Belmont Radio Corp. 878 Schematic

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Product Information:
Model:878
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:

1 PAGE 8-24 BELMONT

Mmm, 8'18, Series B

Schematic ,Vol tags

BELMONT RADIO CORP.

BAA cl(

Pam/7.1 #ro/1:4 fro

fur Aw. uw;

The tube complementI of thls chassm ls as follows: 1_Type 6Q7 duplex diode penwde Second detector, A_v_c_
l-Type 6K7 Remote cut-DH pentode RF. ampliier audio, ?

1_Type 6J7fpent0de first detectorf l-Type SFS-pentode output ampllfier.

l-Type 6C5 Oscillator l-Type 5Y3 or 5W4-high vacuum rectifier.

1_rype 6K7 Remote cut-05 1121110112 IF. amplifier (465 KC.) l-Type 665 Cathode Ray Tuning IndicatorA

No. No. 11155 s c 102-35 one w. 01 3 gan; var. :0110211521 ~
m 130-20 100111 ohms-01s w-2o%-so v,_c|r. c1 100-9 .os-200 von-_25% l. F, FREQUENCY
6 1:0-8111011 onmsfz w1-ao _wife wound s 10041.2s_400 von-207, ,
R7 130-311 2 meg ohms-us w1-2091-1oo M4211 c7 129-5 .0001 Mica-MTMaa C22 '29'57 ~?0?5 MH"'"MT_?*5*
R10 130-22 5000 Ohmwl/s w1~2090~10 vfcan C10 129-50 .0001s Mica-MT-o-zog; PARTS ,
1111 101-47 1 meg 011111-41111. cm 1111111 Ac swam, c11 100-22 .os-200 vom-25% T1 "L54 MW "d 5~W- A111051" le ASW'L
R13 101-311 100m Ohms-*Tone Con with rid, swim., C1: 100-25 ,02f4oo von-257, T5 109'" Bm'km R-F' CWI A5521"
me 106-27 ss ohms-"10% Mum 112515101- C16 103-11 14 111111-400 von Elmmyuc T6 "0'42 M-W- "d s-W- 0"- Cm Assem-
Nmeaus, R17. R18 in one 111111~w1n No. 105-27 F20 100-13 v05-400 Van-45% Il 114-36 s Speaker (led Ream. 1250 Ohms)

S 125-18 'Band Switch
51 lOl-38 Fideliiy Switch on Tone Control

FIG. l-BOTTOM VIEW SHOWING TRIMMERS

(C)John F. Rider, Publisher

Page 2:

BELMONT RADIO CORP.

SERVICE NOTES

Voltages taken from different points of circuit to dismis are
measured with volume control full on, all tubes in their sockets
and speaker connected, with a volt meter having s resistance of
1000 ohms per volt, These voltages are clearly indicated on the
circuit diagram.

IN ORDER. TO PREVENT SIGNAL FROM ACTING UPON
AVC AND AFFECTING ACCURACY 0F VOLTAGE MEA-
SUREMENTS, AERIAL AND GROUND LEADS SHOULD
zEENSTIIIsORT CIRCUITED WHILE MAKING MEASURE-

All voltages are to be measured with 119 volts on the primary of
the power transiorrner.

Resistances of coils_and transformer windings are indicated in
ohms on schematic circuit diagrams.

To check for open by-pass condensers, shunt each condenser with
another- condenser of the same capacity and voltage rating, which
is known to be good, until the defective unit is located

Excessive hum, stuttering, low volume and a reduction in all
DC. voltages is usually caused by a shorted electrolytic con-
denser, open hy-pass condensers frequently muse oscillation and
distorted tone.

ALIGNING INSTRUCTIONS

Dummy Antennas

The following dummy antenna., are used in aligning md are
referred to in the following alignment instructions as "Dummy
1", "Dummy 2", and "Dummy 3"4

Dummy l: (IFJ-Consists of a .1 mfd. condenser connected in
series with the external oscillator.

Dummy 2: (Broadcast)-Consist's of a 200 mmfd. condenser and
a 20 ohm resistor connected in series with each other
and in series with the external oscillator.

Dummy 3:' (Intermediate and Short Wave)-Consists of s .l
mfd. condenser and a 400 ohm resistor connected in
sen'es with each other and in series with the external

Resonance Indicator:

Use as a resonance indicator an output meter connected across
the primary of the speaker input transformer or b means of an
adapter between the plate and screen termina s of the type
6F6 output tube. Maximum deflection of the meter indicates
resonance. Use only enough signal to get a readily readable
output. A low range output meter or the low scale of s multi-
range meter should be used.

CAUTION :

No aligning adjustments should be attempted without first
thoroughly checking over all other pomible sauna of trouble,
such as poor installations, open or grounded antenna systems, low
line voltages, defective tubes, condensers and resistors. In order
to properly align this chassis, an oscillator (generator) is abso-
lutely necessary. No ali ing adjustments should be attempted
with the chamis in the ca inet. To remove the knobs, pull them
oE and to take the chassis out of the cabinet, remove the four
bolts by which it is fastened.

ALIGNING LF. TRANSFORMERS (465 K.C.)

Part No. 108-63 Output LF. Transformer
Part No. 108-64 Input LF. Transformer

These IF, transformers have two adjustments, both of which are
accessible from the underside of chassis (See bottom view, Fig.

1. With volume control full on, (the extreme right of its ron-
tion), the wave changing swi h in the broadcast osition,
(extreme left of its rotation), e tone control on " i" part
of the sharp positionA (as much right rotation as possible with~
out operating the Hi Fidelity switch), and with the variable
condenser set to approximately 1400 kiloqycles, make the
following adjustments:

(a) Connect external oscillator set at 465 kilocycles, in series
with "Dummy 1", to the control grid cap of the type 6K7
tube, located between the two LF. tnnsfonners, and sd-
just the output LF. transformer 108-63 to resonance.

(b) With "Dummy l" still connected, move oscillator output
clip from grid of 6K7 to grid cap to 6.17 and adjust in-
put LF. transformer (108-64) to resonance.

(c) With oscillator still connected to GJ 7, re-adjust output
LF` transformer if necessary.

ALIGNMENT PRHCEDURE

The following adjustments to be made after the I.F.'s have been
aligned as explained above.

BROADCAST BAND ALIGNMENT:

1. Witlr band changing switch in the broadcast position, exA
treme left of its rotation, and with external oscillator set
at 600 kilocycles and connected in series with "Dummy 2"
to the tan antenna and black ground lead, make the folA
lowing adjustments:

FOR SERIES "A"

(a) Adjust broadcast series pad (adjustment number 5)
to resonance with oscillator. Keep set in tune with os-
cillator by slowly rocking to and fro the variable con-
denser until maximum output is obtained. Note: This
adjustment is accessible from the top of the chassis
and is located between the variable condenser and the
108453 output LF. transformer. See top view, Fig, 3.

FOR SERIES "B"

(a) Adjust broadcast series pad (adjustment number Cl)
to resonance with oscillator. Keep set in tune with os-
cillator by slowly rocking to and fro the variable con-
denser until maximum output is obtained. Note: This
adjustment is acmssihle from the top of the chassis
and is located between the variable condenser and the
108-63 output LF. transformer. See top view, Fig. 3.

(b) Re-set. external oscillator to 1400 KC., move dial pointer
to 1400 K.C. and adjust oscillator (adjustment number
4), R.F. (adjustment number 6) and antenna (adjust-
ment number 'l) to resonance._ See bottom view for
location of these adjustments, Fig. 1.

(c) Repeat adjustments "a" and "b" until sensitivity is at its

NOTE: IT IS EXTREMELY NECESSARY IN MAKING
ALL OF THESE ADJUSTMENTS THAT THE FUNDA-
MEN TAL OSCILLATGR SIGNAL BE TUN ED IN AND NOT
THE IMAGE FREQUENCY WHIGB WILL FALL BELOW
THE FUNDAMENTAL.

SHORT WAVE BAND ALIGNMENT:

l. With band changing switch in the short wave position, ex-
treme right of its rotation, and with external oscillator set nl
l7 megacycles and connected in series with "Dummy 3" to
the tan antenna and black ground lead, make the following

(a) Move dial pointer to 17 megacyelm and adjust short wave
oscillator (adjustment number 1), short wave RF. (ad-
justment number 8) and short wave antenna (adjust-
ment number 9) to resonance.

(b) Re-set externall oscillator to 6 megacycles and pick up
signal by rotating variable condenser and check for sen-

MIDDLE WAVE BAND ALIGNMENT'.

FOR SERIES "A"

l. With band changing switch in the middle wave position,
center of its rotation, and,with external oscillator set at 1800
KC. and 'connected in series with "Dummy 3" to the tan
antenna and black ground lead, make the following adjust'`

(a) Rotate variable condenser to approximately 1800 KC.,
tune in oscillator signal and adjust M.W. series pad
(adjustment number 8) (see top view) to resonance.
Slowly rock condenser to and fro while making this ad-
Justment tu he sure maximum output is obtained.

(b) Set _external oscillator at 5 M.C., rotate condenser, pick
up signal and adjust intermediate wave R.F. (adjustment
number 10), intermediate wave antenna (adjustment
number 11) and intermediate wave oscillator (adjust-

1 ment number 2) to resonance.

FOR SERIES "B"

l. With band changing switch in the middle wave position,
center of its rotation, and with external oscillator set at 5
M.C, and connected in series with "Dummy 3" to the tan
:netstnlna and black ground lead, make the following udjuiw
(a) Rotate condenser, pick up signal and adjust middle

wave RF: (adjustment number 10), middle wave an-

tenna (adjustment number 5) and middle wave oscil-

lator (adjustment number 2) to resonance. _
NOTE-'Re-chec'k broadcast alignment and if it is found neces-
sary to re-adjust either R,F. or antenna trimmers, re-
peat the 17 M.C. short wave and 5 MC. middle wav

(C)John F. Rider, Publisher

B MONT PAGE 8-25

Page 3:

BELMONT RADIO CORP.

SERVICE NOTES

Voltages taken from different points of circuit to dismis are
measured with volume control full on, all tubes in their sockets
and speaker connected, with a volt meter having s resistance of
1000 ohms per volt, These voltages are clearly indicated on the
circuit diagram.

IN ORDER. TO PREVENT SIGNAL FROM ACTING UPON
AVC AND AFFECTING ACCURACY 0F VOLTAGE MEA-
SUREMENTS, AERIAL AND GROUND LEADS SHOULD
zEENSTIIIsORT CIRCUITED WHILE MAKING MEASURE-

All voltages are to be measured with 119 volts on the primary of
the power transiorrner.

Resistances of coils_and transformer windings are indicated in
ohms on schematic circuit diagrams.

To check for open by-pass condensers, shunt each condenser with
another- condenser of the same capacity and voltage rating, which
is known to be good, until the defective unit is located

Excessive hum, stuttering, low volume and a reduction in all
DC. voltages is usually caused by a shorted electrolytic con-
denser, open hy-pass condensers frequently muse oscillation and
distorted tone.

ALIGNING INSTRUCTIONS

Dummy Antennas

The following dummy antenna., are used in aligning md are
referred to in the following alignment instructions as "Dummy
1", "Dummy 2", and "Dummy 3"4

Dummy l: (IFJ-Consists of a .1 mfd. condenser connected in
series with the external oscillator.

Dummy 2: (Broadcast)-Consist's of a 200 mmfd. condenser and
a 20 ohm resistor connected in series with each other
and in series with the external oscillator.

Dummy 3:' (Intermediate and Short Wave)-Consists of s .l
mfd. condenser and a 400 ohm resistor connected in
sen'es with each other and in series with the external

Resonance Indicator:

Use as a resonance indicator an output meter connected across
the primary of the speaker input transformer or b means of an
adapter between the plate and screen termina s of the type
6F6 output tube. Maximum deflection of the meter indicates
resonance. Use only enough signal to get a readily readable
output. A low range output meter or the low scale of s multi-
range meter should be used.

CAUTION :

No aligning adjustments should be attempted without first
thoroughly checking over all other pomible sauna of trouble,
such as poor installations, open or grounded antenna systems, low
line voltages, defective tubes, condensers and resistors. In order
to properly align this chassis, an oscillator (generator) is abso-
lutely necessary. No ali ing adjustments should be attempted
with the chamis in the ca inet. To remove the knobs, pull them
oE and to take the chassis out of the cabinet, remove the four
bolts by which it is fastened.

ALIGNING LF. TRANSFORMERS (465 K.C.)

Part No. 108-63 Output LF. Transformer
Part No. 108-64 Input LF. Transformer

These IF, transformers have two adjustments, both of which are
accessible from the underside of chassis (See bottom view, Fig.

1. With volume control full on, (the extreme right of its ron-
tion), the wave changing swi h in the broadcast osition,
(extreme left of its rotation), e tone control on " i" part
of the sharp positionA (as much right rotation as possible with~
out operating the Hi Fidelity switch), and with the variable
condenser set to approximately 1400 kiloqycles, make the
following adjustments:

(a) Connect external oscillator set at 465 kilocycles, in series
with "Dummy 1", to the control grid cap of the type 6K7
tube, located between the two LF. tnnsfonners, and sd-
just the output LF. transformer 108-63 to resonance.

(b) With "Dummy l" still connected, move oscillator output
clip from grid of 6K7 to grid cap to 6.17 and adjust in-
put LF. transformer (108-64) to resonance.

(c) With oscillator still connected to GJ 7, re-adjust output
LF` transformer if necessary.

ALIGNMENT PRHCEDURE

The following adjustments to be made after the I.F.'s have been
aligned as explained above.

BROADCAST BAND ALIGNMENT:

1. Witlr band changing switch in the broadcast position, exA
treme left of its rotation, and with external oscillator set
at 600 kilocycles and connected in series with "Dummy 2"
to the tan antenna and black ground lead, make the folA
lowing adjustments:

FOR SERIES "A"

(a) Adjust broadcast series pad (adjustment number 5)
to resonance with oscillator. Keep set in tune with os-
cillator by slowly rocking to and fro the variable con-
denser until maximum output is obtained. Note: This
adjustment is accessible from the top of the chassis
and is located between the variable condenser and the
108453 output LF. transformer. See top view, Fig, 3.

FOR SERIES "B"

(a) Adjust broadcast series pad (adjustment number Cl)
to resonance with oscillator. Keep set in tune with os-
cillator by slowly rocking to and fro the variable con-
denser until maximum output is obtained. Note: This
adjustment is acmssihle from the top of the chassis
and is located between the variable condenser and the
108-63 output LF. transformer. See top view, Fig. 3.

(b) Re-set. external oscillator to 1400 KC., move dial pointer
to 1400 K.C. and adjust oscillator (adjustment number
4), R.F. (adjustment number 6) and antenna (adjust-
ment number 'l) to resonance._ See bottom view for
location of these adjustments, Fig. 1.

(c) Repeat adjustments "a" and "b" until sensitivity is at its

NOTE: IT IS EXTREMELY NECESSARY IN MAKING
ALL OF THESE ADJUSTMENTS THAT THE FUNDA-
MEN TAL OSCILLATGR SIGNAL BE TUN ED IN AND NOT
THE IMAGE FREQUENCY WHIGB WILL FALL BELOW
THE FUNDAMENTAL.

SHORT WAVE BAND ALIGNMENT:

l. With band changing switch in the short wave position, ex-
treme right of its rotation, and with external oscillator set nl
l7 megacycles and connected in series with "Dummy 3" to
the tan antenna and black ground lead, make the following

(a) Move dial pointer to 17 megacyelm and adjust short wave
oscillator (adjustment number 1), short wave RF. (ad-
justment number 8) and short wave antenna (adjust-
ment number 9) to resonance.

(b) Re-set externall oscillator to 6 megacycles and pick up
signal by rotating variable condenser and check for sen-

MIDDLE WAVE BAND ALIGNMENT'.

FOR SERIES "A"

l. With band changing switch in the middle wave position,
center of its rotation, and,with external oscillator set at 1800
KC. and 'connected in series with "Dummy 3" to the tan
antenna and black ground lead, make the following adjust'`

(a) Rotate variable condenser to approximately 1800 KC.,
tune in oscillator signal and adjust M.W. series pad
(adjustment number 8) (see top view) to resonance.
Slowly rock condenser to and fro while making this ad-
Justment tu he sure maximum output is obtained.

(b) Set _external oscillator at 5 M.C., rotate condenser, pick
up signal and adjust intermediate wave R.F. (adjustment
number 10), intermediate wave antenna (adjustment
number 11) and intermediate wave oscillator (adjust-

1 ment number 2) to resonance.

FOR SERIES "B"

l. With band changing switch in the middle wave position,
center of its rotation, and with external oscillator set at 5
M.C, and connected in series with "Dummy 3" to the tan
:netstnlna and black ground lead, make the following udjuiw
(a) Rotate condenser, pick up signal and adjust middle

wave RF: (adjustment number 10), middle wave an-

tenna (adjustment number 5) and middle wave oscil-

lator (adjustment number 2) to resonance. _
NOTE-'Re-chec'k broadcast alignment and if it is found neces-
sary to re-adjust either R,F. or antenna trimmers, re-
peat the 17 M.C. short wave and 5 MC. middle wav

(C)John F. Rider, Publisher

B MONT PAGE 8-25

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