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Zenith Challenger Model 740 10 Tube Battery-Operated Superheterodyne
Radio Service Data Sheet
June 1934 Radio-Craft

June 1934 Radio-Craft

June 1934 Radio-Craft Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

This Radio Service Data Sheet for the Zenith Challenger Model 740, 10 tube, battery-operated superheterodyne floor console radio was published in the June 1934 issue of Radio-Craft magazine. As was typical of the era, a finely styled wooden cabinet housed the electronics chassis and speaker. Large dials and knobs would be be in vogue for a few more years. Programmable buttons for storing channel tuning were still rare at the time, too. The Zenith Challenger 740 was battery powered, which was popular in rural areas not yet serviced by AC lines. A bonus of that configuration was no 60 or 120 Hz hum from the power supply. A fairly unique feature for this early of a radio was its permanent magnet "reproducer" (aka speaker). The "ballast resistor tube" (V10) is a resistance device used to help limit and regulate current flow. There are still many people who restore and service these vintage radios, and often it can be difficult or impossible to find schematics and/or tuning information. Therefore, I maintain a constantly growing list of all data sheets posted to facilitate searches.

Zenith Challenger Model 740 10 Tube Battery-Operated Superheterodyne Radio Service Data Sheet

Zenith Challenger Model 740 10 Tube Battery-Operated Superheterodyne, June 1934 Radio-Craft - RF CafeSensitive and selective. Incorporates A.V.C.; tone control; only 18 ma. no-signal "B" drain; an 8 1/2 in. permanent-magnet type dynamic reproducer; 3 stages of A.F. amplification, including semi-class B output; console cabinet; 9 "standard" tubes and a ballast resistor tube.

This 10 tube battery-operated superheterodyne incorporates A.V.C., tone control, semi-class B push-push output, and a diode second-detector; the console cabinet, illustrated, houses an 8 1/2 in. permanent-magnet type dynamic reproducer. Tube V10 is a type 10-AB ballast resistor. The "A" is a 2 or 3 V. source; the "B", 135 V.; and "C", a tapped 22.5 V. battery.

Tube socket analysis figures follow: short the antenna to ground, and read voltages to the "A-" filament terminal.

(†) Computed; (††) varies with frequency setting; (*) volume control at minimum; (**) as read at battery.

In the diagram, those circuit elements which are shown dotted do not exist as distinct units but occur as a result of the mutual position of other circuit elements or their parts.

Control-grid bias for V1, the type 34 R.F. amplifier, is obtained by a tap between R1 and R2 in the high resistance potentiometer, R11. R1, R2, and R4.

Separate oscillator V9 is tuned by a shaped-plate variable condenser. This design eliminates the need for a padding condenser - and therefore its occasional adjustment.

The diode second-detector develops across R4 an A.V.C. potential which is applied, through filter resistors R2 and R1, to the grid-return circuits of V1, V2 and V3. Increasing signal strength increases the control-grid voltage applied to these tubes; this reduces the amplification and results in practically uniform output as set by the manual volume control, R6.

Inoperation of the receiver may be due to one of several reasons. Check for batteries incorrectly connected; battery voltages below par; defective tubes; a tulle in the wrong socket; a control-grid cap unconnected; or, a tube shield shorting to the cap of a 34. Also check up the speaker plug; off-on switch; antenna and ground leads, for reversed connections; shorted variable condenser plates; and, open-circuit in transformers.

Low volume may be due to weak batteries, poor tubes, poor antenna system, or misalignment of the tuning and I.F. condensers. Check the reproducer and chassis for opens, shorts and grounds.

Circuit oscillation may be due to tubes having incorrect characteristics. A good ground connection is essential. Firmly seat the tube shields. Check bypass condensers for open-circuit; and the tuning condensers for chassis contact. Unit R.F.C. must not short to the chassis.

Distorted reception can usually be traced to defective tubes. However, check batteries for voltage; the reproducer for mal-adjustment; the A.F. transformers. Tune "dead-center" (as necessitated by the A.V.C. action).

Two factors directly affecting the "B" battery consumption, whose normal no-signal drain is 18 ma., are: (1) Signal strength. Weak station signals cause but little A.V.C. action, with resulting high-plate current drain by V1, V2 and V3; and (2) A.F. volume. Increasing the A.F. volume increases the "B" drain of V7 and V8.

 

 

Posted July 28, 2023
(updated from original post on 7/19/2015)


Radio Service Data Sheets

These schematics, tuning instructions, and other data are reproduced from my collection of vintage radio and electronics magazines. As back in the era, similar schematic and service info was available for purchase from sources such as SAMS Photofacts, but these printings were a no-cost bonus for readers. There are 227 Radio Service Data Sheets as of December 28, 2020.

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