Radio Broadcast (May 1928-Apr 1929)

294 RADIO BROADCAST ADVERTISER DON'T MISS "The Engineering History of Radio" By Donald McNicol Felloic A. I. E. E. Fellow I. R. E. Past President of I R E. It is appearing serially in Radio Engineering beginning with the June 1928 issue Editoi — M. L. Muhleman Managing Editoi — G. C. B. Rowe Associate Editor — John F. Rider Associate Editor — Austin C. Lescarboura A new section covering COMMERCIAL DEVELOPMENTS now appears in each issue. It deals with aeroplane and train communication, talking movies, picture transmission, speech amplifiers, etc. Some of the other articles in the June issue are: The Sulphide Rectifer by Dr. H. Shoemaker Selecting a Band of Radio Frequencies by G. F. Lampkin Radio Set Power Supply by George B. C rouse A. C. Tubes vs. Series Filament Operation by W. P. Lear Mathematics of Badio by John Bider High Voltage D. C. Generators by J. H. Blankenbuehler Radio Engineering is NOT sold on newsstands. If you are not a subscriber already, use the coupon below. NOTE — The June and July numbers ivill be sent gratis to those using the coupon below in subscribing. RADIO ENGINEERING, Inc. 52 Vanderbilt Ave.. New York,N. Y. Enclosed find ! fo*!!!! i-°r.^ ^oar ! subI !s3.00 for 1 years ) scription to Radio Engineering to begin August, 1928. Enclosed find twenty cents (20c) for which send sample copy of Radio Engineering. (PLEASE PRINT NAME AND ADDRESS) Name Address City State Please check classification. □ Manufacturer □ Dealer □ Engineer Q Technician Anything else SHEETS THE Radio Broadcast Laboratory Information Sheets are a regular feature of this magazine and fcave appeared since our June, 1926, issue. They cover a wide range of information of value to the experimenter and to the technical radio man. It is not our purpose always to include new information but to present concise and accurate facts in the most convenient form. The sheets are arranged so that they may be cut from the magazine and preserved for constant reference, and we suggest that each sheet be cutout with a razor blade and pasted on 4" x 6" filing cards, or in a notebook. The cards should be arranged in numerical order. In July, 1927, an index to all sheets appearing up to that time was printed. In the May, 1928, issue we printed an index covering the sheets published from August, 1927, to May, 1928, inclusive. All of the 1926 issues of Radio Broadcast are out of print. A complete set of sheets, Nos. 1 to 88, can be secured from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Orders for the next set following can also be sent. Some readers have asked what provision is made to rectify possible errors in these sheets. In the unfortunate event that any serious errors do occur, a new Laboratory Sheet with the old number will appear. — The Editor. No. 217 Radio Broadcast Laboratory Information Sheet September, 1928 Using a Milliamimeter as a Voltmeter what resistances must be used DY CONNECTING accurate fixed resistances in series with milliammeters it is possible to make very useful voltmeters that may be used to read filament voltages, plate voltages, C voltages, the output voltage of B-power units, etc. The accuracy of such a home-made voltmeter depends upon the accuracy of the milliammeter and the fixed resistance. Resistors accurate to within a few per cent, can be obtained by purchasing them directly from any reputable manufacturer. The table on this sheet gives the values of resistances required with different milliammeters to read voltages from 1 volt up to 1000 volts. For example, if a 5-mA meter is to be used to read voltages up to 50 volts then a 10.000-ohm resistor is necessary. A 1.0-mA meter may be used to read voltages up to 1000 volts if a resistor with a value of 1,000,000 ohms (1 megohm) is placed in series with it. The values of resistance required to read voltages not given in the table, or for use with meters with higher ranges may be determined by dividing the voltage to be measured by the maximum current in amperes of the meter. Suppose that a 50-mA meter is to be used to read voltages up to 300 volts. Three hundred volts divided by 0.050 amperes (50 mA) gives 6000 ohms as the required value of the resistance. Resistors with a wattage rating of 1.0 watt will be satisfactory for all those values given in the table, but it is advisable to use resistors with a rating of about 5.0 VOLTAGE MULTIPLIER FOR MILLIAMMETERS Milli 1,000 10,000 li in, .) 1,000,000 Amperes Ohms Ohms Ohms Ohms 1. 1 . volt 10 volts 100 volts 1000 volts 1.5 1.5 " 15 " 150 " 2. 2. " 20 " 200 " 3. 3.' 30 " 300 " 5. 5. 50 ' 8. 8. 10. 10. watts so that there will be little possibility of the value of the resistance changing due to heating. Also resistors with a rating of 5 watts, operating at considerably below their rated dissipation, will be likely to hold their calibration a much longer time than resistors of lower wattage. No. 218 Radio Broadcast Laboratory Information Sheet September, 1928 Servicing Radio Receivers HOW FAULTS SHOULD BE LOCATED '"THE tracing of faults in a radio receiver is not 1 always an easy matter. There is a tendency to delve at random into the vitals of the receiver rather than to follow a systematic procedure by which the fault may generally be more quickly and easily located. In locating and remedying faults the systematic testing of the circuit and the apparatus in the receiver is essential. Measuring instruments are frequently helpful in making these tests but a great deal may be done with a simple and inexpensive device. In the testing of the component parts in a receiver a pair of telephones connected in series with a small battery is useful in determining where the fault exists. The windings of a transformer may be readily tested by means of this simple circuit. When the two terminals are connected across the transformer winding a click will be heard if the circuit is continuous. Fixed condensers may also be tested, and here a click should be heard when the leads are placed across the terminals of the condenser, but no click will be heard when the terminals are removed unless the condenser is defective. If the insulation in the condenser is poor, however, or the condenser is definitely short-circuited, a click will be heard both when the circuit is closed and when it is opened. Ordinary radio-frequency transformers and superheterodyne intermediate-frequency transformers, audio-frequency or radio-frequency choke coils, eic. may also be tested for continuity by connecting the test terminals across the terminal of the device under test. If the device being tested has a high resistance the click will be of less intensity than that obtained when testing a low resistance device. In any case, no click at all will indicate an open circuit. When a radio receiver fails to operate, such tests as we have outlined here can be applied to the various components of the receiver to determine whether or not a piece of apparatus is at fault.