Radio Broadcast (May 1929-Apr 1930)

THE SERVICEMAN'S CORNER IN "The Serviceman's Corner" we are endeavoring to group contributions on related subjects. Aside from the convenience of future reference, the expression of half a dozen minds on one subject is generally of more value than an isolated opinion. So you will find this month, and in future editions of this department whenever the number of related contributions justifies it, a symposium of service information on one particular receiver. Such considerations will not necessarily reduce publication of comments on unrelated and interesting items in service routine, nor should they be taken of amperes indicated on the meter. In the specific cases under discussion, R will always be so high that the combination is a so-called high-resistance voltmeter (it may be used for measuring B and C potentials furnished by a B-power supply unit) and the meter will be a milliammeter, preferably having a range of from zero to milliampere. The voltage will then be the resistance of R, divided by one thousand, multiplied by the reading on the meter (in factions of a milliampere) . The following table indicates the proper values of resistors for full-scale deflection on T! RECORD OWNER as indicative of an unusual amount of trouble with the receivers made the subject of group discussion. Your comments on this, as well as our handling of other phases of radio servicing, are always welcomed by the department editor. Test Set Reduced to Lowest Terms 1HE necessity for radio voltage and current testing equipment, along with the high cost of the same has presented a problem to many servicemen that has often echoed its way to this department in the form of a request for data on inexpensive and reliable test equipment. As a matter of fact, the entire gamut of d.c. tests, [requiring milliammeters covering ranges from one milliampere to a hundred and high and low-resistance voltmeters reading A, B, and C potentials from batteries or power-supply arrangements, can be covered with adequate accuracy with one meter in conjunction with an inexpensive assortment of wire-wound fixed resistors used in a combination of series and shunt connections. A satisfactory meter which may be used for this purpose is a standard 0-1 milliammeter. Differences in voltmeters and ammeters of various ranges are principally differences in the resistance characteristics of the instrument, based on the simple and fundamental statement of Ohm's law: viz, voltage equals the current in amperes multiplied by the resistance in ohms. More simply, E = I X R. The fundamental circuits of a milliammeter employed as voltage and current indicators are shown respectively in Fig. 2 (a and b). Resistors R are connected exterior to the meters. In the circuit of Fig. 2 (a) R is always so much higher than the internal resistance of the meter, that this latter resistance may be neglected in all calculations. If a source of unknown voltage is connected across the terminals of the circuit, Fig. 2 (a), the voltage will be equal to the resistance of II in ohms multiplied by the number OF SERVICE CALL ncl DATE ADDRESS MAKE AND TYPE OF SET TYPK POSITION OP TUBE IN SET ere. U° "volts PLATE MA WITH EJiT. PLAT £ A. f LINE VOLTS A C VOLTAGE ADJUSTMENT HIGH LOW OR MED REPAIRS OR REPLACEMENTS MADE is broken up into 20 equal divisions, and an unknown voltage is measured with R = 100,000 ohms, ten divisions (0.5 milliampere) indicates a potential of 50 volts, 12 divisions, 60 volts and so on. An ideal instrument for this purpose is the Weston type 301 milliammeter, zero to one milliampere range, which lists at §12.00. Daven " Super-Da vohms " are economical resistors, sufficiently accurate for the purpose of voltage multipliers. The calculation of the resistor values required as shunts in Fig 2 (b), to increase the current range of the instrument, is a bit more complicated, so that only the results will be indicated. The following table shows the current for full-scale deflection on a 1.0-mA. meter with the indicated resistor shunts. CURRENT 1 mA. 50 mA. 100 mA. RESISTANCE no shunt 0 . 57 ohm 0 . 27 ohm THE HICKOK ELECTRICAL INSTRUMENT CO-. CLEVELAND OHIO. Frank T. Carr. Jr., scientific radio service Report of Radio Set Analysis Service record charts that contribute efficiency to the service business by facilitating a check up on repeat calls. the indicated voltages when using a 1.0-mA. meter. VOLTAGE 5 10 100 500 RESISTANCE 5000 ohms 10,000 ohms 100,000 ohms 500,000 ohms Any cates a fraction of full-scale deflection indisimilar fraction of the full-scale volt age.' For instance, if the one-milliampere scale As with the voltmeter arrangement, any fraction of full-scale deflection indicates a similar fraction of full-scale current in milliamperes. (These current multiplier figures are based altogether upon a 0 to 1 milliammeter having an internal resistance of 27 ohms, the characteristics of the Weston type 301 already recommended. While the voltagemultiplying resistors will be correct for any zero to one milliammeter, the current shunts will apply only to a meter of the characteristics indicated.) J It should not be difficult to secure the proper resistors for the current shunts. A simple way is to secure the winding from a one or twoohm rheostat, determine the resistance of the wire per foot, and cut off the correct amount. A simpler way of obtaining various shunts is as follows. So adjust the circuit including the milliammeter to a predetermined value, preferable fidl-scale deflection. Now shunt any low-range variable resistor across the meter and, vary the resistor until any convenient fraction of the original current is shown on the meter. Leaving this resistor so connected, the same fraction will hold true for any indicated current. It is obviously possible to arrange a zero to one milliammeter, by means of suitable resistors and switches, so that the voltages and currents generally encountered in radio testing and servicing can be measured conveniently on the one instrument. An arrangement of this type was described by G. F. Lampkin in Radio Broadcast for June, 1928. Fig. 1 suggests a neat and convenient metho(| • may, 1929 pnge 18 •