Radio and Television Today (Jan-Dec 1940)

SERVICE NOTES {Continued from page 36) shorting jack which mutes the speaker. One thing to do in all installations is keep DC out of the phones. Use an isolating condenser and prevent any possible shocks. Output transformer is shorted as phones are used. Suppressor, not control grid connects to cathode. volts no-load for 15-ma. max. load. TH has a turn ratio of 300/5000 with 20 kv. insul'ation. A word of warning. The Continental Electric Co. states, "this is a high voltage device, and suitable precautions must be taken. We are not responsible for infringement of any patents." Mallory Supplements Issued Automatic Tuning is the title of the eighth of the series of supplements to the MalloryYaxley Radio Service Encyclopedia published by P. R. Mallory Co., Indianapolis, Ind. The new supplement covers the automatic tuning systems that have been introduced since the second edition of the Mallory Encyclopedia. Tables of the models covered in both issues are included to provide quick reference. Diagrams, and drawings explain the various mechanical, and electrical tuning systems, as well as capacity or inductance switching systems. FM COVERED IN NO. 9 The ninth supplement covers frequency modulation in theory and practice. Complete theoretical explanations of FM circuits and schematics of present receivers are real aids to the serviceman. Auto Radio Typical Troubles CHRYSLER 1608 Low sensitivity: Check the antenna connections on the primary coil and see that proper connections have been made. See plan of connections in following diagram. <<4 PS "SKY" JACK j C%s \S^ FOR SKYWAY ANTENNA. ROAD JACK S. FOR ROADWAY 1 // ANTENNA. r J ZEPHYR L-1760 Hum: Check and replace filter capacity if necessary. If hum remains, use either or both changes as shown. For 1, add plate decoupling 10M re Discharge Tube in Fence Control Circuit Electric fence controlling of live stock has become more popular in recent years and their design and installation is a good field for the rural serviceman. The circuit shown uses a Cetron CR-300 discharge tube made by Continental Electric Co., Geneva, Illinois. The '80 rectifier charges the 0.5 mfd. condenser to approximately 600 volts at a rate determined by the resistor R which may be about 500M %watt. The Cetron is a peak voltage discharge tube which only fires when its anode has potentials between 400600 volts, thus as the capacitor builds up to approximately 400 volts, the tube permits the condenser to discharge through the primary of TH. This pulse of current causes a voltage to be induced in the secondary of TH and this is connected to the fence and ground. Transformer T should have windings for 5-volt 2-amp., 2-volt 8-amp., 850 -U— ) 1 i82M r I ( ,*— >. 7r4 A Ai MOVE LEAD TO ^Zy THIS POINT — . TOAF PLATES — ! — S M I A ___— _.__?___J 1 A A A A A A 1 1 ~ 4\DD THESE PARTS 1 > B + © — p—« ■A'A'A.'A JH a '=rREP ) WIT LACE IOOOJ\,UNIT H 1500 J"U RESISTOR ~" : ::":'r :::r :: Two circuits for reducing hum in Zephyr L-1760 auto sets. filter at left adds two parts. Second detector Low voltage power supply charges 0.5 condenser at rate determined by R. Gas tube passes charge through TH. sistor and .25 mfd. capacitor. For 2, replace the 1000-ohm filter resistor with 1500-ohm 10 watt unit and move the supply lead for output tubes to the other side of this resistor. Engineer Tours With FM Lectures Now on a tour of 17 major cities with illustrated lectures on "The Service Side of Frequency Modulation" is Ray Wilson, service engineer for Zenith Radio Corp., Mr. Wilson covers the economic side of FM, as well as the technical, in his talks to dealers and servicemen. 38 RADIO TODAY