Marantz 2245 User manual

SERVICE
MANUAL
model 2245
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” ‘SPECIFICATIONS
Audio Circuits:
Rated
continuous
(RMS)
power
output
per
channel,
both
channels
operating
simultaneously; 20 Hz to 20,000 HZ jaa verse ee eee eke vee eee irene ees 45 Watts at 4 and 8 ohms
5
25
Watts
at
16
ohms
Compatable Total Music Power (IHF) 3 .c 3 cc ew erantetwen nds Gaeta at aewe s 135 Watts at 8 ohms
High-level hum and noise (ref. 4OW at Bohms) ... 16. ee cette eee —80 dB
Phono hum and noise 2... ee en eee eee eee nent teen nnes 1.5 nV equivalent input
Dynamic range (phono input to tape recording Output) . 0... eee eee ete nee 96 dB
i. Me Distortion (SMPTE),.at tated POWEF. a. cae ee aoe cae es we ae Oe peewee eed ewes 0.3%
Distortion decreases as output is lowered
Total Harmonic Distortion, at rated poWer «6. cee ee eens 0.3% Maximum
ue ~~ Distortion decreases as output is lowered
wa. ower Bandwidth (IHF) for 0.3% THD 2.0.0... 0. ccc eee cece cece e ees ene serene 7 Hz to 70,000 Hz
Damping Factor (ref. 8 OPTS)’. <2 Soe Sess Nee eS akg Sb Seether beter oa be ala Ree Rawat eh Greater than 45
Frequency Response
Through phono ttt tt ttt teen ee ene eee e eee +7dB
tnput Sensitivity (for 40W at 8 ohms)
FIG IOV. | wecck Slaieicel ooo Se les a eh See 0 de eeu ets Se ae GR Ea ATE Oe IG Ege Sale wegen ieee a 180 mV
Phono T1000 HZ) Aas este aetna ey et i eaters Rach ae eae aee & KS aha Aa we aa 1.8 mV
Input |!mpedance :
High-level re 100,000 ohms
PHOMO = acdc cook ae woke PA oe en eg BIS ek nde beat peal Taal URE se gaseina la aia ee eee kaka: 47,000 ohms
Channel Separation 20 Hz 10: 20,000 HZ 5. .ae cree raids Beene t ae ee eg dane S 35 dB Minimum
FM Sections:
THE Usable Sensitwityes Gat so aaee 626 ae ade ea oe oo pe ema ete Ma hetie argtaud walneaee ace 2.3 uV
SSFECEIVFEY? atin. wand pas ee row PO Pe ae eee ee Ree E ROMA ee ta ee aee ees 60 dB
INDISEQUTBEING =o vos kor ase ae ak wk oe er a ae UN oe ts i Se A salt arid ea veh eieaea ~55 dB at 5uV
eet —60 dB at 10uV
. , —65 dB at 50ynV
Total Harmonic Distortion, 400 Hz, 100% Mod. ....... 02... cee eee eee (Mono) 0.2% (Stereo) 0.4%
Frequency Response (ref.75u sec. deemphasis) 2.2... 0... eee eee ee +1dB 50Hz to 15KHz
Stereo Separation score bi ha dee ane tesew Sa ee ks Ge net at ele Stew 4 1,000 Hz 40 dB
Sub- Carrier (38-kHz) Suppression sce. sea si ae eee ee se ore Hobs pidlne a Seated eaees 60 dB
General:
Power Requirements sc vadie sy ete eed dua eenree ee bee ee eton be hee aes 100/17 20/200/220/240V AC
~ 50to 60 Hz
At
rated
output,
both
channels
operating
.........
0.0
cee
eee
eee
e
eee
n
eee
ues
310
Watts
idling Power (Volume Control at zero) oo... ec ent e ee nee eens nees 34 Watts
Dimensions
PARC
UAW
ors
aerate
velueuoue
Rin
use
Pe
tem
hg
ad
same
Mae
Ales
e
Ls
17
2"/g4
Inches
Panel HEIN 24h anv washes fire tt ate ae at dt pests, Whar ace lanes 5 25/4, Inches
SRO RI ti Sake once ib ME a Ne 5 oO deat Nanelgd wu eae t ee uia BAT y tandltras 14 Inches
Weight
ME
ANON
i
ail
ny
iG
Sa
on
ole
ich
dae
dis
cao
Chan
Akan
etait
Sto
ale
34.5|bs
Packed for shipment 3... Py dea ese At Pact Cece e ee ess. 44.5ibs
* These speciticati j : . . .
SE SPECHNCATIONS and exte: yr designs may he cnanged for improvement without advance notice.

TABLE OF CONTENTS
SECTION
PAGE
IMtroduCtion
=
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8
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dcene
Vale
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Sw,
BAG
aw
Mme
W SEE
SES Ree Se
FEE
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1
Service
Notes
..
0.0...
cece
eee
i
cdane
alta
UasSahoace Meats
whe
ete
te
erdsal
qual
vata
gx
ht
1
AME
TUNG
Fete
ec
thee
Gee
fen
A
ae
Rea
E
adh
ee eo
G
Saige
Saw
en
T
IVE
Thee
co
hhc
Oe
ated
ee
sec
nears
Medan
easel
ae
pat
De
REIS
NG
Ces
OR
A
ra
ae
Sed
dae
aes
-2
Phono
and
Tone
Amplifiers
..
0.0.0...
0.
cece
eee
eee
eee
enna
4
Power Amplifier
si4 san
aoe
pba
eed
a ss
ERR
ew UE Eee
oe
eae
he
5
Power
Protection
Circuit
©...
0...
ce
ee
ee
eee
eee
eens
5
Speaker
Protector
Relay
Circuit...
0...
0.
ee
te
te
tenes
5
Suggestions
for
Trouble
Shooting
of
Power Amplifier
..........
Site
Mech dhe
haMihitaiataanead
6
Voltage
Conversion...
0.0...
eee
ees
ng aca
a
ah
erat
OE
RE
a ow
RAD
HS WS 6
Test
Equipment
Required
for
Servicing
©...
1...
cee
eee
ete tenes
8
AM
Alignment
Procedure
...
0...
ee
ete
eee
eee
9
FM Alignment
Procedure
2.1.1...
cc
ene
teen
9
STEREO
Separation
Alignment
.........
dal
cuss
gusdiae CNG
oyneeg,
a
vara e
terete
AG Sarat
tans
9
Muting
Circuit
Alignment
si.
5.0.
ees
ak
es
3
2
we
PRE
ee
wee
as
10
Audio
Adjustment
2.0.2.0...
cc
ee
ee
en
ee
ene
10
Parts
Lest)
sa4
pact tel ot
avn
ate ites
eas eae
oe Rete
ae
arate
ae, fe
ala aie
alta
ne ete
21
SpeciHGatlONs
occ
¢.cee
eae terse
Ske
Ss
eg
ee
ee
BS
OOS ate
eMac
ha Xe
29
LIST OF ILLUSTRATIONS
FIGURE PAGE
1. Remove the Terminal Cover .. 0.0... ccc eee eee ee ee 6
2. Voltage Conversion Chart ...............45 Rig ate a gee Wieriaha tg Yate edo aah es nea Os 7
3. Lissajou Figure on Oscilloscope .. 0.6... et tte ees 11
4. Dial Stringing s 3. sccicha ae diate walls or Oh Mi eae Gn ae Dee Eels Bie ag ee hee 11
5. Front Panel Adjustment and Component Locations ...........0 cece cece eee 12
6. Main Chassis Component Locations (Top View) .............0 000s e eee Bec Nghtits, 12
7. Rear Panel Adjustment and Component Locations ....... 6... cece cee eee es 13
8. Main Chassis Component Locations (Bottom View) ...... 0... cc cece ee eee eee 13
9. FM Front End Assembly P100 Component Locations. .......... 0.0.00. e eee 14
10. FM IF Amplifier Assembly P200 Component Locations ................0..00005 14
11. FM Detector Assembly P500 Component Locations ........ 0.00: cece eee ene 15
12. MPX Stereo Decoding Amplifier Assembly P300 Component Locations............ 15
13. Muting Control Amplitier Assembly P550 Component Locations ...............45 16 -
14. AM Tuner Unit Assembly P150 Component Locations ......... 0.0.0.0... 0b eee 16
15. Phono Amplifier Assembly P700 Component Locations ...................00005 17
16. Tone Amplifier Assembly P400 Component Locations ............... 00.00.00 17
17. Power Amplifier Assembly P750 Component Locations ...........0. ee cee eee 18
18. Regulated Power Supply and Protection Relay Circuit Assembly
P800 Component Locations ©2000... 0. eee eee ees 19
19. Loudness, High and Low Filter and Muting Switch Unit Assembly
P600 Component Locations ........ 00.0. eee eee ee eee 19
20. Schematic Diagram ©... 2.2 ee ees 20

INTRODUCTION
This
service
manual
was
prepared
for
use
by
Authorized
Warranty
Stations
and
contains
service
information
for
Marantz
Model
2245
Stereophonic
Receiver.
Servicing
information
and
voltage
data
included
in
this
manual
are
intended
for
use
by
the
knowledgeable
and
experienced
technician
only.
All
instruction
should
be
read
carefully.
No
attempt
should
be
made
to
proceed
without
a
good
understanding
of
the
operation
in
the
receiver.
The
parts
list
furnish
information
by
which
replacement
part
may
be
ordered
from
the
Marantz
Company.
A
simple
description
is
included
for
parts
which
can
be
usually
be
obtained
through
local
suppliers.
1. Service Notes
As
can
be
seen
from
the
circuit
diagram
the
chassis
of
Model
2245
consists
of
the
following
units.
Each
unit
mounted
on
a
printed
circuit
board
is
described
within
the
square
enclosed
by
a
bold
dotted
line
on
the
circuit
diagram.
1.
FM
FrontEnd
1...
00.
ee
ee
_
mounted
on
P.C.
Board,
P100
2.
FMIF
Amplifier
2...
0
ee
_...
mounted
on
P.C.
Board,
P200
3:
(PM
Detector
Gover
eee
Boe
ke
HA
ee
Dw
ks
mounted
on
P.C.
Board,
P500
4,
MPX
Stereo
Decoding
Amplifier
2...
00.
ee
ee
eee
mounted
on
P.C.
Board,
P300
5.
Muting
Control
Amplifier...
--
6-6-0
ee
eee
ees
mounted
on
P.C.
Board,
P550
6
SAM
TunerUAit
o
4c8
Sida
we
ce
ae
Ge
We
eS
mounted
on
P.C.
Board,
P150
7.
Phone
Amphtier
..ccwe
Ge
eee
So eo
Rees
ssestan
as
ec
mounted
on
P.C.
Board,
P700°
8.
Tone
Amplifier
cts
tues
oad
ae
i
ahs
Bn ee
Oe
ee
mounted
on
P.C.
Board,
P400.
9.
Power
Amplifier
.
50.
eb
de
eee
mounted
on
P.C.
Board,
P750_
10.
Regulated
Power
Supply
and
Protection
Relay
Circuit...
.
mounted
on
P.C.
Board,
P800
11,
Loudness,
High
and
Low
Filter,
and
Muting
Switch
Unit
. . .
mounted
on
P.C.
Board,
P600
2;
AM
Tuner
All
components
except
Tuning
capacitor
and
ferrite
bar
antenna
are
mounted
on
a
printed
circuit
board
P.150.
The
AM
signals
induced
in
a
ferrite
bar
antenna
are
applied
to
the
base
of
RF
amplifier
transistor
H151
through
a
capacitor
of
C151
and
amplified
to
the
level
required
for
overcoming
the
conversion
noises,
thus
giving
good
S/N
performance.
The
tuned
circuits
inserted
in
both
out
and
input
circuit
of
the
RF
amplifier
assure
very
high
image
and
spurious
rejection
performance.
Thus
amplified
and
selected
AM
signals
are
then
applied
to
the
base
of
converter
transistor
H1i52
through
a
coupling
capacitor
C156.
While
the
local
oscillator
voltage
is
injected
to
the
emitter
of
H152
through
a
capacitor
C157.
Both
AM
signals
and
oscillating
voltage
are
mixed
at
the
base-emitter
junction
and
converted
into
455KHz
intermediate
frequency.
The
resulting
IF
signal
is
applied
to
the
first
IF
transformer
1153
consisting
of
one
ceramic
filter
and
two
tuned
circuits.
The
output
of.
L153
is
led
to
the
transistor
H153
which
in
turn
apply
its
output
to
the
transistor
of
next
stage
H154.
The
fully
amplified
IF
output
is
then
applied
to
the
diode
H157
to
detect
audible
signal
through
the
detector
transformer
L154.
The
detected
audio
signal
is
filtered
and
amplified
and
the
final
audio
output
is
obtained
from
the
collector
of
H155
and
applied:
one
to
the
tape
out
jacks
through
monitor
‘switch
on
the
front
panel
and
the
other
to
the
function
rotary
switch,
Se:
:
The
DC
component
of
the
detected
1F
signal
is
used
as
a
AGC
voltage
to
control
emitter
current
of
H153
which
in
turn
control
the
bias
current
of
the
RF
amplifier
through
the
resistor
R179
and
R751.
A
part
of
IF
signal
output
is
also
applied
to
the
diode
H158
through
a
capacitor
C167
and
rectified
to
obtain
DC
current
for
energizing
the
AM
signa!
strength
meter
Mo01,

Suggestions
for
AM
Tuner
Trouble
Shooting
Check
for
broken
AM
bar
antenna,
next
try
to
tune
station
by
rotating
fly-wheel
tuning
knob
slowly
and
observe
the
AM
signal
strength
meter
whether
it
deflects
or
not.
If
the
signal
strength
meter
gives
a
deflection
at
several
frequencies
received,
no
failure
may
exist
in
the
stages
at
least
preceding
final
1F
transformer
L154.
Next
connect
a.oscilloscope
to
the
pin
terminal
J162
or
J157
and
check
for
audio
signals
with
the
tuning
meter
deflected.
If
the
signal
strength
meter
does
not
deflect,
check
the
local
oscillator
circuit.
Normal
oscillating
voltage
at
the
hot
end
of
the
oscillator
tuning
capacitor
is
about
2
or
3
volts,
varying
with
tuning
capacitor
position.
When
measuring
oscillating
voltage
use
a
RF
VTVM,
no
circuit
tester
gives
correct
indication.
!f
the
local
oscillator
voltage
is
normal,
check
all
voltage
distribution
in
the
AM
circuits
by
using
a
DC
VTVM
and
compare
the
measured
values
with
those
given
in
the
schematic
diagram.
3. FM Tuner
The
FM
Tuner
section
of
Model
2245
is
divided
into
five
functional
blocks:
FM
Front
End,
IF
Amplifier,
Detector,
Muting
Contro!
and
MPX
Stereo
Decoding
Circuit.
,
FM
signals
induced
by
a
FM
antenna
are
led
to
FM
antenna
coil
L101
through
an
attenuator
switch
and
a
Balun
coil.
These
signals
are
then
applied
to
the
FET
RF
amplifier
which
in
turn
applies
its
output
to
the
next
EET
Mixer
H102
through
the
double
tuned
high
selective
circuits.
The
FET
Mixer
convert
its
input
signal
into
10.7MHz
intermediate
frequency
and
amplifies
it
at
the
same
time.
The
H103
is
a
local
oscillator
and
its
output
is
injected
into
the
source
of
the
FET
Mixer,
the
injection
voltage
is
about
700mvV.
The
10,7MHz
front
end
output
is
led
to
the
next
IF
amplifier
unit
through
a
coaxial
cable.
:
The
IF
amplifier
unit
consists
of
five
stages
of
1F
amplifier
and
one
stage
of
AGC
amplifier.
Six
pieces
of
ceramic
filters
are
also
used
to
obtain
high
selectivity
four
stages
of
symmetrical!
diode
‘limiters
are
also
employed
for
the
best
limitting
characteristics,
improved
capture
ratio
and
good
AM
suppression.
A
part
of
FM
Front
End
output
is
applied
to
the
AGC
amplifier
H206
and
rectified
its
output
is
fed
back
to
the
gate
of
FET
RF
amplifier
to
decrease
the
gain
with
increased
signal
strength.
The
IF
signal
sufficiently
amplified
through
every
stage
of
1F
amplifier
is
finally
applied
to
the
IC
limiter
on
the
Detector
Unit.
The
detected
audio
output
is
fed
to
the
buffer
amplifier
H502
and
its
buffered
output
is
led
to;
(a)
noise
amplifier
H551
through
resistor
R551
and
capacitor
C551,
(b)
Quad
Radial
Jacks
on
the
rear
panel
through
resistor
R564,
(c)
MPX
stereo
decoding
circuit
through
R563.
The
DC
current
caused
at
the
third
windings
of
the
discriminator
transformer
is
directly
applied
to
the
FM
center
tuning
meter.

}
‘ea
proportional
to
the
noise
components
in
the
audio
signals.
Audio
Muting
and
Stereo
mode
auto-selecting
circuit
The
muting
circuit
consisting
of
all
solid-state
electrical
switching
has
been
incorporated
in
the
Model
2245.
Three
inputs
control
the
muting
function.
The
first
is
related
to
signal
strength,
the
second
to
the
noise
condition
at
the
detector
and
the
third
is
derived
from
the
DC
component
of
the
detector
output.
These
inputs
are
properly
matrixed
and
gated
to
provide
muting
free
from
noise
and
transients.
The
first
input
of
DC
voltage
obtained
by
rectifing
a
part
of
IF
output
signal
from
the
H205
is
applied
to
the
base
of.
H306
and
turns
on
it,
if
the
1F
output
is
greater
than
predetermined
level
(muting
threshold
level).
When
the
H306
is
turned
on
the
H307
is
turned
off,
allowing
the
emitter-collector
resistance
increasing
and
the
collector
voltage
rises
about
9V.
The
increased
collector
voltage
increases
the
gate
bias
voltage
and
turns
on
the
switching
FET
H308,
decreasing
the
source-drain
resistance
to
near
zero
ohm
and
allowing
the
audio
signal
applied
to
the
source
to
flow
to
the
center
of
38KHz
switching
transformer
through
the
source-drain
path.
;
When
the
input
signal
is
lower
than
predetermined
level,
the
DC
output
obtained
is
small
and
can
not
turn
on
the
H306,
thus
the
H306
keeps
its
turn-off
state
and
this
makes
H307
turn
on,
decreasing
the
collector
voltage
and
turning
off
H308.
Thus
no
audio
signals
can
pass
through
the
FET.
This
is
the
fundamental
principle
of
the
muting
operation
but
for
more
elaborate
muting
.
operation
the
second
and
the
third
inputs
are
necessary.
The
second
input
is
used
to
protect
the
muting
operation
and
MPX
stereo
beacon
lamps
from
“misoperation
due
to
undesirable
noises.
The
high
frequency
noises
included
in
the
detected
audio
signals
are
separated.
by
a
small
capacitor
C551
and
amplified
by
the
noise
amplifier
transistor
H551
and
its
output
is
rectified
by
the
two
diodes.
The
rectified
DC
output
is
When
there
are
excessive
noises
in
the
audio
signals
such
as
obtained
with
a
station
incorrectly
~
tuned
in,
the
rectified
DC
output
turns
on
the
transistor
H522,
decreasing
the
emitter-collector
resistance
to
zero.
This
means
the
collector
of
H307
is
short-circuited
to
the
ground,
therefore
.
the
H308
is
turned
off
and
any
audio
signals
having
excessive
high
frequency
noises
can
not
go
through
the
FET’s
sourcedrain
path.
The
transistor
H303
connected
in
series
with
the
19KHz
pilot
signal
amplifier
transistor
H302
is
also
turned
off
(when
the
transistor
4522
or
H307
are
turned
on.)
and
no
current
flows
in
the
H302,
resulting
in
turning
off
the
stereo
beacon
lamps.
Thus
misoperation
due
to
undesirable
noises
is
also
avoided.
The
third
input
is
obtained
from
the
EM
discriminator
circuit.
The
DC
output
so
called
“gr
curve
is
applied
to
the
gate
of
H553
through
a
resistor
R523
and
deviding
network
(R565
&
R566).
The
DC
output
is
zero
with
a
station
correctly
tuned
in,
but
will
vary
from
negative
to
positive
values
or
vice
versa
when
the
tuning
point
is
deviated
toward
either
plus
or
minus
frequency
from
the
correct
tuning
frequency.
When
the
DC
output
is
increased
to
a
greater
level
than
that
of
predetermined,
the
increased
source
potential
of
H558
makes
the
transistor
H561
turn
on,
and
this
makes
the
H306
turn
off,
...
H307
turn
on,
....
H308
turn
off,
....
1303
turn
off
(this
means
no
19KHz
pilot
signal
is
amplified
and
no
stereo
beacon
is
turned
on.)
When
the
DC
output
is
increased
to
the
negative
predetermined
level,
the
decreased
source
potential
turns
off
the
H559
which
in
turn
makes
the
4560
turn
on
and
the
H306
is
turned
off.
The
subsequent
changes
are
exactly
the
same
as
that
just
described
above.
Thus
when
the
tuning
is
shifted
or
deviated
to
the
certain
frequencies
in
which
undesirable
noisy
side-audio
signals
are
produced,
both
muting
and
19KHz
switching
transistors
are
operated
automatically
and
open
the
circuits.
-
oa
ae
With
the
station
correctly
tuned
in,
the
bias
current
of
the
FET
H558
is
adjusted
so
that
both
transistor
H560
and
H561
are
not
turned
on,
giving
no
effect
on
the
transistor
H306.

MPX Stereo Decoding Circuit
The buffered and non-equalized audio signals are applied to the first amplifier H301 which
serves as a tuned amplifier for the pilot signal in the composite signals and as a buffer amplifier for
the audio signals. The amplified 19KHz pilot signal is led to the second 19KHz amplifier H302
and further amplified if switching transistor H303 in turned on by the controlling BC signal as
described in the preceding chapter. The final 19KHz pilot signal is rectified by the doubler circuit
consisting of the H315 and H316 to obtain synchronized 38KHz amplifier driving signal.
The H304 is the 38KHz tuned amplifier and supplies its output to the switching matrix circuit
consisting of four diodes. While the composite signals are applied to the center tap of switching
transformer 1/2 L302. The right and left stereo signals decoded by the switching circuit are led to
the crosstalk cancelling amplifier which utilizes complementary configuration with NPN and PNP
transistors through de-emphasis network consisting of C315 and R335, and C316 and R336.
L305 is a low-pass filter networks having very sharp cut off characteristics and eliminates
undesirable residual switching signals. Transistors H313 and H314 are buffer amplifiers and their
outputs are led to the function switch.
Suggestion for Trouble Shooting of FM Tuner
Symptom: No FM Reception
First turn on the power switch and try to tune FM stations. Rotate the fly-wheel tuning knob
slowly and observe the FM signal strength meter and FM center tuning meter. |f the center tuning
meter deflect at several frequencies received, the tuner circuits preceding the discriminator circuit
may have no failure. |f the signal strength meter deflect but no deflection is obtained on the
center meter, there may be sore defects around the detecting circuit consisting H501, L501,
H503, H504, etc. When no reading is obtained in both meters, check FM local oscillator circuit,
using a RF VTVM. The normal tocal oscillator voltage is one or two volts (rms) at the tuning
capacitor, depending on the tuning capacitor positon. If the local oscillator voltage is normal,
next check all voltage distribution in the FM Front End and !F amplifier unit and compare them
with those shown in the circuit ciagram. When both meters deflect but no sound is obtained,
check audio circuits, using high sensitive oscilloscope.
Symptom: No Stereo Separation:
First check the MONO switch is in normal out position. Connect a FM RF signal generator
output modulated by. a stereo modulator to the rear FM antenna terminals, and check the stereo
beacon is turned on or.not. If not turned on, check for 19KHz pilot signal and 38KHz switching
signal, using an oscilloscope. :
4, Phono and Tone Amplifiers
Program source signals from the PHONO jacks on the rear panel are supplied to the input
circuit of the Phono Amplifier through the selector switch and the output of the Phono
Amplifier is applied to another section of the selector switch. This amplifier provides a gain of
40dB.
All signals selected by the funetion switch (S002-3F, 4F) are led to the balance and volume
controls through the MONO switch.
Signals .properly attenuated by the volume control are applied to the tone amplifier and
subjected to the tone control networks such as bass, mid, treble control and high and fow cut
filters.
Thus controlled audio signals are then led to the PRE OUT jacks on the rear panel.

5. Power Amplifier
The
signal
from
the
tone
amplifier
is
applied
to
the
differential
amplifier
(base
of
H751)
through
the
coupling
capacitor
C751.
The
differential
amplifier
provides
very
high
input
impedance
and
its
collector
output
(H752)
is
applied
to
the
base
of
H753
which
in
turn
applies
its
Output
to
the
next
stage;
H756
through
the
network
R766,
C762
and
R771,
and
to
the
H757
through
the
network
R776,
C763
and
R772.
The
outputs-of
H756
and
H757
are
applied
to
the
H758
and
H757
respectively.
HOO1
and
HOO2
are
power
transistors
used
in
complementary
symmetry
configulation
and
mounted
on
the
heat
sink.
To
maintain
overall
amplifier
stability
and
linearity,
degenerative
feed
back
is
utilized
throughout
the
amplifier.
This
feed
back
is
also
necessary
to
reduce
distortion
to
within
specified
limit.
The
RC
network
R775
and
C756
condition
the
feed
back
signal
for
the
audio
signals.
R759
and
C755
are
also
a
feed
back
loop
provided
to
obtain
a
stable
zero
DC
off
set
voltage
at
tne
speaker
output
terminals.
The
R762
is
a
trimming
resistor
to
adjust
the
DC
off
set
voltage.
Dynamic
bias
is
applied
to
the
base
of
driver
transistors
H758
and
H757.
This
dynamic
bias
circuit
is
comprised
of
H761,
H760
and
R763.
This
provides
a
variable
base
bias
for
driver
transistors
that
automatically
maintains
the
proper
base
voltage
with
temperature
change.
The
ternperature
sensitive
biasing
components
of
the
dynamic
circuit
are
thermally
couppled
through
a
heatsink
to
the
power
amplifier
transistors.
6. Power Protection Circuit
Protection
circuit
for
the
amplifier
is
provided
by
sensing
resistor
networks
and
two
switching
_
transistors.
When
the
output
transistors
are
over-driven,
the
current
increase
through
the
power
-
output
transistor
causes
an
increased
current
flow
through
R789
(or
R788)
and
the
potential
“across
the
R789
will
be
increased.
This
increased
voltage
potential
is
applied
to
the
base
of
H755
~
through
the
resistor
R783
and
turns
on
the
H755.
Since
the
collector
of
H755
is
directly
connected
to
the
base
of
H757,
this
means
that
the
base
of
H757
is
bypassed
to
the
ground
through
emitter-collector
path
of
H755.
Thus
the
input
signal
to
the
H757
is
restricted
to
the
value
which
maintains
the
operation
of
power
transistor
with
in
the
safety
area.
A
resistor
network
R777
and
R781
also
works
as
a
sensing
network.
When
the
center
voltage
(collector
voltage
of
power
transistors)
is
excessively
increased
to
a
positive
value
by
certain
troubles,
the
voltage
applied
to
the
base
of
H755
makes
the
H755
turn
on,
making
bypass
circuit,
and
protects
the
power
transistor.
For
the
other
half
cycle
of
driving
signal,
the
eperatig
principle
is
applied
provided.
7. Speaker Protector Relay Circuit
The
speaker
protection
circuit
consisting
of
H808,
H809,
H810,
etc
protects
the
speaker
systems
against
any
loud
“‘pop’’
sound
developed.
This
circuit
is
so
designed
that
no
sound
is
heard
for
the
first
three
or
five
seconds
after
the
power
switch
is
turned
on
by
the
time
constant
circuit
consisting
of
C807
and
R816.
This
circuit
also
protects
the
speaker
systems
against
some
troubles
due
to
DC
off
balance
between
the
speaker
system
terminals
by
instantly
operating
the
relay
and
cut
off
the
speaker
systems
from
the
circuit.
When
DC
off
balance
voltage
(positive)
is
developed
between
speaker
terminals
by
possible
defects
such
as
broken
power
transistor,
short-circuits,
or
broken
potentio
meter
R762,
as
the
base
of
H808
is
connected
to
the
speaker
terminal,
the
transistor
H808
is
turned
on
by
this
offset
voltage
developed
and
this
makes
the
transistor
H809
and
H810
turns
off,
thus
cutting
off
the
relay
and
disconnecting
the
speaker
from
the
output
circuit.
When
negative
offset
voltage
is
developed,
this
voltage
directly
turns
off
the
H809
and
H810,
thus
speaker
is
cut
off
from
the
circuit
and
protected.
Thecircuit
also
protects
the
speaker
systems
from
the
possible
damage
when
the
amplifier
is
aver-driven
by
very
bow
frequencies
such
as 7
or
Rad
cycles.

8.
Suggestions
for
Trouble
Shooting
of
Power
Amplifier
8.1
Excessive
Line
Consumption
3.
Check
for
shorted
rectifiers
HOO5;
also
check
C007
and
CO08.
b.
Check
for
shorted
transistors
H758
and
H759,
HOO1
and
HO02,
or
check
H760.
Check
for
open
control.
R763,
and
bias
diode
H761.
Check
L004
for
short.
CAUTION:
BECAUSE
THE
DRIVER
AND
OUTPUT
STAGES
ARE
DIRECT
COUPLED
COMPONENTS
MAY
FAIL
AS
A
DIRECT
RESULT
OF
AN
INITIAL
COM-
PONENT
FAILURE.
IF
A
SHORTED
TRANSISTOR
OR
ZENER
DIODE
IS
FOUND,
OR
CONTROL
OR
BIAS
DIODE,
BE
SURE
TO
CHECK
THE
REMAINING
DRIVER
AND
OUTPUT
COMPONENTS
FOR
SHORT
OR
OPEN
CIRCUIT
BEFORE
RE-ENERGIZING
THE
AMPLIFIER.
8.2
No
Line
Consumption
or
Zero
Bias
a.
Check
line
cord,
fuse,
transistors
H760, HOO1,
HOO2.
HOO3
and
HOO4,
bias
diode
H761.
b.
Check
for
open
rectifier
HOOS,
or
open
L004.
8.3
No
DC
Balance
3.
Check
R762
and
Zener
diodes
H762
and
H7/63.
9.
Voltage
Conversion
This
model
is
equipped
with
a
universal
power
transformer
to
permit
operation
at
100,
120,
200,
220
and.240V.
AC
50
to
60Hz.
To
convert
the
Model
2245
to
the
required
voltage
perform
the
following
steps:
(1)
Remove
the
top
cover.
(2)
Remove
the
Transformer
Wire
Connection
Terminal
Cover,
loosen
two
Cover
mounting
‘screws
on
the
rear
panel,
see
Fig.
1.
(3)
Change
the
jumper
wires
as
illustrated
in
Fig.
2
for
the
required
AC
voltage
and
replace
the
fuse as instructed.
CAUTION:
DISCONNECT
POWER
SUPPLY
CORD
FROM
AC
OUTLET
BEFORE
CONVERT-
ING VOLTAGE.
REMOVE SCREW
ene aan .
ea
je &|
bs
—*
Be
»REMOVE
THIS
COVER
_
Figure
1.
‘Remove
the
Terminal
Cover