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

August 1932 Radio-Craft [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.

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.

• AMRAD Model 81 "Bel Canto"
• GE Model 250 Radio Service Data Sheet
• Hoffman Model A300
• Emerson Model 505
• Olympic Models 6-501, 6-502, 6-503
• Radiola Models 61-5, 61-10
• Farnsworth Models ET-060, ET-061, ET-063
• General Electric Model 321
• Garod Model 6AU-1
• Truetone Model D4620
• Westinghouse Model H-148
• Wards Models 54BR-1501A, 1502A
• Majestic Models 8S452, 8S473
• RCA Models Q22A, Q32
• Zenith Model 5G003ZZ
• Mantola Models 92503 and 92504
• Emerson Model 508 Series 8-7434351 and Up
• Belmont Model A-5D118
• Wards Model 74BR-2707A
• Crosley Model 56TP-L
• Admiral Model 7C60 Chassis 6B1
• 336 Belmont Radio Model 6D111, Series A
• 333 General Electric Models 100, 101, 103 and 105
• RCA Victor Models 54B1, 54B-N, 54B2, 54B3 Radio Data Sheet 335
• National Union "Presentation" Radio Model G-619
• Zenith Radio Models 8H032, 8H033, 8H050, 8H052, 8H061
• General Electric Farm Radio Model 280
• Admiral Model 6RT44-7B1
• Montgomery Ward Airline Model 04BR-1105A Radio
• Belmont Model 678 Auto-Radio Set
• Sentinel Model 217-P Portable Radio Set Radio
• Remler Model No. 36 Dual-Wave Auto-Radio
• Stromberg-Carlson No. 82 All-Wave Receiver
• Majestic A.V.C. Model 290 Chassis
• FADA 9 Tube Model 190 "Metal" All-Wave
• RCA Victor Models 9T and K2 9-Tube, 5- to 566-Meter
• Motorola "Golden Voice" Model
• RCA Victor Model H-6
• Simplex Model TA
• Automatic "Magic Eye" Model A1
• Silvertone Models 4488 and 4588 (Chassis No.101412) and 4488A and 4588A (Chassis No. 101412A)
• RCA Victor Model M109 "De Luxe" 7-Tube Auto-Radio Receiver
• Crosley Model 6625 6-Tube 3-Band Receiver
• International Model 77 Series 7-Tube Dual-Band Receiver
• Belmont Model 6D121
• General Electric Models 60, 62
• Admiral Model 7C64
• Radiola "28" Super and "104" Power Speaker
• Sonora Model TW-49
• Stromberg-Carlson Models 1020, 1120, Series 10
• Air King Model 4604D
• Sparton Models 526, 526X, 526PS
• Truetone Model D2624
• Admiral Models 6EI, 6EIN
• Detrola Models 571A, 571B
• General Electric Model 250
• Howard Model 920
• Colonial Model 652 5-Tube Broadcast-Short-Wave
• Fairbanks-Morse 9-Tube All-Wave Model 91
• International Model 500 5-Tube Dual-Range Battery
• Emerson Model 678 "Auto-Dynamic" 5 Tube
• Stromberg-Carlson Nos. 230 and 231 Series
• Atwater Kent Model 649 All-Wave
• Howard Model G-26, and "Airplane 4" Model AA25
• Montgomery Ward "Airline" Series 7GM 7-Tube High-Fidelity Receiver
• RCA Victor Model T5-2 5-Tube, 2-Band A.C. Superheterodyne Receiver
• Majestic "Models 50," "51" and "52"
• Bremer-Tully Model 7-70 and 7-71
• General Electric Model M-49 4-Tube Radio-Phonograph Dual-Wave Superheterodyne
• RCA-Victor Radiola "Superette" Model R7 Superheterodyne
• Crosley Model AC-7 and AC-7C
• Westinghouse "Columnaire" Models WR-8 and WR-8-R (Remote Control)
• Characteristics of Metal Tubes - and Other "Octal" (8-Prong) Base Types
• Kolster K20, K22, K25, K27 and K37 Six-Tube Receivers
• Stromberg-Carlson Nos. 62 and 63, 8-Tube High-Fidelity Chassis
• RCA Model 103, 4-Tube A.C. Compact Dual-Wave
• FADA "Special" Model 265-A and FADA "7" Model 475-A
• General Electric Model C-62 6-Tube Battery
• Emerson 5A Automotive
• Zenith 666 Automotive
• Motorola 100 Automotive
• Crosley Roamio 4-A-1 Automotive
• American-Bosch 524A Automotive
• Crosley Model 1316 (in Model 167 Console)
• RCA Victor "High-Fidelity Electrola," Model R-99
• AMRAD Model 81 ("Bel Canto" Series) Receiver
• Fada 103 Fadalette, Stewart-Warner Series 108, DeWald 54 Dynette Sets
• RCA Victor R-27 and Philco 53 Ultra-Midget A.C.-D.C. Radio Receivers
• Majestic Models Fairfax and Sheffield 8-Tube
• Stromberg-Carlson No. 29, 9-Tube Superhet
• International Kadette Model 400 4-Tube Battery-Operated Superhet
• RCA Victor Model 5M 5-Tube Auto Superhet
• Majestic Model 11 Short-Wave Converter
• Silver-Marshall Model 727-DC Battery-Operated Superheterodyne
• RCA Victor Model VHR-307 Home Recording - Phono-Radio Combination
• Delco 32-Volt Radio Receiver Chassis Models RA-3, RB-3 and RC-3
• Majestic Chassis Models 380 A.C. T.R.F., and 400 A.C.-D.C. Superheterodyne
• General Motors S1A, S1B
• Admiral Model 7C63, Chassis 7C1
• Westinghouse Model H-133
• Arvin Models 150TC, 151TC
• Kadette Model 90 Duplex
• RCA-Victor "Magic Brain" Model 281
• Grunow 11A Chassis 11-Tube All-Wave Superheterodyne
• Sears, Roebuck & Co., Silvertone "Rocket" Models 6110 and 6111
• General Electric Model GD-52
• Zenith Models 6D302, 6D311, 6D326, 6D336, 6D360
• Allied Radio, Knight Model E10913
• Arvin Model 140P
• Emerson Models 501, 502, 504
• Crosley Model 56TD-W
• Hoffman Model A500
• Stewart-Warner Model 9003-B
• Zenith Models 6D014, 6D029
• Coronet Model C-2
• Sparton Models 7-46, 7-46PA, 8-46, 8-46PA
• Stewart-Warner Models 9001-C, D, E, F
• Zenith Models 5D011-5D027
• Bendix Models 636A, C, D
• ECA Model 108
• International Model 66 and 666, 6-Tube Superhet
• Ford-Philco Radio, Model FT9, 6-Tube Auto-Radio Receiver
• Howard Explorer Model W Deluxe 19 Tube All-Wave Superhet
• RCA Victor Portable Table Electrola Model R-95
• Atwater Kent Model 305Z 5-Tube 32 V. D.C. Superhet
• Kadette Jewel Model 40 Chassis 3-Tube Ultra-Midget Receivers
• General Electric Model N-60 6-Tube Auto Superheterodyne
• Sparton Model 40 6-Tube T.R.F. Automotive Receiver
• Clarion "Replacement" Chassis, Model AC-160 A.V.C. Superheterodyne
• Emerson Models 20A and 25A
• General Electric K-40A
• Pilot Model B-2
• RCA-Victor Radiola Model M-30 Automotive Radio
• Motovox Models 10A All-Electric and 10E Battery-Operated "Moto-Tetradynes"
• Kennedy Superheterodyne Short-Wave Converter
• RCA Victor Model R-78 B1-Acoustic 12-Tube
• Philco Model 15 Series, 11-Tube Superheterodyne Chassis
• Zenith Challenger Model 740
• Sparton Selectronne Receivers Models 1068 and 1068X
• Fada Model 155 Super Fadalette A.C.-D.C. Set
• Clarion De Luxe Models AC-280 and 25-280
• Crosley Model A-157 (River Roamio) Auto Radio
• Philco Model '37-116 Codes 121 (Shadometer) and 122 (Dial Tuning)
• Arvin Model 28
• Philco Model 818
• Fada Model 266 Motoset
• Bosch Models 736, 737, 738
• RCA-Victor Model 15U, Radio-Phonograph
• Sparton Models 566 ("Bluebird" Mirror), A.C.-D.C. 5-Tube 2-Band Midget Superhet
• Atwater Kent Model 776 6-Tube Auto Radio
• Stromberg-Carlson No. 61 4-Band 7-Tube A.C.-D.C. Receiver
• Arvin Model 182TFM
• Crosley Model 58TK
• Westinghouse Model H-165
• General Electric Models G-105 and G-106
• Silvertone "F," "FF," "G," "H," and "J"
• Stewart-Warner Model 03-5A1 to 03-5A9 (Chassis 03-5A) Senior Varsity Radio
• Radiola Models 61-6, 61-7
• Westinghouse Models H-104, H-105, H-107, H-108
• Farnsworth Models EC-260, EK-262, EK-263, EK-264, EK-265
• United Models 980744, 980745
• Stewart-Warner (R-127 Chassis) Models 1271 to 1279 All-Wave
• ERLA Model 4500 Dual-Wave T.R.F. 4-Tube A.C. Receiver
• Clarion No. TC-31 5-Tube A.C.-D.C. Superhet.
• Detrola Model 105C 5-Tube Dual-Band A.C.-D.C.
• Zenith 6-Tube All-Wave Chassis No. 5634
• RCA Victor Model 261, 555 to 107 Meter
• Philco Model 38-116; Code 125
• Stewart-Warner "Ferrodyne" Chassis Model R-136
• American-Bosch Model 43OT 5-Tube 3-Band Superheterodyne
• RCA Victor Model C9-4 9-Tube 3-Band Superheterodyne
• Kennedy "Model 826B" Combination Receiver
• Steinite 50-A and 102-A
• Pilot Model 63 All-Wave 6-Tube Superheterodyne
• Stromberg-Carlson No. 69 4-Tube All-Wave Superhet. Selector (Converter)
• RCA Victor Model 102 4-Tube A.C.-D.C. T.R.F. Receiver
• Bosch Models 60 and 61
• Atwater Kent Models 30, 33, 35, 48 and 49
• Crosley Model 120 Senior Superheterodyne (Pliodynatron) Chassis
• Columbia Screen-Grid 8 Receiver
• General Electric Models A82 and A87, 8-Metal-Tube All-Wave A.C. Superhet.
• Colonial 31 and 32 D.C.
• Zenith 5-Tube Triple-Wave Chassis nos. 5508 and 5509
• Remler Model 46 ("Scottie")
• General Electric FA-60 and FA-61
• Stewart-Warner Series 900
• Howard Model B-5 (715), Series 1 and 2 (Sheaffer Radio-Clock-Pen Desk Set)
• Ford-Philco Car-Radio Models F-1440 and F-1442
• Brunswick Model 31 Combination Radio and Panatrope
• Emerson Models 38, 42 and 49, 6-Tube Dual-Wave (Chassis U6)
• General Motors Chevrolet No. 601574 Automotive
• RCA Victor M-104 (and M-108) Automotive
• Arvin-Ford 17-A Automotive
• Westinghouse Model WR 207 & WR 208 5-Tube Dual-Band Superheterodyne
• Radiolas "Super VIII" (AR-810, "Semi-Portable" (AR-812), 24 and 26
• Howard Model 45 A. V. C.
• Majestic Model 25
• Galvin Motorola Model 61
• Arvin Model 6
• Admiral Models 7T06, 7T12
• Garod Model 5A1
• Hoffman Model A301
• Knight Model E10716 Battery Portable
• Arvin Models 555, 555A, 552N, 552AN
• Grantline Models 605, 606
• Truetone Model D2616
• Belmont Model 5D128
• Arvin Models 444, 444A
• International Kadette Model 1019
• Stewart-Warner Models 97-561 to 97-569
• General Electric Model 280
• Zenith Models 5R080, 5R086
• Truetone Models D1747, D1748
• Crosley Roamio Automotive T.R.F. Models 90, 91, 92
• Crosley Roamio Automotive Superheterodyne Models 95, 96
• Wells-Gardner Series 062
• Emerson Model AZ-196
• Belmont Model 5P19
• Crosley Fortyfive
• Crosley Model 56FC
• Emerson Models 507, 509, 518, 522, 535
• Garod Model 6AU-1
• General Electric Models 219, 202, 221
• Crosley "Chairside" Model 567
• Belmont Model 408 Battery "Farm"
• Wards Model 74BR-1055A
• Farnsworth Models EK-081, EK-082, EK-083, EK-681
• Philco Model 200-X Radio
• Admiral "Aeroscope" Models 161-5L, 162-5L and 163-5L
• Philco Model 59, 4-Tube A.C. Midget Superheterodyne
• Zenith Farm Model 6V 27, 6-Tube Superhet
• Ward 10-Tube All-Wave High-Fidelity Superhet, Series ODM
• Philco-Packard Deluxe
• Canadian Westinghouse Model 175
• Crosley Model 1155
• Philco Models 39 and 39A
• Arvin Model 35 8-Tube Car-Radio
• Hetro Air-Ace Series M
• Westinghouse Models H-161, H-168, H-168A
• Garod Model 5A4
• Arvin Models 152T, 153T
• Belmont Model 5240
• Mantola Models 92505, 92506
• General Electric Models 102, 102W, 107, 107W, 114, 114W, 115, 115W
• Crosley Model 555 (A.F.M.)
• Crosley Model 515 (Fiver)
• Crosley Model 425 (Travo)
• Firestone-Stewart-Warner Model R1332
• Fairbanks-Morse Model 81 "Farm" Set
• Clarion Model 423, 470, 471, 472, 480
• International Radio Corp. Model 90
• Belmont Model 578 Series A