Radio Broadcast (May 1929-Apr 1930)

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RADIO BROADCAST by the detector grid requires an amplification of 100,000/150 = 667 times. If the induced antenna voltage had been applied directly to the first r.f. grid through a resistance to ground, the required r.f. amplification would be 100,000/10 = 10,000 times. Case 2: A 227-type tube power detector with 90 volts on the plate is used to feed a 250-type tube which has a plate voltage of 300 and a grid bias of — 54 volts. How much radiofrequency amplification is required to give full output when the signal field strength is 1 microvolt per meter? The maximum audio voltage that can be applied to the grid of the 250-type tube is 54 volts, and assuming the 227 tube amplifies its audio grid voltage 25 times, the power detector must have 54/25=2.16 audio volts produced on its grid by the detection process. With complete modulation (m = 1 ) and assuming 80 per cent, of ideal detection, then according to the formula already given 0.80 X 1 X Es = 2.16 and the required radio signal is Es=2.7 volts, which is a value that can just be handled at 90 plate volts without Table I DETECTION CHARACTERISTICS OF THREE-ELEMENT TUBES Type Mu Detector Voltage Leak Resistance Ceff for70% Constant V g Recommended reproduction of 5000 cycles 201a 9 — 0 47 .3.20 mcohins ft ftftftllO mfrt>3 240 30 —0^47 3^20 0.000212 199 6 —0.50 1.50 0.000255 120 .3 —0.45 1.67 0.000255 171a 3 —0.28 7.20 0.000160 112a 8 —0.26 5.8 0.000212 226 8 —0.29 1.6 0.000212 227 8 —0.2.3 7.8 0.000318 12 6 —0.27 8.0 0.000318 Note: Values of Vg are averages for a number of tubes. Values of effective grid condenser capacity, Ceff, are values of grid condenser capacity plus tube input capacity to audio frequencies, which is usually about 0.00007 mfds. for tubes with a mu of about 8. The recommended values of grid leak are such as to make the detector input grid-filament resistance at least 100,000 ohms. All tubes are R.C.A. or Cunningham. undue distortion. If the antenna is 10 meters and a tuned antenna circuit with a step-up of 15 is used, the radio-frequency amplification required to put 2.7 volts on the detector grid is 2,700,000/(10 X 15) = 18,000 times. This could be obtained easily with three screen-grid stages. Without a tuned antenna the amplification needed is 15 times as much, or 270,000 times. Case 3. A 210-type tube with 300 volts on the plate is used as a power detector and operates the loud speaker directly from its own power output. How much undistorted power is obtainable, and how much radio-frequency amplification with a 1 microvolt per meter field is needed to give this power? A 210-type tube used as an a.f. amplifier at 300 volts would require a grid bias of — 22J and would put out 600 undistorted watts. As a power detector the tube will handle a fully modulated carrier wave of about 12 radio-frequency volts (which is just more than half of 22 1) and will then put out about 600/4 = 150 undistorted audio-frequency milliwatts. This 12 volts of radio frequency will put about 0.85 X 12 = 10.2 audio volts on the grid of the detector to produce the 150 milliwatts output, and, with the 10-meter tuned antenna with a step-up of 15, will require a radio-frequency amplification of 12,000,000/(15 x 10) =80,000 times. This could be supplied readily by three good stages of screen-grid radio-frequency amplification. N. E. M. A. ATTACKS SERVICE EDUCATION PROBLEM THERE has been a general recognition on the part of the radio industry that satisfactory radio reception requires more than the making of good radio receivers. If it is to give the consumer the utmost satisfaction the receiver must be installed in a manner which assures the maximum performance in its particular location with respect to selectivity, sensitivity, and tone quality. While the potentiality of maximum performance in these respects may be built into the receiver by proper design and construction, only when the installation is intelligently and scientifically made is there assurance that the customer will enjoy the highest standards of reception. The mere realization that the dealer must learn the practical aspects of installation and maintenance is no solution of the complex problem of raising the standards of service rendered the consumer both with the initial purchase and the subsequent maintenance and attention required. The difficulties of dealer education by the manufacturer are increased greatly by the fact that he often has thousands of outlets distributing his product, so that there is no practical way of establishing contact with all of those having a hand in making the radio receiver perform in the ultimate purchaser's home. Many manufacturers issue service bulletins of great value to trained men but they do not fulfil the broad function of teaching servicing. With a view to meeting the problem of service education the Radio Division of the National Electrical Manufacturers Association has issued a course especially for servicemen and dealers responsible for installations. This course was prepared in collaboration with the Radio Institute of America, a pioneer organization in training men for the v arious branches of the radio field. Four booklets giving the sort of practical information required in servicing, periodic examinations which are marked and rated, and direct correspondence help are included as a part of the course. The text covers not only installation but all the details of repair and maintenance of radio receivers. It discusses how to hunt for trouble with a view to conserving the time required with each service call. Scores of diagrams of commercial receivers are included in the text so that repairs can be made in the minimum of time by a study of the circuits of all popular models of radio receivers. While the subject of installation is treated in a very thorough fashion, the course does not place undue emphasis on that phase of the radio serviceman's work. Repair problems, trouble hunting and similar questions are presented in a comprehensive manner. The first of the four textbooks discusses the fundamental principles of electricity involved in broadcasting and explains the mag Students receiving instruction in servicing at the Radio Institute of America netic field, electromagnetic induction, transformers, condensers, oscillatory circuits, audio-frequency amplification, and reproduction by the loud speaker. This volume also gives definitions and standard radio symbols for all parts used in radio reception and required of servicing, as well as an outline of the general types of radio receiver including tuned and untuned radio-frequency sets, regenerative detector circuits, super-heterodyne systems, and all types of audio amplifiers. The second volume contains a discussion of direct-current receivers and the associated battery supply, and it goes into the subject of a.c. receivers exhaustively; also repairing magnetic and dynamic loud speakers. Among many other subjects is that of the installation of a broadcast receiver. This text also includes the construction of a testing set for general repair and test purposes and a modulated oscillator for shop tests. Servicing, trouble shooting, vacuum tubes, and the elimination of electrical interference are some of the important questions taken up in the third text. Rook four is unique in that it contains complete circuit diagrams of all the leading models and makes of radio receivers on the market. This is the first encyclopedia of radio circuit diagrams which has been prepared to aid the serviceman in the application of the practical service methods explained in the first three text books. In preparing t his service course for the radio industry, the National Electrical Manufacturers Association has endeavored to achieve a high degree of practicability and the widest possible distribution by enlisting the cooperation of the Radio Institute of America which has devoted itself since 1909 to training men for work in all branches of the radio field. • may, 1929 page 40 #