Belmont Model 578 Series A, 5-Tube A.C. Superheterodyne
Radio Service Data Sheet
March 1936 Radio-Craft

March 1936 Radio-Craft

March 1936 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 is another Radio Service Data Sheet that appeared in the March 1936 edition of Radio-Craft magazine. I post this schematic and functional description of the Belmont Model 578 Series A, 5-Tube A.C. Superheterodyne radio manufacturers' publications for the benefit of hobbyists and archivists who might be searching for such information either in a effort to restore a radio to working condition, or to collect archival information.

Belmont Model 578 Series A 530, 5-Tube A.C. Superheterodyne Radio Service Data Sheet

(Dynamic reproducer; A.V.C.; available with transformers for any A.C. line; 7-tube performance; full-vision vernier dial)

Voltages and resistance values (latter in parentheses) for this circuit are shown on the diagram. These voltages are all measured to chassis with a 1,000 ohms-per-volt meter. All tubes must be in their sockets, volume control full-on and the speaker connected. Voltages are measured with 119 V. input to the power transformer.

Belmont Model 578 Series A 530-1,720 kc.) 5-Tube A.D. Superheterodyne Radio Service Data Sheet, March 1936 Radio-Craft - RF Cafe

Transformers are available for universal operation on 40 to 60 cycles and with primary taps for use on 108, 125, 150, 220, and 250 V. Also 25-cycle transformers are made for 105-115 V. or 220 V. primaries, not universals.

When aligning the I.F. transformers, the volume control must be in the maximum position and the gang condenser at the minimum position. Connect the external oscillator to the cap of V2 in series with the "I.F. dummy antenna" and to ground, and align the condensers in I.F.T.2. Move the clip to the cap of V1 and align I.F.T.1, then check I.F.T.2 again to get an exact setting. The "dummy antenna" is merely a 0.1-mf. condenser in series with the lead to the cap.

The R.F. alignment is accomplished with the oscillator connected to the antenna and ground leads of the set through the broadcast dummy antenna. With the plates of the gang condenser entirely out of mesh, and the service oscillator set at 1,720 kc., adjust the receiver oscillator trimmer (rear of gang condenser). Reset external oscillator to 1,400 kc., turn gang condenser to pick up signal and adjust antenna trimmer to resonance. Next, reset external oscillator to 600 kc., tune signal in, and adjust series padder to resonance, while rocking gang condenser back and forth slightly at the same time to insure correct setting. The padder is accessible from the top of the chassis, and is located between the gang condenser and the power transformer. The dummy antenna for broadcast alignment consists of a. 200-mmf. condenser and a 20-ohm resistor in series with each other and the external oscillator.

No alignment should be attempted with the chassis in the cabinet. The chassis is held in place by 3 bolts. The knobs are of the pull-off type. The use of a signal generator is an absolute necessity for correct alignment. Beginning with series 5K173250A, the antenna circuit is connected as shown in dotted lines, the solid-line antenna wires being omitted.

Open bypass condensers frequently cause circuit oscillation and distorted tone. The simplest way of finding the defective unit is by shunting each bypass condenser with another of the same capacity until the bad one is found.

Defective or shorted electrolytic filter condensers cause excessive hum, motor-boating, low volume, or a reduction in all D.C. voltages. Open or shorted electrolytic and bypass condensers (across bias resistor of V4) will cause low volume and distorted tone.



Posted July 13, 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 220 Radio Service Data Sheets as of February 2, 2018.