Radio broadcast .. (1922-30)

.RADIO BROADCAST. making contact with points AA' except when the push button is depressed. By omitting contacts AA', a double-pole single-throw switch results. It is often desirable to lock a button when making certain tests. As indicated in Fig. 2, by means of a groove cut in the panel and a pin in the button, a slight turn of the button when pressing it, keeps it down. Turning the button in the direction opposite to that used in locking it, allows the pin to sink into the groove as the spring of the switch lever brings the button up to its normal position. To indicate when the button is locked, a line is engraved on one end and parallel to the pin. Since it is essential that the pin be in the exact center of the rod and at the proper distance from the ends, and that it must go through the rod at right angles to it, a hint on the best way to do it wul not be amiss. In this connection the writer has found that best results can be obtained only by the use of a jig. Fig. 3 shows how to make one. The rod should fit snugly in the jig and it is inserted so that the engraved line is parallel to the pin hole. If care has been taken to make the distance from the pin hole to the end of the jig exactly the dis- tance desired between the pin and the engraved end of the push button, all the buttons will come out identical and accurate. As contact points |-inch $* s round-head brass machine screws are used. The ends of the screws should be sandpapered until they are bright. It has been found that the brass contacts give service for about a year before tarnish begins to interfere with their proper operation. The switch levers themselves can be cleaned by scratching with a needle through the con- tact screw holes. All the contacts of the set tester can be gone over in less than half an hour and the instrument will then be good for another year of service. The B.C. Meter The d.c. meter has a 1-mA. full-scale movement. This will allow the use of 1000 ohms per volt in the multiplier re- sistors; that is, when used as a voltmeter, the resistance for a particular scale is 1000 ohms multiplied by the highest voltage on that scale. For instance, the resistor for the 10-volt scale of the meter has a re- sistance of 10,000 ohms. Meters marked with multi-range scales are obtainable from meter manufac- turers or a 1-mA. scale may be used and the readings multiplied by the proper conversion factor. Shunts are easily calibrated against another meter hav- ing the proper ranges. Be certain that the wire used for the shunt has Pin Hole -\UYCable [Heater Leads R, 5000 ohms Rz 5000 » Ba 50,000 • R« 50.000 R, 75.000 UX Cable, Fil.Leads Dotted lines indicate that both levers are operated by the one push button G-UX Cable G-UY , Cable 100mA. 10mA. Pin Jack for 222 •••..-. Adapter Lead + 10V. 100 V. 500V. +1 amp. +100 mA. Fig. 1 — Schematic of Diagnoser. IUY Cable PL Lead tors, since the maximum dissipation is 1 mA. at 500 volts or 0.5 watts. Practically all wire-wound resistors will dissipate at least this much wattage. If multipliers are made for the a.c. voltmeter the matter of Panel sufficient carrying capacity for the cur- rent it is to handle. Since the shunts are in series they must each be capable of carrying the maximum current any tube might draw. There should be no trouble due to overloading the multiplier resis- Topviewof . . Push Button sufficient wattage capacity be- stowing En- comes very important since graved Line many a.c. movements draw as much as 300 mA. A 150-volt multiplier for such a movement would have a resistance slightly under 500 ohms (500 ohms minus the resistance of the move- ment) and would have to dissipate 45 watts at full-scale deflection. A built-in oscilla- tor is an absolutely necessary compo- nent of a set-tester. And it should be completely self con- Fig. 3 tained. If it must be powered by the Pin Hole through exact center of Hole Hole diam of " Push Button 0.0005 mfd Twenty turns wound between four screws in panel to form loop 7 by.10 inches Fig. 4 a.c. lines, its usefulness is largely curtailed. The oscillator used in this diagnoser con- sists of a tuned circuit, shock-excited by a buzzer. About 20 turns of magnet wire wound as shown in Fig. 4 and shunted by an 0.0005-mfd. condenser will give a wave of about 400 meters. The buzzer used should be of the high-resistance type. (Federal or Mesco). At 3 volts these buz- zers draw less than 75 mA. and a couple of flashlight cells will, in ordinary service, last about a year. The test plugs can be made from tube bases. If you haven't access to a lathe, a wood-turner will make handles for them for twenty-five cents. The picture shows the panel layout fol- lowed by the writer in the construction of his diagnoser. Unless there is some manifestation of the trouble to indicate in just which cir- cuit the fault exists, it is advisable to check through the receiver in the same order that the signal follows— first radio-frequency stage, second radio-frequency stage . . . , detector, first and second audio stages. Insert the test plug (with an adapter if necessary) into the first socket of the set under test, and place the tube in the diagnoser socket. Pressing the button marked "A" (Fig. 1), will show the filament voltage at the socket. (Not the voltage of the A supply). Should the pointer move backwards, release this button and press the one marked "A rev." Alternating voltage is indicated when the "3V" or 15V" a.c. button is pressed. Some few sets using a.c. tubes have a re- sistor in series with the tube filaments. (The Garod EA and one of the Zenith models are the only ones the writer knows of.) In such a set, the voltage shown by the diagnoser's a.c. voltmeter will not be the actual voltage applied to the tube, but will be less because of the increased voltage drop across the filament series resistor occasioned by the added current of the voltmeter being passed through the resistor. Grid circuit continuity is ascertained by pressing the "C" button. In a detector using grid-circuit rectification, the grid condenser must be shorted out to obtain a reading, since this condenser isolates the grid of the tube from the rest of the cir- cuit, so far as direct currents are con- cerned. The values of C bias have been arbitrar- ily taken with reference to the negative side of the filament. However, as may be seen from the circuit diagram, the grid return in the diagnoser is brought to a center-tapped resistor of 1000 ohms, across the filament. If the filament is sup- plied with a.c. there is no d.c. potential difference between the center tap and the filament terminals, and C bias may be read accurately with respect to it. If the filament is energized by d.c. this mid-tap, of the resistor is at a positive potential with reference to the negative end of the filament by an amount equal to one half of the filament voltage. In this way there is always a potential difference between the grid and the negative filament to indi- cate grid continuity, even when the set has no C battery. (There are still some antediluvian battery-operated relics, inno- cent of a C battery, in existence!) The ac- tual C bias then, is the value indicated on the meter, less one half of the filament voltage, if d.c. is used on the filament. There is another effect, though, that tends NOVEMBER 1929 •