Radio Broadcast (May 1929-Apr 1930)

-40' Bare Stranded Copper Inside Ship -(Km*-" Ground Clamp to Water Pipe Fig. 1 — Wiring diagram of the test ben^h picui ed on page 87 If the workers like, a permanently installed arrangement of testing meters may be mounted on a bakelite or metal panel and added to such a work bench. This has some advantages and some disadvantages, depending on individual preference. A few models of complete testing tables, factory built, are on the market. One made by a well-known firm contains seven meters mounted on a steel panel 24 by 42 inches, placed horizontally across two wooden posts 36 inches high. The meters are as follows: 0-7.5 volts d.c; 0-75 volts d.c; 0-150-300 volts d.c, 1000 ohms per volt; 0-15-150 d.c. milliamperes; 0-4-8-16 volts a.c.; 0-250-750 volts a.c; and 0-1.5-15 microfarads. Binding posts and switches are provided so that each meter may be used individually and for all ranges. The panel is wired up with a tube socket and a long cord with a tubeprong plug at the end. By means of these, each circuit in a radio set may be tested with the tube in a socket. Outlets may be connected to a 110volt 60-cycle line. Voltage may be read and a radio set then plugged into the outlets. The 750volt a.c. range is for testing the highvoltage secondary winding of a power pack and the capacity meter is useful for determining condenser values and in keeps no-charge service calls at a minimum and develops confidence for the shop and men employed by it. While individual cases frequently arise, calling for ordinary common sense in the handling, the following list of troubles and probable causes may be considered a general guide: No Ej>: Bad B batteries, bad power pack, open plate circuit. The latter may be due to a burned-out plate resistor, a burned-out transformer coil, or a poor or broken connection. A shorted by-pass condenser, or bad rectifier tube, in case of an electric set. No Es: Bad C battery, or open grid circuit which may be due to burned-out grid resistor, burned-out transformer coil, or bad connection. In electric sets where lp through a resistor gives Ee, check plate circuit also. High //.• Wrong C bias (too low or reversed), shorted tube, shorted plate resistor, or wrong connection batteries or power pack. No E/: Bad filament battery, bad power pack, open rheostat, or bad connection. Testing Equipment The testing apparatus employed, and the cost of it, is largely a matter of choice with the shop-owner. A large amount of expensive equipment is not actually required in order to conduct a successful servicing business, if such apparatus as is purchased is well chosen. Each outside serviceman must have a neat box with a small number of carefully selected tools. Superfluous tools only get in the worker's way. The kit should contain two styles of pliers, wire-scraping knife, soldering iron, two screw drivers, and a couple of octagonal wrenches. If one of the screw di'vers is magnetized it will' be a great help. This is easily accomplished by inserting the blade inside of a coil of about 50 turns of No. 18 insulated wire which is connected across the terminals of a sixvolt battery and closing the circuit for a few minutes. It can also be done by holding the screw driver, with a good tight grasp, against the field of a d.c. motor while running. A small mirror, such as carried in women's hand bags is often very useful in looking at crowded wiring and connections under circumstances where it is not desired to dissemble the set, and this is small and of light weight. [A dentist's mirror might prove even more satisfactory — Editor.] Besides the tool kit, the outside serviceman should carry an assortment of standard tubes in a separate box, and a 0.00035 Power Pack Fig 2 — I oil meter continuity testing. 7.5 150 Fig. 3 — Oscillator-voltmeter test combinations. standard tube and set-tester, or set of testing meters. Inside the service shop, there should be one completely outfitted work bench for every three or four outside men. In our former article we showed a picture of the actual layout of a service-shop bench which is in continual practical commercial use. The heading picture of this article gives a general idea of the double test bench in use in the Loomis laboratory. This is located in the center of the room in order to permit students to gather around it while trouble-shooting demonstrations are going on. Otherwise it would be much better backed up against windows. A complete wiring diagram of this table is shown in Fig. 1. Batteries and a.c. generator are installed beneath the bench, with wires brought up t hrough drilled holes to conveniently arranged terminals. Fig. 4 — Diagram of the oscillator tester shown in Fig. 5. locating condenser shorts. The bench described above is the Jewell radio test bench, No. 580, price $278.50 Standard Testing Methods It would be rather out of the question, in an article of this space, to undertake to cover tests on every known make of receiver. Individual tests for various makes of sets, and specific uses of various standard test sets, are at all times available from manufacturers. Each receiver requires its own particular tests, and the makers, in their service manuals, thoroughly explain the correct way of making tests which are best adapted to the characteristics of their machines. In addition to this information some test-set manufacturers supply with their testers complete testing data for every standard receiver. Certain tests apply in a general way to all sets, regardless of make. These may be set forth as follows: Test for plate voltage: Touch leads from a high-voltage voltmeter to the plate and filament prongs of each tube socket, with the set connected to its usual power supply. If no voltage is indicated, there is an open circuit in a transformer coil or a shorted by-pass condenser in the circuit attached to the plate prong of that. tube. If several tubes show no plate voltage there may be a broken or loose connection. If a power pack is used, the output voltage of this should be checked, and, if trouble is indicated there, the power pack nee:ls to be overhauled. locating open circuit: If the power input to the set is o.k., and a voltmeter gives no reading as referred to above, the same voltmeter may be employed in locating the open which is probably the reason for no voltage on the plate of the tube. The open may be either the primary of an audio-frequency transformer, a coil in the 8 8 e • JUNE 1 929 •