Radio broadcast .. (1922-30)

THE SERVICEMAN'S CORNER Screen-Grid Servicing S SCREEN-GRID problems are begin- ning to crop up regularly in the Corner's mail. DONALD F. SAMPSON, of Sampson's Radio Laboratory of Central City, Nebraska, writes on the subject as follows: " One rather interesting ex- perience I had with a defect- ive screen-grid tubejwas when I found a set (Silver) that went off and on intermit- tently, and which could be either started or stopped by jarring the receiver. This generally indicates a loose connection somewhere in the circuit. The trouble, however, was found to be a short be- tween the elements of the second r.f. tube. "Although the Silver re- ceiver is designed so that matching of screen-grid tubes is not necessary to prevent oscillation or to obtain the F roper degree of selectivity, have found that all screen- grid tubes are not good de- tectors. This is especially true where the tube is used as a power detector. A few cases have also been found where the detector introduced a con- siderable amount of a.c. hum. The most common symp- tom of an inferior detector tube is lack of "kick," and this can almost always be remedied by interchanging the detector tube with one of the other tubes in the set. A poor de- tector tube may also cause a certain fuzzi- ness in the tone quality of the machine. Of course, at times a noisy screen-grid tube will be found, but so far as the writer has been able to determine, the symptoms and the method of finding the defective tube are the same as for the 227 tube. "My experience has been, using R.C.A. and Cunningham tubes, that screen-grid tubes are not 'short lived,' but, on the contrary, they will stand just as much abuse as any other heater-type tube. I have carried a Silver receiver on the floor in the rear of my car for over seven thou- sand miles, and not a screen-grid tube has been changed. "I found one set in which all the volt- ages were about forty per cent, below normal, excepting the grid voltage of the 227 tube in the first a.f. stage, which was fifteen volts, instead of one volt. The trouble was found to be in the 227 tube and on replacement all voltages were correct, and the receiver operated properly. "I have found more trouble with 227 tubes, because of a gaseous condition, than with 224 tubes, and believe that screen-grid tube troubles will be no greater than those encountered with the 227 tubes." A complete test bench: The interesting test bench and panel illustrated on this page is part of the equipment used by L. C. Wingard, service manager of the Cleveland Talking Machine Company, jobbers of Cleveland. With it, it is possible to test every piece of apparatus which is handled by the company. A General Radio The test equipment pictured above is the apparatus em- ployed by the Cleveland Talking Machine Company. It is designed so that all types of tests may be performed on the apparatus which they sell as wholesalers. oscillator designed to test Radiola 64's but remodeled so that it will cover the broad- cast band; two loud speakers, a magnetic and a Radiola 104, switches for using either low- or high-impedance pick-up units; complete power apparatus furnish- ing various voltages and currents—all these are but part of the apparatus built into, on, behind, and under the bench shown in the picture. A soldering kink: DOUGLAS B. SEVIN borrows a thought from welding technique in devising an arrangement for soldering in fairly inacces- sible places. He uses a carbon rod, taken from a flashlight cell, aflixed to any suitable holder. One side of a low- potential source —such as an A battery or a 2.5-volt fila- ment winding—is connected to the carbon and the other to the connection to be sold- ered. The carbon is pressed lightly an eighth of an inch or so away from the point of soldering, and the joint is al- most immediately heated to a solder-melting temperature. The Crosley Bandbox: "From observation, I have noticed that the Crosley Model 602 Bandbox con- denser gang, gets out of line after a few months' of use. It seems that tightening the con- denser drive bands too tightly has a tendency to warp the two end condensers gradu- ally. If you examine the de- fective condenser closely in a case of this kind you will see that the rotor plates are not properly centered all the way across the condenser when it is at maximum capa- city. This causes it to be The serviceman is consistently confronted with the mechanical problems of installation and main- tenance. A specific case is the in- stallation of a moving-coil loud speaker so that it offends neither the eye nor the ear. It is unfortu- nately\almost a corollary that for an eleclrodynamic loud speaker to be efficient acoustically it must present to the eye a cross between the smoke- slack on Peter Cooper's locomotive and an ironing board. We should appreciate contribu- tions from servicemen describing the reconciliation of electrical and mechanical efficiency with the dic- tates of good taste in the well- appointed home — from antenna lead-ins to loud speaker camouflage. — THE EDITOR. 'in line' at one place and not at another. You will also note that the condenser has two adjustment screws on each side. Line these condensers up properly and balance the set with an oscillator and 'dummy' tube and you will find that the set has the original selectivity and volume." H. ODELL PUHLES, Service Department, Vaughn's, Inc., Greensboro, N. C. Data on Zenith sets: WALTER STRAUSS, JR. (w9cwx) runs into screen-grid and other troubles with Zenith sets: "The early model Zenith screen-grid receiver employed a 224 for the first r.f. stage and either a 250 or 210 in the last a.f. stage. Two small 25,000-ohm resistors in series were used to reduce the 450 volts going to the plate of the last a.f. tube. The reduced voltage, about 135 volts, went to the screen grid. After several weeks of operating one of these resistors is apt to 'go west' and a good indication is: weak os- cillations over the entire band; low vol- ume; and broad tuning. "To remedy this, use a 50,000-ohm re- sistor with the good 25,000-ohm unit and take out the bad one. The insertion of the 50,000-ohm resistor will cut down the screen-grid voltage to some extent but the volume will come to about par and so will the selectivity. I haven't had any trouble since the insertion of the 50,000-ohm re- sistor which was about 5 months ago. 104 • • DECEMBER 1929