Radio Broadcast (May 1928-Apr 1929)

cv.i.O The Service Man's Corner "3? THE many practising radio service men who see Radio Broadcast regularly have praised highly the articles for and by service men which are now a regular feature of this magazine. Those of our readers working in this field seem to like "The Service Man's Corner" especially. "Although I feel confident to face almost any kind of service problem," writes one reader, " I enjoyed reading the first 'Service Man's Corner' and am looking forward to future issues. All of us need to keep in touch with what other workers are doing and I feel I can always learn something from reports of other's experiences." Even though some of the suggestions appearing in these pages may seem self-apparent and too simple to deserve mention, it is possible that the point covered is so obvious that many readers have never thought of it. . A.C. Hum: Some service men tell us that where they have replaced a.c. tubes in an "electric" set an unusual amount of hum developed. The answer is apparently in the fact that some a.c. tubes or circuits vary slightly in some characteristic and adjusting the resistor responsible for balancing out the hum will cure the trouble. In other words, the resistor in question should have the tap adjusted in the exact center, but in the case of some tubes, the lowest resulting hum in operation results when the resistor is adjusted slightly off-center, the exact point being a matter of experiment. Many commercial receivers now are equipped with variable "center-tapped" resistors, which makes the solution of this trouble simple. In servicing less modern sets, it may be wise where possible to replace the fixed centertapped resistor with a variable unit. Getting the "lows": It is curious how a condenser across the loud-speaker (leads helps to bring out these lower notes that everybody is yelping for. Try different values until the customer yelps out loud. [This stunt will be effective on low frequencies, but will cut off most of the highs. — Editor] There is another suggestion I wish to make at this time: have a routine in checking a receiver and don't vary from it. I spent several days learning that a voltage-divider system was "open" in the detector supply before I relearned the value of an invariable routine. The groping was unproductive, but the trouble was almost at once apparent when the set was checked systematically. We never seem to learn: I blew three tubes a few days ago, and repeated that performance the next day. And yet it's so easy to remove tubes while working on a set! [What test routine do readers prefer? The best contributions will be printed. — Editor.] H.J. Goddard, Ellendale, N. D. R.T.F. Set Trouble: In servicing an Atwater Kent, the following trouble presented itself. The set worked on local stations but the signal strength was weak. Having eliminated the batteries, tubes, antenna, and loud speaker for faults, the set itself was inspected. It was a one CTJ HIS page marks the second appearance of our ■*■ department exclusively for the practising service man. It is unique, we feel, because for the most part it is written by the service man himself. These pages will be a forum where the service man can discuss his problems, get his pet idea into print, and see now and then a hint which will he useful in his daily work. Contributions which should preferably be short, to the point, and typewritten are solicited and will be paid for if used. Address your articles to the editor, "Service Mans Corner." — The Editor. dial, three-condenser set. The puzzling thing was that all continuity tests showed the set to be ok. By placing my finger on the stator plates of the first condenser the signals faded out completely; the same result was found in touching the third condenser. On the center condenser the signals remained the same. Moving the second condenser caused little change. Grasping the center or second r.f. coil, and forcing it slightly from side to side, the set worked ok at times. The continuity test showed no open circuits. By moving each wire connected to this coil a bad connection was located where the lead was soldered to the coil. The connection was slightly corroded. The condition was this: even though this connection passed 22 volts in the continuity test, the corroded joint would not pass r.f. current because of its high resistance. After the connection was cleaned and resoldered the set worked satisfactorily. Such a condition might not occur in a new set, but this possibility is well to remember when working on receivers which have been in use for some time. — Thomas Glose, Allentown, Pa. Filament voltage on the CA"-35o: A service man writes us that he "is having trouble with a new CX350 tube arcing across between the elements and wonders if the trouble is general with others." He continues, "I am using the drop across a 1 500-ohm resistor for grid bias and the tube draws 50 mils. On loud signals, there is , an arc inside the tube and the milliammeter in the plate circuit deflects toward the high end of the scale. I have had three tubes and they all do the same thing." Roger M. Wise, chief engineer of E. T. Cunningham, has cast some light on the probable cause of the trouble. He says: "In using the cx-350 we find that the important precaution of operatingthe filamentat approximately the rated voltage is often overlooked. We investigated recently a complaint of flash-over in an amplifier 18] in which two of these tubes were being used. When tested in our laboratory, the tubes in question operated normally at rated maximum plate voltage, but when placed in the amplifier giving the trouble, one of the tubes arced. A check on the operating conditions showed that while the plate voltage in this amplifier was only 375 volts, the filament voltage was 6.0. As soon as the filament potential was raised to the rated figure, 7.5 volts, the tube operated satisfactorily." Items of Interest SOME manufacturers tell us that the demand for power amplifiers for public address work in its various possible applications is one of the year's most astounding developments in radio accessories. The service man and professional set-builder who is interested in this work ought to have — in addition to the catalogs of the various makers — the General Radio Experimenter vol. 3 no. 4 for September, 1928, for the article "Notes on Group Address System," by C. T. Burke. Silver-Marshall's The Radiobuilder, vol. 1, no. 6, dated October 9, 1928, describes in interesting detail the new S-M rack-and-panel "P. A." amplifiers. Jenkins & Adair, of Chicago have just issued Bulletin No 7 on a microphone mixing panel which should be useful in more pretentious public address systems. fl The Weston Electrical Instrument Corporation have just released descriptions and prices on testing apparatus for the service man. Their publications describe Model 537 a.c.-d.c. set tester, Model 533 Counter Tube Checker (which is a.c.-operated) and circulars Y and X describing portable a.c. and d.c. testing instruments respectively. What list of equipment do service men feel is the minimum for field work? Most service men feel rather strongly on this point and we should like to have lists submitted. The results will be tabulated and be published in this department. ^1 Practising service men, especially those who are working out of a radio store, will find the excellent loose-leaf tube data book issued by E. T. Cunningham, 370 Seventh Avenue, New York, of constant value. These sheets, supplied in a binder give the following data: name of receiver, manufacturer's name, model number, a chart showing location of tubes, socket number and what part in the circuit each tube plays. Space is provided on each sheet for notes, and remarks containing useful data on the set in question. We are advised that Cunningham will supply service men with the book, on request.