Radio today (Sept 1935-Dec 1936)

SERVICE NOTES-RIDER (Continued from page 47) COMPLETE DESIGNS of 10 Public Address AMPLIFIERS! A manual of circuits of Audio Amplifiers. One for every purpose. From 3 watts to 80 watts. A I 1 tried and tested d esigns.With compl ete lists of parts and their values. FREE! VALUABLE REFERENCE BOOK This advertisement and 10c IX STAMPS, to cover mailing expenses, will bring your FREE copy of this valuable Reference Book. There is no obligation. STANDARD TRANSFORMER CORPORATION 853 Blackhawk St., Chicago FREE -> RADOLEK DEALERS make the most PROFITS. Here is the Reason Why! £ The New Radolek 1935 Fall Edition of the Profit Guide is the most complete Radio Parts Catalog ever published — new, bigger and better. Everything in radio — at the right prices. Over 160 pages of valuable, money-saving "radiobuying" information. Over 10,000 separate Repair Parts — hundreds of new items — a complete, new selection of Radio Receivers and Amplifiers. Contains the most complete, exact duplicate, replacement parts listings, of volume controls, condensers, transformers, vibrators ever compiled. Nowhere, ever, has there been a Radio Parts Catalog comparable to this superb book. Every page brings you extra profits. This is your book — it's FREE. If you want the best Radio Parts Catalog — if you want to give better service at bigger profits — then send for this NEW Radolek Profit Guide. RADOLEK restricts distribution of this catalog to active and legitimate Radio Men. Please enclose your business card or letterhead — THE RADOLEK CO. 638 West Randolph Street, Chicago, III. Send me FREE the Big New RADOLEK PROFIT GUIDE Address Are you a Serviceman? □ Dealer? n Expm? D sistor, flows through the resistor. Replacement of the condenser in exactly the same manner as used before invites trouble, in the event that failure again results. Experience shows and the recommendation is made by several receiver manufacturers that a preferable connection for the tone control circuit is across the output transformer primary, instead of from plate to chassis. Time and again, tone control resistors which will not be completely damaged have become noisy after being subjected to the comparatively high current flow occasioned by the breakdown of the tone control condenser. With the change in the connections named, breakdown of the condenser greatly reduces the hazards to the resistor, because the voltage applied across the resistor is the voltage existing across the winding only and is seldom enough to damage the resistor, particularly in view of the low d-c resistance shunt of the winding itself. Arvin Model 7 * In this receiver the 6F7 is used as an r-f. amplifier, employing the pentode portion and the triode portion is employed as the first stage a-f. amplifier. This is not reflexing, since the tube is a dual purpose tube, consisting of a triode and a pentode portion. The cathode is common. The i-f. peak is 170 kc. The balance of the receiver employs a 6A7 as a combination mixer-oscillator, a 6B7 as an i-f. amplifier and half wave diode rectifier and also AVC. The output tube is a 41 and the power supply employs an 84. The following changes have been made in the receiver. A 200-ohm %watt resistor has been added across the vibrator points in the power supply. The grid circuit of the 6A7 oscillator system originally employed a 100,000-ohm resistor. This has been changed to a 50,000-ohm resistor, rated at J4 watt. Oscillators in superheterodyne receivers * Time and again, servicemen attempt to increase the signal output of a superheterodyne receiver by increasing the operating voltages applied to the oscillator tube. This change is intended to increase the amplitude of the local oscillations fed into the mixer tube, and thereby increase the amplitude of the resultant signal and total receiver output. Such an increase in operating voltage is beneficial only if the output of the receiver oscillator is insufficient. However, if the output of the oscillator is sufficient, further increase will have no beneficial effects. The reason for this is that in heterodyne operation, the level or amplitude of the resultant signal is a function of the weaker of the two mixed signals and not of the stronger. The resultant output signal has an amplitude nearly proportional to the amplitude of the weaker signal. It has been found that if the locally generated signal level is increased from equality, to several times the level of the incoming carrier, the increase in resultant voltage is less than 20 per cent. At the same time, there exists the possibility of overloading of the mixer tube, with consequent distortion. Considering all factors, such a change is not worth while. New metal tube sets * Highlights, pertaining to circuit design of the new metal sets, as relating to service data, will be considered next month. NEW BOOKS "The Story of Radio" By Orrin E. Dunlap, Jr. * ORRIN E. DTJNLAP, Jr., radio editor of the New York Times, authors a new edition of "The Story of Radio," first brought out in 1927. Current edition reviews the history of radio from early experiments to the present status of facsimile and television. Okay for anyone wanting radio in a nutshell available for quick reference. Book's best feature is readability. Written as though "Radio," personified, is telling its own life story. Nice idea. Gives facts like dates and places with much less than the usual dryness. Well worth the $2.75 if you are at all interested in the development of radio as a science and a business. New chapters on recent experiments especially notable. Published by The Dial Press, New York City, 1935. Price $2.75. — Radio Today 48 Radio Today