Radio Broadcast (May 1928-Apr 1929)

116 RADIO BROADCAST DECEMBER, 1928 nected in series with the primary in the antenna circuit so that the primary is never tuned by the capacity of any antenna length within the 200-550 meter band. Three stages of resistance-coupled audio amplification are recommended, although other types may be used if desired. It is unnecessary here to give constructional details for the kit-set as these may be obtained directly from the Browning-Drake Corporation, who manufacture the 1929 Browning-Drake A.C. Shield Grid Kit. The Kit-Set has been carefully designed by mathematical and laboratory methods, and the writer feels that it is the best Browning-Drake ever presented to the public. Not only does it outperform all previous models from the standpoints of selectivity and sensitivity, but its tone quality is as nearly perfect as can be obtained at the present stage of the radio art. Coupled with this is the feature of simplicity of operation, due to its being absolutely single control. LIST OF PARTS A COMPLETE list of the apparatus employed in the construction of the A.C. Screen-Grid Browning-Drake Receiver follows: Ai, A2 One Browning-Drake 1929 kit (a.c. screen-grid type); Li, La Two radio-frequency transformers; Ri Three Browning-Drake center-tapped resistors; R2 Three Aerovox or Durham resistors, 0.05megohm ; R3 Two Aerovox or Durham resistors, o. 1megohm; R4 One Aerovox or Durham resistor, 0.5-megohm; Rs One Aerovox or Durham resistor, 8-megohm; Re One Frost volume control and 1 10-volt switch; R7 One Yaxley resistor, 1000-ohm, type 71,000; Rs One Aerovox resistor, 2000-ohm, type 992; Ci One Aerovox or Sangamo moulded fixed condenser, o.oooi-mfd.; C2, C3, C4 Three Aerovox or Sangamo moulded fixed condensers, 0.001-mfd.; C6 One Aerovox or Sangamo moulded fixed condenser, 0.00007-mfd. ; Cf, One Aerovox or Parvolt by-pass condenser, i-mfd.; C7 Three Browning-Drake special amplifier coupling condensers, o. i-mfd.; C8 One Aerovox moulded condenser, 0.5-mfd.; Two Eby vacuum-tube sockets, UY-type; Four Eby binding posts (Ant., Gnd., Speaker and Speaker); One Browning-Drake Foundation Unit consisting of drilled front and base panels with amplifier sockets and resistor mounts, twisted a.c. filament wire, connecting cable and all hardware. The total cost of the parts in the above list is $5945 The additional apparatus required for placing the receiver in operation follows: One screen-grid tube, a.c. -type; One detector tube, UY227-type; Two amplifier tubes, ux226-type; One power tube, i7iA-type; Filament transformer, B-power unit, aerial, ground, loud speaker, etc. FIG. I. COMPLETE SCHEMATIC DIAGRAM Book Reviews " A Treatise on 25 Testing Units for Service Men." Published by Radio Treatise Company, New York City, 26 pages. Price — $1.00. HT H E title of this book defines clearly and defi* nitely its scope and purpose. In the description of the various testing units the author has also given considerable general data on how to test receivers. The service man, endeavoring to service a radio receiver by simply following instructions, has before him a task comparable to that which Diogenes had. To successfully service a set one needs, besides the mechanical tools of the trade, a clear understanding of how receivers and set-testing devices work. The educated service man will not hesitate to try this strange food — for an understanding of why the wheels go round is strange to many service men. The testing devices described in this book are many and include the following: Tube Reactivator and Filament-Emission Tester. Vacuum-Tube Bridge, by which one can measure the amplification constant and plate impedance of a tube. Tube Tester, for measuring the electrical characteristics of all types of a.c. and d.c. tubes. Voltage Tester, for measuring the A, B, and C potentials in any modern receiver. Oscillators, both audio and radio-frequency, to be used as local sources of signals in servicing sets or in making laboratory tests. Laboratory Oscillators, producing frequencies throughout the audio range and up to about 300 kilocycles. This is useful in measuring audiofrequency apparatus intermediate-frequency transformers, etc. Indicating Devices, which include descriptions of several vacuum-tube voltmeters. On page 17 is described a vacuum-tube voltmeter of thedesign generally known as the "slide back" type, in tvhich the unknown voltage is balanced against the C-battery voltage. It is not generally the case, however, that the unknown voltage is equal to the change in grid voltage, although the author states that such is the case. When very large voltages are being measured the slide-back method can be used with quite a small error, but with small voltages, such as one frequently must deal with in amplifiers, the slide-back method, in the reviewer's opinion, is quite inaccurate. However, the service man generally is interested in qualitative rather than quantitative values and in such cases this type of vacuum-tube voltmeter probably can be utilized satisfactorily. As we indicated previously, the appeal of this book is confined generally to the dealer or service man, although many set builders should find the constructional data on laboratory instruments very helpful. — H. E. R.