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RCA Victor Model R-78 B1-Acoustic 12-Tube Superheterodyne
Radio Service Data Sheet
August 1932 Radio-Craft

August 1932 Radio-Craft

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

For several years I have been scanning and posting Radio Service Data Sheets like this one featuring the RCA Victor Model R-78 B1-Acoustic 12-Tube Superheterodyne floor console radio in graphical format, and run OCR on them to separate the textual content and make it searchable. 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. I will keep a running list of all data sheets to facilitate a search. The RCA Victor Model R-78 B1-Acoustic 12-Tube photo was found on the RadioMuseum.org website.

RCA Victor Model R-78 B1-Acoustic 12-Tube Superheterodyne Radio Service Data Sheet

(Also, General Electric "Convention" Model J-125 Chassis.)

This is the first commercial receiver to incorporate the new "super-phonic" line of tubes which have recently made their appearance on the market. The tubes of this series incorporated in the R-78 (and J-125) chassis are the 58 R.F. pentode, 56 general-purpose, 46 Class Band 82 mercury-vapor rectifier. (The type 58 tubes are of 6-prong-base design.)

A feature of the receiver is the tone control, which is designed to maintain even reproduction of the low and high frequencies, regardless of the volume setting. Thus, bass reproduction at low volumes is not attenuated as when, non-compensating circuits are used.

The resistance and capacity values of the respective units are indicated by figures within parentheses.

The following operating voltage and current readings are for a 120-volt line, the volume control set at "minimum," and no signal being received.

Filament potential, all tubes, 2.5 volts. Plate potential (to cathode or filament), V1, V2, V4, V6, V7, V10, 210 volts; V3, 70 volts; V5, 200 volts; V8, V9, 400 volts; V11, zero. Plate current, V1, VI0, 3 ma.; V2, 1.5 ma.; V3, V6, V7, 5 ma.; V4, 2.5 ma.; V5, 1.0 ma.; V8, V9, 6 ma.; V11, zero. Control-grid potential (to cathode or filament), V1, V2, V3, V4, V8, V9, VI0, V11, zero; V5, 12 volts; V6, V7, 8 volts. Screen-grid (to cathode or filament), V1, 100 volts; V2, V4, V10, 95 volts. Cathode (to heater) potential, V1, V3, V10, 7 volts; V2, 10 volts; V4, 8 volts; V5, 12 volts; V6, V7, 11 volts; V11, 15 volts.

The input signal potential for the I.F. amplifier is applied also to the A.V.C. amplifier tube due to the grids of both being coupled together by means of C32. The output of the I.F. amplifier V4 is applied to second-detector V5 through a sharply-tuned transformer I.F.T.2; however, the output of A.V.C. amplifier V10 is coupled to A.V.C. tube V11 through a broadly tuned unit.

Although too much selectivity ahead of V11 is undesirable, since it introduces excessive distortion and overload as a station signal is tuned in, still, a certain amount is essential; otherwise, the A.V.C. will be caused to function by a local station when it is desired to tune in a weaker station on an adjacent channel.

The voltage developed across resistors R4, R21, R222, furnish control-grid bias for V1 ; the drop across R4, R22, is the control-grid bias for V2; and the drop across R4, control-grid bias for V4.

As the drop in these resistors is due to the signal potential applied to the A.V.C. tube and this voltage is in turn dependent upon the bias of the R.F. first detector, and I.F. amplifier, an automatic action is obtained; greater voltage is applied to the I.F. and first-detector than to the I.F. to prevent overloading of these tubes due to a strong, undesired adjacent carrier.

The undistorted power output of the R-78 is rated at 10 to 20 watts, depending upon the percentage of modulation of the incoming signal; consequently, to compensate for variations in sound intensity over the audio frequency band as the output is varied within these limits the volume control circuit is arranged to produce substantially flat response between the range of 35 and 5,000 cycles.

The trap circuit A.F.C.1, C11 tunes to approximately the middle of the A.F. response range and as the volume is reduced to one point, it causes greater attenuation of the middle register than at either end. From this point to the minimum position the volume control acts as a potentiometer across the trap circuit and reduces the volume without changing the frequency response to any greater degree.

This completes the description of the first half of the volume control; the second, which functions only over the last 20 degrees of the angular movement of the volume control, is resistor R1 connected between the R.F. and first-detector cathodes and varies the overall sensitivity.

Push-pull voltage amplifier V6-V7 is the driver stage for push-push amplifier V8-V9.

Cabinet resonance bas been nullified by means of two side chambers; the baffle area is large.

To prevent excessive hum and noise, it is essential that a good ground be connected to the yellow lead of the chassis; considerable hum also may be caused by insufficient twist in the volume control leads, due to pickup by A.F.C. 1.

Schematic circuit of the RCA Victor Model R-78 receiver. The same circuit is used in the G.E. "Convention" Model J-125 set. All the tubes are of the new "superphonic" series. The power consumption of the set averages 110 watts; and varies between 70 and 130 watts, depending upon the degree of output volume. The undistorted power output may reach 20 watts during heavy passages in the program of a strong station.

In localities remote from strong stations it may be desirable to increase the A.V.C. action to obtain better than 100 mv, sensitivity. This is accomplished by shorting out R1, as indicated by the dotted line, "short."

To realign the chassis, an output meter will be necessary. (This mar be a current-squared galvanometer connected to the secondary of T3 in place of the reproducer voice coil; an 0-5 ma. meter in the plate supply lead to V2; or a low-range A.C. voltmeter across the reproducer voice coil.)

A "dummy" 56-type tube having an open heater circuit is required to replace V11; make certain that the dial pointer reads exactly at the short line on the scale when the gang condenser plates are fully meshed. Then, align the circuits at 1,400 kc., with the volume control in the "maximum" position.

Follow this with the alignment procedure at 600 kc., then repeat the procedure at 1,400 kc. Condenser C4A, the 600 kc. trimmer, is reached through a hole in the top of the chassis, and about half-way along a line drawn from the tuning dial to the socket of the first-detector.

To adjust the I.F. circuits, set the service oscillator at 175 kc., replace the regular type 56 tube with the dummy 56, as previously described, couple the oscillator to the control-grid of the first-detector, and set the volume control at "maximum"; adjust first I.F.T.2, then I.F.T.1. Repeat the procedure. Looking at the rear skirt of the chassis, and reading from left to right. the trimmers of the I.F. transformers are arranged in the following order: C8, C7, C6, C5. Terminal panel P1 is below these adjustments. At the left of P1 is the "fidelity'" switch, SW.1.

It is a good plan after making the I.F. realignment adjustments to repeat the oscillator and R.F. adjustments.

Following is the color code of the power transformer: 1, black, red tracer; 2 black-red; 3, red; 4, 5, yellow; 6, 8, brown; 7, brown-black; 9, 11, blue; 10, blue-yellow; 12, 14, green; 13, green-yellow.

Posted September 27, 2022
(updated from original post on 7/27/2015)

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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RCA Victor Model R-78 B1-Acoustic 12-Tube SuperheterodyneRadio Service Data SheetAugust 1932 Radio-Craft

RCA Victor Model R-78 B1-Acoustic 12-Tube Superheterodyne
Radio Service Data Sheet
August 1932 Radio-Craft