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.
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 227 Radio Service Data Sheets as of
December 28, 2020.
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while typing up your telephone line, and a nice lady's voice announced "You've Got
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