Belmont Radio Corp. A-7DF21 Schematic

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Product Information:
Model:A-7DF21
Manufacturer:Belmont Radio Corp.
Description:SERIES A

Schematics Content:

Please note: this content is a computer-generated interpretation of the above schematic and is provided only to help assist you in locating this schematic. For the actual text, please consult the schematic above. Thank you.

Page 1:

BELMONT YPAGE 20-5?

APPLYING POWER TO RADIO

This receiver, unless otherwise marked must be operated
on an AC voltage of 105 to 125 volts, 50 to 60 cycles,
or on a DC voltage of 105 to 125 volts. If you are in
doubt as to the voltage of your power supply, consult
I your local power company. Receivers of this same

model which are for use on voltages other than those
specified above are so marked.

BROADCAST BAND
This is the tuning band in which the standard broad-
cast stations operate. The upper scale on the dial covers
the broadcast range of 535-1620 Kc., and is calibrated

REPLACEMENT

REPLACEMENT OF DIAL CORDS
GENERAL-A dual track drum pulley and two indivi-
dual cords are used on this model.

The rear track carries the Drive String (see Fig. 1
'and 2) while the front track carries the Pointer string
(see Fig. 5 and 4).

DRIVE STRING: Using approximately 20 inches of dial
cord, fasten one end to the tension spring and pass
around drum and drive shaft (Fig. 1). Continue cord
I around drum and pass through hole in rear track (see
Fig. 2). Tie end of cord to spring so that spring is ex-

POINTER STRING: Use approximately 40 inches of dial
_cord, fasten one end to tension spring and pass around
drum pulley (see Fig. 5). Continue cord (Fig.'4) from
point B around idler pulleys, around drum, then pass
ithrough hole in front track, and tie cord to tension
spring so that the spring is extended 1A inch.

POINTER CALIBRATION: Adjust pointer as shown in Fig.
E4, then loop Pointer string once around upright ear on

MODEL A-7DF2T,

Series A
in channel numbers. To obtain the kilocycle reading,
multiply the number on the dial by 10; thus 80 on the
dial corresponds to 800 kilocycles.

FM ANTENNAS

The noise-reducing capabilities of FM are noticeably
greater when strong FM signals are obtained. There~
fore, we recommend, whenever expedient, the use of an
outside "folded dipole" aerial with a SOO-ohm line lead-
in, The aerial must be carefully installed according to
the directions furnished with it. The radio is shipped
from the factory with_the built-in FM aerial connected
by means of, the long jumper wire to the left-hand FM
Antenna Terminal. (See Chassis view).

IMPORTANT: The built-in Antenna is part of the
line cordfand, therefore, should be fully extended.
Changing the angle of drop of the line cord will IM-
PROVE RECEPTION. If HUM is objectionable reverse
the line cord plug in the power receptacle. Select the
position giving least amount of hum.

To connect the lead-in from the outsideV FM aerial,
remove the wire from the built-in FM antenna and con-
nect the twin lead-in wire to the two screws. Either wire
of the twin lead-in may be connected to either screw.
A ground connection is not required for FM reception._

lt should be remembered in coniunction with the
erection of an FM folded dipole aerial that the signal
strength from an FM transmitting station is less and'
less at greater distances from the transmitter and that
FM reception is hardly ever possible beyond uline of
sight" distances between transmitting and receiving`
aerials. This maximum limit is usually about 45 miles
but consistently satisfactory reception is frequently'
limited to 30 miles or less depending on the height of
transmitting and receiving aerials and the intervening

comer :mouse

wlm me mrge drum turned compvexely
D015 wunrer-emrwtse me up me len sage
oi nw some' wm. me am shown

Pointer Stringing and Allgnmenll

o John F. Rider

Page 2:

PAGE 20-6 BELMONT

Series A

AllGNMENT PROCEDURE

Broadcast Band Section I. F. and R. F.

The alignment procedure below includes the sensiti-
vities at the inputs of various stages, All signal input
values are basedvon an output of 50 milliwatts. This
may be measured by disconnecting the speaker voice ,
coil and substituting a 3.2-ohm resistor across the secon- |2AT7
dary winding of the output transformer. A reading of T5
.40 volts AC across this resistor will be approximately
equivalent to 50 milliwatt output with the speaker con-
nected. The volume control must be set at maximum.
The tone control must be set for maximum treble.

The signal source must be an accurately calibrated |25A6
signal generator capable of supplying the frequencies 352152,*
designated, modulated 50% with a 'SOO-cycle audio sig- "NETQLRTAL '
nal. A 400 cycle audio signal is required for the audio (50? WM) is '2AU6 mt,
measurement. Variations in sensitivities of plus or minus
25% are usually permissable. Chassis View
Band Switch in AM Position, Gang Open, Dummy Antenna .1 Mfd.
SIGNAL
GENERATOR CONNECTION
FREQENCY T0 RADIO ADjUSTMENTS TO BE MADE ADUST FOR
55 IKOSOUse PmIIFOquPBAG Primary and Secondary of T8. Mgiggilglbgugut
microvolts and B minus See Chas" "ew' Milliwms
455 Kc. Use Ping of 12BA7 Primary and Secondary of T6. Maximum output
30 microvolts onvertFr See chassis view. Shmilcl be 50
and B minus Millrwatts
High Side of Maximum output
400 clicks' Use Volume Control None Should be 50
and B minus Milliwatts

BROADCAST BAND-R. F'. AllGNMENT

Check pointer so that the right hand edge of the pointer skirl coincides with #be
right hand edge of dial marker at the extreme left when gang is closed.
For Adjustment, .ree dial mechanism illustration.

SIGNAL GENERATOR
FREQUENCY SET POINTER AT CONNECT TO RADIO ADH/ST
1620 Kc Extreme Right AM Antenna Clip Oscillator trimmer
' Calibration Marker and B minus CZ-B for maximum
1400 Kc Second Calibration AM Antenna Clip Antenna trimmer
l from Left md B minus CZ-A for maximum I

Check tracking at 1000 Kc, 600 Kc, and 555 Kc t0 be sure oscillator is set correctly.

o John F. Rider

Page 3:

IMPORTANT

No alignment of the FM section of this radio should be
attempted unless you are positive that the circuits are in
need of adjustment and you have the necessary equip~

All components used in this radio are extremely stable
and the tuned circuits should require no adjustment
over a long period of time.

AllGNMENT PROCEDURE
FM Band Settion I. F. 11nd R. F.
A non1metallic alignment tool must be used.

Band Switch in FM Position. Dummy Antenna .1 Mfd.

BELMONT PAGE 20-7

MODEL A-7DF21,
Series A

NOTE

The following alignment is based on the use of the new
Simpson vacuum tube voltmeter which has a "floating
ground". In other words, the meter, when used as a
vacuum tube voltmeter, can have both the positive and
negative sides connected to points above ground and
still give true readings,

A standard AM signal generator is required.

SIGNAL H
GENERATOR CONNECTION Ig?gggclir%? ADJUSTMENTS AD UST FOR
FREQUENCY To RADIO To RADIO To BE MADE l
10.7 Mc. P- N 1 - Bottom Core Resonance
Use about 0? 12 Pm lisa ,IBOfm-EQLS Primary of T9 should be about
.1 volt Ratio Detector 3 volts
10.7 Mc. Pi N 1 Top Core Zero. Use zero
Use about 0:12 [gi/16 See note "A" Secondary of T9 center scale
.1 volt Ratio Detector See note "B"
10.7 Mc. P~ N _ 1 - f Primary and Secondary Resonance
Use about 350 OfmIZBXG Pm 213A 7Bomi1?iL5 of T7. FM Driver IF should be about
microvolts ` See chassis view. 3 volts
10.7 Mc. T d f P- f 12A Primary and Secondary Resonance
Use about soo Dpcgflc ? 1" Eiga 7B?mmuSL5 of Ts. FM Input 1F should be about
microvolts See chassis view. 5 volts

NOTE "A"-Connect two resistors in series, 100K OHMS each,
from Pin No.7 oi 12AL5 to B minus(pin no.5).These resistors
must be matched within 5%. Connect vacuum tube voltmeter
between the midpoint of the resistors and point zz.

NOTE "B"--If T9 has been tampered with, it is possible that
no crossover point will be found at first. Careful adjustment of
both primary and secondary is necessary.

NOTES ON FM--L F. ALIGNMENT

Check pointer ,to that the fight hand edge of the pointer skirt coincides with the
right band edge of dial marker at the extreme left when gang is elated.
For Adjustment, .tee dial mechanism illustration.

NOTE "C"-To use a VTVM which does not have the "float4
ing ground" feature, in step 2 above connect "ground" side of
VTVM to midpoint of resistors (Note "A") and "high" side

GENERAL-Input signals should be adjusted to give approx-`
imately 3 volts. The ratio detector is operating at a resonable
level at this point and will 1give the truest indication of cor-
rect alignment with the procedure specified.

GE?616352190R POINTER CONNECTION AD UST V T V M
FREQUENCY T0 RADIO I CONNECTIONS
los MC. 108 MC FM antenna FM Osg Cs
Marker terminals fOr maximum Pin N0_ 7 of
98 MC Tune in See Note "B" FM Mixer CZ-C 12AL5 t0 B minus
` Gen. Signal below for maximum

NOTE "A"-lf a signal generator with the above fundamental
frequency is not available, it is sometimes possible to use har-
monics. Use extreme care .in picking harmonics. An alternate
procedure is to use a local station carrier of known frequency
to align the FM Band and to use the vacuum tube volt-meter

as above for resonance indication. A weak carrier, however,
will not produce 3 volts.

NOTE "B"-Connect 500 ohms in series with "hot" side of
generator and connect to left hand screw of external FM An-
tenna Terminals. Connect cold side of generator to right hand

o John F. Rider

Page 4:

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Rider

(C) John F.

Page 5:

BELMONT PAGE 20-9

MODEL A-'ZDFZl ,

Series A

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(C)John F. Rider

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