Radio Broadcast (May 1929-Apr 1930)

THE SERVICEMAN'S CORNER To start off the "Corner" this month we have a short article describing a peculiar short circuit trouble. This contribution comes from B. B. Alcorn, of the Kew Badio Electric Company, of Kew Gardens, Long Island, N. Y. Most readers of this department will remember Mr. Alcorn as the author of a series of articles on "Practical Service Methods" which was published in Badio Broadcast. "An unusual complaint which will be of interest to the readers of the "Serviceman's Corner" has recently come to the attention of the writer. It concerns a Zenith receiver having a Jones type plug used for the connection. The same type of trouble could easily develop in any set using this connector and might for a time prove as baffling as it did for the writer. "The set was found to be inoperative with all the indications of a short. It was checked carefully, but no fault could be found in any of the parts. Then the power pack was checked and here the test showed a decided short, yet each individual part seemed to be O.K. Finally the short was run down to the cable and finally to the connection plug. "These plugs are made of small brass cotter pins run through holes in a block of insulating material. If the ends of these split pins are open a trifle too wide it is easy for them to catch on the edge of their socket and bend over. This is what happened and, in bending, it had become firmly wedged against the next pin, effectively shorting out the filament supply for the ux-226's. This particular point was about the last place that one would look for trouble and in the writer's experience it is the first time that a short of this kind has come to his attention." Metal Eyelets Cause Trouble: H. W. Hendelson, Badio service and merchandiser, of Vandalia, Mo., cracks another hard nut. "We recently had a short that was a hard one to locate. We knew this short was in the r.f. plate circuit, but could not locate its position. We tested the by-pass condensers and found themO.K. The r.f. coils were attached to the subpanel with metal eyelets and one resistance strip in the filament circuit was also attached by an eyelet. These eyelets also furnished the hole for the wiring to pass through to the r.f. stages. The wires going through these holes were rubber covered with a cotton covering between rubber and wire, and this insulation appeared to be O.K. where it went through subpanel. I lowever, it seemed to me that there was no other place for a short, so I opened one wire at a time, watching the meter as I did so. As 1 opened the wire going through the eyelet that held the filament resistance I got a zero reading on meter, thus locating the short. I placed a piece of spaghetti tubing around all wires going through eyelets and in this way corrected the trouble." On the Use of Relays: J. P. Kennedy, senior student in E. E. at Notre Dame, Screen-grid receivers have been on the market just about long enough to develop characteristic faults and ailments. The screen-grid set by now has become a part of the servicemans daily routine. We particularly solicit contributions from servicemen on the problems of screen-grid servicing, which direct attention to characteristic troubles and their correction. These troubles seem to be lack of selectivity, noisy tubes, short life, and marked fluctuation in signal strength. We will pay special rates for unusually interesting contributions on screen-grid trouble shooting. — The Editor and radio serviceman of South Bend, Ind., sends through a couple of notes on relays: "The most troublesome feature about relays used in conjunction with sets having fixed resistors for filament control has been the excessive voltage drop across the series actuating coil. As three-quarters of This m It is de odel, No. 547, is Weston's latest lest equipment, signed to run every practiea! test on d.e. and a.C. receivers, including screen-grid sets. an ampere is usually enough to work these relays, a shunt of nicrome or other resistance wire may be bridged across the series coil terminals. The amount of wire will vary for different sets and relays but can always be determined by leaving the set turned on and attaching the resistance wire to one terminal and touching various points on the wire to the other terminal. When the relay clicks, it means the resistance wire shunt has too low a resistance and the desired length is that which just permits the relay to operate when the set is turned on and off. "One of my customers has a double outlet plug in the B-power unit side of the relay box and a floor lamp alongside the radio connected, to the extra outlet plug. When the radio is turned on, of course, the floor lamp lights. The effect is charming." Reducing QRN in D.C. Districts: "The writer, whose job is servicing radio sets around the West End Avenue section of Manhattan, has had quite a bit of experience in curing artificial static caused by electric refrigerators, sparking motors, heating pads, and other electrical appliances. Sometimes it has been difficult to locate the offending instrument and then again there have been so many of them that it would have been impossible to fix them all. "Most of this artificial static is picked up by the lead-in wire and the writer has found that a marked reduction of static is obtained, especially where the set itself is of the shielded type, by raising the antenna as high as possible above the roof and replacing the ordinary rubbercovered lead-in wire by some Beldenamel shielded hook-up wire ($3.50 for a 100foot roll) and connecting the shielding to the ground binding post of the set." J. Futterman, Bronx, N. Y. A Few Fireworks: Albert Corideo, radiotrician and authorized S-M service station, of Waterbury, Conn., finds hurry calls diverting. "A woman living two doors away from me came dashing into my laboratory pale with fright shouting that her radio was on fire. I made a dash for her house, and got there before her. There was a curl of black smoke rising from the receiver in the dining room. I pulled the plug from the socket, pulled the console away from the wall, and through the smoke in the set observed a bright red spot. " After giving the receiver and power pack a testing which lasted an hour I found it in good condition. The part that was burning was the insulating material on the power rheostat. • O C T OBEK 1 9 2 <> •