the usual plug-in testing outfit would show this up as a very high plate current through the tube having the reversed grid bias. Servicing Cheap Receivers: L. R. Arnold, of the Bichards Badio Company, Providence, R. I. comments on the difficulties of servicing inexpensive receivers. These are often characterized by fairly good reception on local stations, but are insensitive to distant stations and stations covered by the upper section of the tuning dial. These receivers can often be improved, as far as sensitivity is concerned, by running all r.f. tubes, with the exception of the first, from 135 volts through a bypassed variable resistor, using the additional knob as a sensitivity and volume control. A Portable Receiver To Check General Conditions: The Kolster Radio Company provides its dealers with a portable demonstration set possessing several points of interest that recommend similar outfits for the serviceman. Thecompleteapparatus is pictured in Fig. 1, and consists of two carrying cases, one holding the receiver, tubes and power supply, and the other the loud speaker. A portable receiver of somewhat similar design is of inestimable value to the serviceman in solving the more general problems of poor reception. The inability of a receiver being serviced to receive certain stations can be checked against a standard receiver, the characteristics of which are well known to the serviceman, to determine whether it is the location or the receiver that is at fault. • march. 1929 page 319