April 1932 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
This Radio Service Data Sheet for the RCA-Victor Radiola Model M-30 Automotive Radio Set is an example of the dozens of similar schematic and alignment instruction sheets that have been posted on RF Cafe over the years. Obtaining technical information on most things, even readily available items, prior to the Internet era was often very difficult - if not impossible. Service centers had what was need provided by manufacturers and distributors, but if you wanted to find a part number or service data on a refrigerator, radio, lawn mower, garage door opener, etc., and did not have the original paperwork, you were usually out of luck. Nowadays a Web search will quite often get you what you need thanks to people (like me) who go to the trouble of making the information available. The stuff doesn't just magically appear or get posted by benevolent governmental entities. You're welcome.
(Automatic Volume-Control - Push-Push Power Amplification - 9 Tubes)
Automotive radio receivers have been undergoing a change, with the primary objective of obtaining in automobiles comparatively the same over-all efficiency which exists in the less mobile "home" installation. A close approach to this degree of perfection is obtained from the Radiola Model M-30 receiver, manufactured by RCA-Victor Co., Camden, N. J.
The values of the components of this receiver model are as follows:
Condensers Ct , C2, C3, tuning condensers, 18-325 mmf.; C1A, C2A, C3A, trimmers, 4-50 mmf.; C4, padding condenser, 720 mmf.; C4A, padding trimmer, 10-75 mm£.; C5, trimmers, 15-75 mmf.; C6, trimmers, 140-220 mmf.; C7, 745 mmf.; C8, C10, 0.25-mf.; C9, C11, C12, 0.1-mf.; C13, C14, 0.0024-mf.; C15, C16, 0.5-mf.; C17, 4 mf.; C18, C19, 0.018-mf.
Resistor R1, 10,000 ohms; R2, 6,000 ohms; R3, 40,000 ohms; R4, R5, 30,000 ohms; R6, 170 ohms; R7, 0.5-meg.; R8, 28,000 ohms; R9, 0.1-meg.; R10, 1.0 meg.; R11, 3,500 ohms; R12, 4 ohms; R13, R14, 270 ohms; R15, 1,200 ohms; R16, 70,000 ohms; R17, 50,000 ohms.
The Model M-30 receiver has been designed with particular regard to ease of installation and service. Properly installed, freedom from interference should be experienced with the receiver operating at full sensitivity. Approximate operating voltage and current values (obtained on a Weston Model 547 test set), taken with the volume control set at the "minimum" and "maximum" (maximum values shown in parentheses) positions are as follows:
Filament potential, V1 to V6, V9, 6 volts; V7, V8, 4.5 volts. Cathode-to-heater potential, V1, V4, 18 volts; V2, 1.0 volt; V3, 6 volts; V5, 12 volts; V6, 15 volts; V9, 10 (5) volts. Control-grid-to-cathode (or filament) potential, V1, 0.5-volt; V2, 3 volts; V3, zero; V4, 1.0 (0.5) volt; V5, 10 volts; V6, 2 volts; V7, V8, 20 volts; V9, 1.0 (9) volts. Screen-grid-to-cathode potential, V1, V4, 100 (70) volts; V2, 42 volts. Plate-to-cathode (or filament) potential, V1, V4, 136 (135) volts; V2, 150 volts; V3, 45 volts; V5, 110 volts; V6, 165 volts; V7, V8, 155 volts; V9, 15 volts. Plate current, V1 ,V4, zero (4 ma.); V2, 0.25-ma.; V3, V6, 3.5 ma., V5, 0.5·ma.; V7, V8, 1.5 ma.; V9, zero. Screen-grid current, V1, V4, zero (1.0 ma.); V2, 0.1-ma.
Automatic volume-control tube V9 is connected to the cathode circuit of second-detector V5. The change in the bias voltage of V5, due to fluctuation of the signal, is applied to the control-grid of V9, resulting in a drop across plate resistor R7 which constitutes the control-grid bias for the R. F. and I.F. tubes. As the value of the plate current in a tube is a direct result of the voltage applied to the grid, a greater plate current in V9 gives a greater voltage drop across the resistor in its plate circuit and therefore a higher bias on the I.F. and R.F. stages, resulting in less sensitivity, and vice versa; previous A. V.C. systems have operated on different principles. Manual volume-control resistor R17 varies the bias on the control-grid of V9.
The total "A" current is 2.85 amps.; "B," 12 ma. min., and 25 average max. The power output rating of' this receiver is 2 watts. This high rating is a result of "push-push" (or class B") power output, which has been described in detail in past issues of Radio-Craft; notably, the January and February, 1932 issues.
To adjust these circuits, dismount the chassis but do not remove its connections or the flexible cable. Balance the R.F. circuits at 1400 and 600 kc., using a No. 5 Spintite, and an insulated screw driver. When the dial indicates 150, the tuning condensers should be fully meshed. Padding condenser C4 comes into consideration when the 600-kc. adjustments are being made; its trimmer C4A is then adjusted, while the main condenser gang is rocked back and forth for maximum deflection on the output test meter. During these adjustments, V9 must be removed from its socket.
For the I.F. adjustments, at 175 kc., it is also necessary to remove the chassis from its mounting brackets.
Insufficient volume-control by means of R17 may be due to the use of a tube of wrong constants for V9.
In the receiver, the tubes are arranged in two rows. In one (left to right) they are:
V3, V2, V1, V4, V5, V8; in the other: V9, V6, V7.
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.
Posted August 7, 2015