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MECA Electronics

Belmont Model 678 Auto-Radio Set
Radio Service Data Sheet
August 1940 Radio-Craft

August 1940 Radio-Craft

August 1940 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.

Unlike even the vacuum tube type AM radio in the dashboard of my parents' car in the early 1960s that were self-contained units, even earlier radios designed for cars and trucks had their bulky electronics mounted under the sea or in the trunk, with a remote volume and tuning control mounted in the dashboard. That greatly complicated the installation as well as the design of the radio. This circa 1940 Belmont Model 678 Auto-Radio is a prime example. Note the unique cylindrical shape of the radio chassis, and that the remote control is a pushbutton assembly with rotating knobs for tuning and volume. Operating from a 6 volt DC car battery (12 volts came later), these radios required a "vibrator" circuit to convert DC to AC (and back to a higher level DC) in order to transform to a couple hundred volts for the plate voltage of the tubes.

Belmont Model 678 Auto-Radio Set Radio Service Data Sheet

Belmont Model 678 Auto-Radio Set Radio Service Data Sheet, August 1940 Radio-Craft - RF Cafe6-Tube Superhet.; 6-Button Automatic Remote Tuner Unit; Automatic Volume Control; Range 535 kc. to 1,565 kc.; Synchronous-Type Vibrator; Single Stud Mounting.

Fig. 1. Complete schematic diagram of the Belmont model 678 auto-radio receiver.

Fig. 2. Chassis view showing locations of trimmers and components.

Fig. 3. Bottom view of remote tuner

Fig. 5. Belmont auto-radio model 678. Note Remote Tuner at right.

Alignment Procedure

Turn volume control to maximum for all adjustments. Connect radio chassis to ground post of signal generator with a short, heavy lead. Connect dummy antenna in series with generator output lead;- 0.1-mf. for the I.F. band and a 125 mmf. for the broadcast band. Use an output meter across the primary of the output transformer. Allow chassis and signal generator to heat up for several minutes.

I.F. Alignment

Feed a 465-kc. signal to the grid of the 6SK7 I.F. tube. Set dial at 1,400 kc. Adjust trimmers C19 and C20 for maximum output. To align this output I.F. unit without using cathode-ray oscilloscope, a 10,000-ohm resistor must be shunted across the diode tuned circuit as indicated by points X and Y on the schematic (Fig. 1) and in Fig. 4. Trimmer C19 is identified by a red dot on top of the I.F. can.

Fig. 4. Bottom view of chassis giving socket voltages.

After alignment of these 2 trimmers, remove the 10,000-ohm resistor and align trimmer C21 for maximum output. Do not readjust trimmer C19 or C20 after the resistor has been removed. Shift the signal generator lead to the control-grid of the 6A8 and adjust trimmers C14 and C15 for maximum output.

Broadcast Band

Feed a 1,565-kc. signal to the antenna lead with the set dial adjusted to 1,565 kc. Adjust trimmer C5 for maximum output. Reset the dial to 1,400 kc. and adjust trimmers C1 and C3 for maximum output using a 1,400-kc. test signal. Finally, reset the dial to 600 kc. and adjust trimmer C2 using a 600-kc. test signal in the antenna lead. Maximum gain for this adjustment depends on the capacity of the antenna system of the car in which the radio receiver is installed.

Power consumption is 7.7A. at 6.3V. No suppressors required on the spark plugs; only a distributor suppressor is needed. The output I.F. coil has 3 tuned circuits giving superior band-pass qualities and selectivity as compared to the conventional 2-tunedĀ­circuit coils. Antenna, R.F. and oscillator circuits are permeability tuned, offering automatic tuning applications that are both accurate and stable. The entire coil assembly is mounted in the Remote Tuner control head being connected to the oscillator and R.F. circuits by an R.F. transmission cable.

The R.F. oscillator, and I.F. and A.F. amplifiers, including the power supply, are contained in the speaker case.

This unit has been designed to facilitate servicing.

 

 

Posted July 10, 2019

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 222 Radio Service Data Sheets as of July 10, 2019.

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