Radio broadcast .. (1922-30)

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V THE SERVICEMAN'S CORNER Testing Screen-Grid Tubes BORIS S. NAIMAHK, of the Riverside Auto and Supply Company, New York City, dealer in Colonial and Steinite, sends the following interesting contribution in recognition of the test requirements of screen-grid tubes and the almost universal distribution of test sets designed before these tubes became popular. "The advent of screen-grid tubes has rendered virtually obsolete thousands of perfectly good tube testers. Almost every radio work shop and certainly every radio store has one or more of these testers, which, while perfectly satisfactory in all other respects, are not suitable for testing screen-grid tubes. It is the purpose of this article to describe just how such tube testers may be modernized so that they may be employed to test the four-element tubes. Such a program of modernization involves only a trifling expenditure, under no circumstance exceeding fifty cents for the two units described. " If the reader has a battery tube-tester and wishes to test the 222-type screen- tube, he can make the necessary adaptor from one ux socket, one ux tube base, two small clips, and about two feet of hook- up wire. If an a.c. tube tester is available, all one needs to do in order to test the 224-type tubes is to assemble an adaptor consisting of one UY tube socket, one UY tube base, two small clips, and hook-up wire. The circuits are shown in Figs. 1 and 2, respectively, and the construction should not present any difficulties what- ever. "All of the above mentioned materials, the reader, no doubt, will find in his 'junk-box.' The construction, of course, is very simple, all data being indicated in the self-explana- tory diagrams. The operation of the adaptors is also prac- tically obvious. "The correct procedure is to place the tube base in the tube-tester socket, place the tube to be tested in the ex- tra socket, connecting the clip marked' to top of screen- grid tube' to the control grid, and the clip marked 'to B plus' to some suitable posi- tive potential. Then the tube is tested in the usual man- ner. What constitutes a suit- able positive potential for the screen grid? A little ex- perimenting seems to indi- cate that one third of the applied plate voltage is just about right. This voltage need not be supplied by an independen't source of potential and may be easily diverted or tapped off the voltage normally applied to the plate of the tubes ordin- arily under test. "The average tube-tester contains at least two meters, a voltmeter and a milli- ammeter. The first makes it possible to secure the correct filament voltage. This, of course, should be adjusted to 3.3 volts for the d.c. screen-grid tube, and 2.5 for the a.c. screen-grid tube. The milliam- meter indicates the plate current flow in the tube. It is through a comparison of The serviceman is often called in to shoot trouble on a strange re- ceiver — a set with which he is not over-familiar and one for which the owner has mislaid all operating and circuit data. On such jobs, when the trouble is not obvious, the serviceman's first task is to trace the leads — identifying the coded network of wires as those leading to definite supply voltages. The extent to which this decipher- ing can be expedited often deter- mines the efficiency with which the repairs can be effected. We should like from our readers a description of the systems, if any, recommended for the rapid identification of unknown leads. — THE EDITOR. readings obtained on this meter under dif- ferent control-grid conditions that the actual operating worth of a tube is de- termined. In the average tube-tester this change in control-grid condition is ob- tained by means of a specially provided button. With the button up, we have a certain definite grid voltage condition The portable service laboratory of Waldo Todd Pratt, which makes it possible for him to carry on almost any service work under the actual conditions experienced by the set user. with its corresponding plate current flow as indicated on the milliammeter. When the button is depressed a different bias condition prevails and, accordingly, the value of the plate current, as indicated on the plate milliammeter, is either greater or smaller than the original reading. It is the difference between the two readings that denotes the utility of a tube in the set. It is impossible for the writer to in- dicate just what plate current readings will determine a good, fair, or poor screen- grid tube because it is impossible to fore- tell under just what conditions the test will be conducted. A slight difference in any of the potentials involved in the test affects the readings obtainable. All we can suggest, then, is that the reader test several new tubes and take the readings thus available as indicative of a good tube. Any tube giving readings that fall short of the amounts obtained from a rep- resentative group of new tubes is either poor or fair, depending upon the amount of deviation from the standard values. "We want to enter a word of caution at this time: when a tube test is conducted it is absolutely essential that the filament potentials be adjusted accurately and ex- actly the same for all tubes in order that the readings obtained may be relied upon." A Portable Service iMboralory: WALDO TODD PRATT, West Hartford, Connecti- cut, specializing in Radiolas, is one of the service school that believes in doing the job on the spot. To work effectively on almost any job outside of his shop, he has accumulated the automobile laboratory, shown in the picture on this page and com- prising the following equipment: 1. (At left) Phonograph turntable and pickup, for demonstration, and audio tests. 2. (Above) Western Elec- tric 540w for checking loud speakers. 3. (Next to turntable) Phonograph records. 4. (Above) Oscillator. This has the necessary connectors, with suitable cords, to run as (A) a.c. with 60-cycle modulation from lighting outlet. (B) d.c. plugged into tube testing socket of Jewell No. 117 kit, which thus provides readings of filament voltage and plate current. This provides (1) Modulated oscil- lation, 550 kc. to 1500 kc. (2) Unmodulated os- cillation, 550 kc. to 1500 kc. (C) An external socket- is provided where- by tubes may be "matched" by noting the filament voltage at which they go into oscillation. 5. Radiola 26, altered to 38 • • NO VEM BER 1929