Radio Broadcast (May 1929-Apr 1930)

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~2L RADIO BROADCAST of mounting and adjusting a combination volt-milliammeter of this design. AN INEXPENSIVE TEST SET R. K. Wheeler, of the Wachstetter Radio Company and General Radio Laboratory, of Indianapolis, Ind., solves the equipment problem of the serviceman in a similarly economical manner. He writes: "Many excellent suggestions have been made in reference to radio set testing and servicing. However, all have employed comparatively expensive apparatus, and there are, no doubt, many servicemen, and set owners, who would be glad to have an inexpensive, reasonably accurate outfit, such as I have been using for the past year. The entire test set cost less than $8.00, and it will do practically everything that the more expensive ones will do. "The main item of the test set is a combination tube tester, milliammeter, and two-range voltmeter, Figs. 3 and 4, composed of the following parts: One Readrite 0-25 raA. meter One 2000-ohm resistor, Bradley One 250-ohm resistor, wire-wound Four. Binding posts One Wood case, home-made One tube base, ux-type $1.00 .50 .25 .20 .35 $2.30 "This is wired as per diagram, Fig. 3 and is plugged directly into a receiver to test tubes. The 0 — 6 and 0-50-volt taps are provided for battery checking and continuity tests, and are, of course, worthless for powerpack testing. However, for this purpose an excellent plug-in meter is made by Beede, 0-300-volt range, which shows voltages at the various sockets and is also a good check for continuity of the various plate circuits. This meter is sold at the local Kresge store for $1.75, and, while readings cannot be made with an exactness of 2 or 3 volts on account of the 0-300-volt scale, the writer's meter has been checked several times with Weston and Jewell high-resistance meters and found sufficiently accurate for the serviceman's purpose. "The other item in this kit is a Beede, 0-7.5 a.c. voltmeter, (price $1.00), also plug-in type, for checking filament voltages at the socket. This meter has been found accurate and has been an important item in the writer's kit. As the voltage reading at the filament is one of the most important considerations, it was not felt necessary to invest in an 0-150 a.c. voltmeter." Service Hints on Radiolas WHEN receivers are .as widely circulated, as the various models made by the R. C. A., it is logical and inevitable that the service problems should increase in some way proportionate with their popularity. The serviceman will find it worth while jotting down this page number for future reference. Frank M. Coates, with the McGraw Elec 2000 ohms O A Neg. 0-25 06 V. 050 V. mA. Fig. 1 — A neat and efficient mounting arrangement for a universal volt-milliammeter. trie Company, Sioux City, Iowa, has run into the following interesting cases: "Probably the most baffling troubles in radio receivers are those which do not affect the normal voltages at the tube sockets. The following are very peculiar and interesting cases of this nature which I have found in service work. " 1. The antenna lead of a Radiola 18 be o o Fig. 3 — Circuit diagram of Mr. Wheeler's test set costing $2.30. To Measure Current (B) Fig. 2 — (A) An elementary voltmeter consisting of a low-range milliammeter with a series resistor. (B) A low-range milliammeter, with a series of shunt resistors, R, can be made to cover a variety of high-current ranges. came pinched in such a way that the wire broke inside the insulation which was intact. The trouble was located by connecting the antenna to the volume control where the antenna lead terminates. "2. On a Radiola 60 it was found that by moving the oscillator grid resistor the set would work. Inspection showed that where the bus wire lead connects to the tube socket the paint had not been removed, and this insulated the wire from the solder around it. As the oscillator tube has zero d.c. grid voltage, a meter test would be useless in this case. "3. Another case of intermittent trouble on the model 60 was an open connection to the output condenser. The metal tab became broken under the insulating cover. The trouble was found by connecting a good 0.5-mfd. condenser across the two terminals of the output condenser. "4. The most freakish case I have seen was a power unit that was connected to a light line which was struck by lightning. The transformer, choke, and filter condensers were not damaged but the flexible wiring had the strands fused into little globules inside the insulation which was not burned in the least." HELP FROM THE G. E. COMPANY M. G. McCarroll, with the Radio Engineering Department of the General Electric Company at Schenectady helps the cause along: "I have run across the following trick faults while trouble shooting radio receiving sets. One of the commonest causes of 'no signals' in a Radiola 60, super-heterodyne, is found in a short circuit between one of the r.f. coils and a socket prong. The coils, not being mounted very rigidly, get pressed over sometimes so that when the tube is inserted in the socket there is a contact made between the socket prong and a terminal lug on the top of the coil. It usually is the coil located under the sixth socket from the left end of the set, and sometimes a person can reach down behind the chassis and with his fingers move the coil away from the socket prong without removing the chassis from the cabinet. "The hardest case ever encountered of a loose connection was on a Radiola 60, and after exhaustive searching, in which everything checked out ok. it turned out to be a faulty tuning condenser on one of the intermediate-frequency transformers. The screw holding the plates of this small condenser had come out and while the set operated fairly well, the scraping sound of a loose connection was present with any slight vibration. The variable condenser tab had to be removed in order to get the i.f. transformer out of the chassis for repair, the small defective condenser being located inside the case of the i.f. transformer. "Another unusual incident on Radiola 60's was a case of extreme fading on local signals. The volume would go clear down and then gradually come back to full strength, repeating this several times a minute. The trouble was a dirty contact on the potentiometer used as a volume control. Since the r.f. and i.f. tubes get their bias through this potentiometer it is obvious that when poor contact occurred with the arm the grids of these tubes were left free and consequently caused the fading mentioned above." DATA FROM N. Y. C. SERVICEMAN A categorical source of trouble in the Radiola 17 has been located by J. C. Yaegar, man a 4 © Neg. 0-25 0-6 0-50 mA Volts Volts Fig. 4 — Constructional tletails on another single-meter test set. • may, 1929 page 19 •