Pioneer SX-750 User manual

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sx-75ct
This
Service
Manual
is
applicable
only
to theKC,KUmodel.
CONTENTS
1. SPECIFICATIONS
BLOCK
DIAGRAM
FRONT
PANEL
FACILITIES 5
7
9
11
2.
3.
4.
5.
CONNECTION
DIAGRAM
CIRCUIT
DISCRIPTIONS
5.7 Protection
Circuit
5.6 Tone
Control
5.9 Power
Supply
5.8 Power
Amplifier
Circuit
5.3 FM
Muting
Circuit
5.5 Microphone
Circuit
5.4 Phono
Equalizer
Amplifier
5.1 AMTuner
5.2 FM
Tuner
DISASSEMBLY
13
13
15
15
15
15
16
17
17
18
19
20
21
6. ADJUSTMENTS
6.1 AM Section
6.2 FM
Section
6.3 MPX
Adjusrment
6.4 Power
Amplifier
Section
7.
8.
9.
10.
DIAL
CORD
STRINGTNG 22
23
24
25
27
29
31
33
35
37
38
39
40
41
42
43
LEVEL
DIAGRAM
PARTS
LOCATION
10.1 FrontPanel
View
1O.2 FrontPanel
View (with
removdfrontpanel)
10.3 TopView
10.5 Rear
View
10.4 BottomView
1I. EXPLODED
VIEW
11.1 Part
I
11.2 Part
ll
1
1.3 Part
Ill
11.4 Part
lV
11.5 Part
V
11.6 Part
Vl
11.7 Part
VlI

44
11.8 Packing.
12. SCHEMATIC
DIAGRAMS
P.C.
BOARD
PATTERNS
AND
PARTS
LIST
12.1 SchematicDiagrams
12.2 MiscellaneousParts
12.3 Internal
Circuitryof
Integrated
Circuits. .......... 48
12.4 Filter&
MutingAssembly
(AWM-094) .......... 50
12.5 ToneControl
Assembly
(AWG-046) '...- 52
12.6 PowerAmplifier
Assembly
(AWH-046) 54
12.7 PowerSupply
& ProtectionAssembly
(AWR-099) . . . 58
P.A PowerSupply
Assembly
(AWR-1001 62
12.9 HeadphoneJack
Assembly
(AWX-106) ......... 64
12.10Tuner,
AF
& Control
Assembly
(AWE-073) ....... 65
12.11 Amplifier
Assembly
(AWK-065) 73
12.12
De+mphasisSwitch
assembly
(AWXO95} .-..-'... 74
45
47

-
1.
SPECIFICATIONS
Semiconductors
FET
lCs
Transistors.....
Diodes........
AmplifierSection
Continuous
Power
Output
from20 Hertzto 20,000Hertz
(Both
channelsdriven). . 50watts
per
channel
(8
ohms)
60watts
per
channel
(4
ohms)
Total
HarmonicDistortion
(20
Hertzto 20,000Hertz,fromAUX)
ContinuousRatedPowerOutput . ..
Nomorethan
O.1o/o
25watts
per
channel
power
output,8
ohms .. Nomorethan0.05%
1wattper channel
Power
output,8ohms .. .
Nomore
than
0.05%
lntermodulation
Distortion
(50
Hertz:7,000
Hertz=4:1,from
AUX)
ContinuousRatedPowerOutput . . Nomore than
0.1%
25
watts
per
channel
power
output,8ohms .. .
Nomorethan0.05%
1watt
per
channelpower
output,8ohms........
DampingFactor
(20
Hertzto 20,000Hertz). .
Input
(Sensitivity/
lm
pedance
)
PHONO
. .. 2.5mV/50kohms
l\41C.... ... 5mV/50kohms
AUX... .. 150mV/50kohms
TAPEPLAY1...... 150mV/50kohms
TAPEPLAY2...... 150mV/50kohms
TAPEPLAY2
(DlN
connector)... l50mV/50kohms
PHoNooverloadt"""' tt n o .o. t."':l
.. 200mV
(1kHz)
Output
(Level/l
mpedance)
TAPERECI
.............
TAPEREC2...... .........150mV
TAPEREC2
(DlN
connector)
..... 30mV/80kohms
SPEAKER ....... A,
B,A+B
HEADPHONES. . . ..... Lowlmpedance
FrequencyResponse
PHONO
(RIAA
equalization)
.. .... 30Hertzto 15,000Hertz !O.2dB
AUX,TAPEPLAY l0 Hertz
to 50,000Herrz .-?
dB
I
7
42
.....40
ToneControl
BASS
.....
TREBLE... ..
+8dB, 7dB (100
Hz)
..
+9d8, 7dB
(10
kHz)
Filter
H|GH... ... 6kHz(6dB/oct.)
LoudnessContour
(Volume
controlset
at 40dB
position)
.... +6dB(100
Hz),
+3dB(10
kHz)
Humand
Noise
(lH
F,short-circuited,A Network,rated
power)
PHONO. ...70dB
AUX,TAPEPLAY... ..........9OdB
FM Section
usable
sensiriviry l\4oNo . . .. . . .. .10.7d8f(1.99v)
STEREO....
... .19.0dBf
(9.8sV)
50dB
Quieting
Sen5itivity
i\4ONo......... 17.2dBf
(4.0pV)
STEREO....
.... 39.2dBf
(50gV)
Signalto NoiseRatioat
65dBf
MONO.. .....72d8
STEREO. ....67d8
Distortionat
65dBfl00Hz...MONO........ O.15y.
STEREO.......
O.3yo
lkHz....MONO........ O.15Yo
STEREO.......
O.3v.
6kHz....lvlONO....... . OA%
STEREO_......0A%
Frequency
Response
.. -... . 30Hzto 15,OO0Hz
+-8:E
dB
Capture
Ratio .........1.0d8
AlternateChannelselectivity ........ 80dB
SpuriousResponseRatio.
.. .......90d8
lmageResponseRatio... .........80dB
lFResponseRatio... ...100d8
AlVlSuppressionRatio... ......... 55d8
l\4utingThreshold ladBf (2.8t/V)
Stereo
Separation . . 40dB
(l kHz),30dB (30H2
- l5kHz)
SubcarrierProductRatio .......... 62dB
SCARejectionRatio... .......... 62dB
Antennalnput .300ohmsbalanced
75ohmsunbalanced
150mV
5

six-75c'
AMSection
Sensilivity
(lH
F,Ferriteantenna)..
(lH
F,
Ext.
antenna)
Selectivity
Signalto
NoiseRatio . .. .. .
lmage
Reiection
lFRejection
Antenna.......... Built-in ......65dB
FerriteLoopstickAntenna
. .13.7kg(30lb2oz)
'l
s.3kg
(33
lb1
'1
oz)
.300s
V/m
....15ttV
. . . .
35dB
.. . .50dB
. . . .40dB
Miscellaneous
PowerRequirements... ...120V,60H2
PowerConsumption 160W
(UL),
350W
(max.)
Dimensions .. 480(W)x149(H)
x371(D)mm
18-718
(w)
x
s-7l8(H)
x 14-sl8(D)in.
Weight
withoutpackage........
with package
FurnishedParts
FN4T-typeAntenna.
OperatingInstructions
NOTE:
Specifications
and designsubjectto possible
modificotion
without notice,dueto improoements.

2.FRONT
PANEL
FACILITIES
POWER
INDICATOR
LAM
Lightswhen
SPEAKERSswitch
ismovedto any
position
from POWEROFF, and AC power is suppliedto the
receiver.
SPEAK.ERSSWITCH
Functionsboth as powerswitchand speakerselection
switch.
POWER
OFF:ACpower isoff.
A:
OFF:
B:
AiB:
PHONES
OUTPUT
Insertheadphone
plug into this jack when headphone
listening
isdesired.
Inthiscase,
SPEAKERSswitchshould
bein
OFF
position.
BASS,
TREBLE
CONTROLS
For toneadjustment
when
TONE switch
is in ON posi-
tion.
Whenknobsareturned
clockwisefrom "0" position,
responsein bass
or treblerange,respectively,isboosted.
Turningcounterclockwise
attenuates
response.
TONESWITCH
ControlsON-OFFooeration
of tonecontrolcircuit.
lnthe
ON position, tone control by meansof BASS and
TREBLEknobs
maybeaccomplished.
In the
OFF posi-
tion, the tone controlcircuit is bypassedandfrequency
responseinhigh
andlowrangesis
flat.
HIGH
FILTER
SWITCH
For highfrequencynoisedueto scratcheson the record,
place
switchin 6kHz position.
This will provide 6dB/
octaveattenuation
atfrequencies
above
6kHz.
Fornormal
operationswitch
shouldbeinOFF (upper)
position.
Sound will be heardfrom speakerscon-
nectedto A speakerterminals.
No soundwill be heardfrom speaker
sys-
tems.
This position shouldbe usedwhen
listening
withheadphones.
Sound will be heardfrom soeakerscon-
nectedto Bspeakerterminals.
Sound will be heardfrom speakerscon-
nectedto A terminals
andfrom speakers
connected
to Bterminals.
STEREOINDICATORLAMP
Lights
whenFM
stereobroadcast
is
being
received.
(\D
r'IONCGT. BT
EPEAKEFg .* ;'l?."
BALANCE
CONTROL
For adjustment
of relative
output levels
of L and R
channels
of speakersystems
or headphones.
Clockwise
rotation
from center
position
increases
volume
of R over
L channel.
Counterclockwise
rotation
increasesvolume
of
L channel
over
R.
VOLUME
CONTROL,
For adjustment
of speakeror headphone
output level.
Levelincreases
withclockwise
rotationof knob-
LOUDNESS
SWITCH
SIGNAL
METER
For FM and AM reception,
needle
of the SIGNALmeter
theright.
turn
TUNINGknobuntil
isdeflecteda maximumto
level,
placing
this
switchin
response
in low and high
FALANCI
FMTUI
With thr
right, fi
TUNIN(
MODE
For sel
playbac
STERE(
channel
speaker:
NOTE:
Recordi
MONO,
rate.
DUPLIC
Withsu
two tap
onto th
operatic
For listening
at low volume
the ON position
will boost
frequency
ranges.
The response
of the human
earto low and highsound
volumes
is different. At low volumelevels,
the ear is
relatively
insensitive
to sounds
at either
extreme
of the
frequency
scale.
By means
of the LOUDNESS
switch,
these
sounds
are
given
additional
amplification.
luutJ
FM lillmilillril'
AM 55
,$?,94,96,0g,100,102
illilrltl |ltlillt]rIrlilttlilttliltl | iltrn,trtt
ilillitiltiltl

ii.
FM
TUNINGMETER
Withthe needle
of the SIGNALmeter
deflectedto the
right, fine-tuneFM broadcastby centeringneedleof
TUNINGmeter.
PROGRAM
SOURCEINDICATORS
Indicate
program
sourceselected
by meansof FUNCTION
switch.
TUNING
KNOB
For
selectionof FM
orAMstations.
FM MUTINGBUTTON
Foi selectionof FM broadcasts,buttonshouldbein ON
(released)position.
When button is in ON position, unpleasantinterstation
noiseis suppressed.When
signal
strengthispoor, it may
notbe
possible
to bringinthedesiredstationif MUTING
isON. In thiscase,
press
buttonto place it intheOFF
position.
FUNCTION
SELECTOR
Forselectionof program
source.
AM:
FM: AM broadcasts
FMbroadcasts
PHONO: Playing
records
AUX/MIC: For useof component
connectedat AUX
terminalsof receiver,
or microphoneswhich
maybe
plugged
intoMlc jack.
NOTE:
AUX and MIC programsourcescannot be usedsimul-
taneously. When using AUX hi-fi component,micro-
phone
should
bedbconnected.
rc
JAcK
Acceptsstandard6mm{ plug.Microphoneinput signal
entersbothRandL channels.
TAPEMONITOR
SWITCH
1: For program sourcesother than tapedeck
(playback).
(REC
or
PLAY).
SOURCE: For programsources
otherthantape deck
(playback).
2: For monitoring
of playback
or record mode
of tape
deckconnectedto TAPE2 terminals
(REC
orPLAY).
NOTE:
When
not monitoringrecord
orplayback mode
of a tape
dech,
switchshouldbein SOURCE
position.
Inposition
1 or 2, the
programsourceindicatedby theFUNCTION
selectorwill not be heardthroughspeahersysfemsor
headphones.
i',,1
,,,t
* ;s-
andR
ockwise
'Rover
lume
of
rt level.
vitchin
nd
high
l sound
3ear is
r
of the
switch,
to*s ueoiwer: !
MODE
SWITCH
For selection
of stereophonic
or monophonic
modeof
playback.
In normaloperation,
switch shouldbe in
STEREO
position.
In the MONO position,
R and L
channel
signalswill be mixed,and
soundscomingfrom
speakersof bothchannels
will bethesame.
NOTE:
Recording
stereophonicallywith theMODE switchin the
MONOposition may causechannelseparation
to deterio-
rate.
DUPLICATE
SWITCH
With switchin ON position, signalsrecordedon oneof
two tapedecksconnectedto thereceivermayberecorded
onto the other, with or without editing. For normal
operation,thisswitchmustbein
OFF
(upper)position.
a
lilllllllllllrllll

3.CONNECTIONDIAGRAM
REAR
VIEW
Outdoor FM antenna
Centerwire
-tl
mesh
75 ohm coaxial cable
{indoor
oroutdoorl
k
A*ry
ANTENNA
PLACEMENT
Makenote
ot rhelollowing
po'nts when choosing e location
for the FM antenna.
300 ohm leeder cable
Direction ot signals
1.5meter (5 ft.) above
More than 4 meters
(15ft.) above
ground
Please
seetollowing section,
"Antenna and Ground
Connections,"
for instructions
regarding
Fl/1
antenna
FM do-rmph6i5 3witch: For normal FM recebtion rhis switch
shorrld
be seton "75!S" (unir is set to rhis when leavingfactory).
The "25pS" setting is used only when listening to FM Dolby*
transmissaons.
When listening
ro FM Dotby transmissions,
it wi
be
necessarylo use an NR adaptor wiich can be purchased separately.
More detailedinstructions
witt be found in the section,
,,FM Dotbv
Reception."
Antennashouldnot
tte placed where it
will be subiect to elec-
tracalinterlerence
from motor vehicles.
Fig.I
s
r L_t
6E E
B E^A tr
Feeder wire and cSle a Length should b€ a
should be insulated at short aspossible.
mounting points, a Feeder
wire should not
b€ coiled.

TOP
VIEW
six-750
Accessory AC outlets:
Switched ...... Power to this outlet is controtted by the SX-750
pow€r switch.Wh€n power to the receiver
is "On," this oudet will
provide
AC currenr{10Owattsmaxamuml.
Unslvitchod.....
Power
to thisoutlet isnot controtted
by theSX,75O
power swilch. As long as the power cord is plugged
into a tive
outlet, thisoutlet will supply AC current{maximum 300 warts).
AC outl€ts
L channel R chr
A SPEAKER SYSTEMS
L channel
m
R channel
m
0000
0
0
000000
00[|
00000000rt000000000000000000000000000000||0000000
BSPEAKERSYSTEMS
Carrridg€ tape deck Tape deck
Fig.2

4. BLOCKDIAGRAM
ANTENNA
FI1
TSQ
FM
300si
GND
Af1
IF
amp
CF . HA12O1CF HAl13
?T410614P
Ratio
detecfon
SIGNAL
meter^
vlu
CONTROL
SECTION
TAl136P1
TAl 136Pi
A11
TUNER
*,;t-l
AMP
TA1136Pl
!
IlICAH
P
c! c!
35
Tone
control
ON
I
I
EQ
t-- rF
,#
{rr
=
U)
(./)
(-/)
p
-.
a
ON
OFF
I
I
I
I
I
I
I
I
I
=
U)
IY
P
=
=
t-rl
o_
g
1
-lI
I
I
I
I
J
7
I
L
PHONO
R
L
AUX n
K
tl
lc
0N6.l
OFF
t,
16ng
gQnTfol
ON
DCPOWER
SUPPLY
.l
,aF
:;-<
(Yh
AC POWER
C\J C!
,:k
LLJ _l
q,
Dl_'_i
FI1 TUNER
rPFRI F
#
bA))
TAl
1
36
P1
I
I
I
tf-
tl
il,
i!t
tl,
I
I
=
c/)
Lri
-)
o_
f
-IRFRIF
:tV€
J}}L
_--5. Local
oscillafor^
@g l--
f,tr i
_;
Hi
oa o-
+
3O
2"1
1
Sro
AC
outlet AC
ouilet
AC].5V
Dia
I
illunrination

T4T0614p
Ratio
detecton NPXdecoder
HA'I136
0FFo
fM l4uLiny
T
7V
4o .
3o )1d
SIGNAL
meten TUNING
meter FM
MUI
NGi_
STERE0
indicaton
,-l
+13V i
ONl I k srs
5-I
ej'
TION POW
ER AIl
P
+
41V
-+1v
+
13V
*35V
- 38V
PROTECTION
CIRCUIT
AC'I.5V
Tone
contr-ol
ON
Tone
controi
ON
-ll--
!i
SUPPLY
Hrqr
I
FlLTERy'..-,
swri
i
tl
ftsl2yidnive
Sze
q--
VZU
Szu
Over-ourr^ent
!
over-I
oad I
detec-fion
_-J
DCPOWER Pro
X
ram
indicator^ eLo
2oo
5e
1l
I n I L__R
PHONES SPEAKERS
n
K
Sr
' FUNCTI0N
switch Sz
, SPEAKERS
switch
1.AI.1 1. POWER
OFF
2.
FM 2,
A
3 PHONO 3. OFF
4. AUX/Y|C 4. B
5. A+B
AM
rM
l-ll
PHONO
AUX
'b6
ob A
)-
5
Powen
indicaion
ReXu
lator'
Sre

5. CIRCUIT
DESCRIPTIONS
5.1 AM TUNER
Composedof single
IC (HA1138) combining a 1
stageRF amplifier and a 2 stageIF amplifier (Fig.
1).
5.2 FM TUNER
Front End
The outstanding performance, exemplified by the
remarkable 80dB imaging and 90dB spurious
rejection, originates in the dual-gate MOS FET
RF amplifier and 4-gurg variable capacitor tuning
circuit. A modified Clapp circuit is employed in
the local oscillator, leading to high frequency
stability. Since the output is taken from the
oscillator tuning circuit, higher harmonics in the
oscillator signal and spurious response become
reduced.
sx-750
lF Amplifier and Detector
Comprise 2 dual element ceramic filters,
sistor, and 1 IC (integrated circuit).
(HA1137) circuit isillustrated in Fig. 2.
FM lF Amplifier and DetectorCircuit
Threedual elementceramicfilters, an IC (HA1201)
containing a differential amplifier, and an IC
(HA1137) containing a 3-stagelimiter amplifier
compose the FM IF amplifier. Fig. 2 shows the
HA1137 block diagram (see circuit diagram on
page
65).
In addition to limiter amplifier. the HA1137 IC
includes detector, meter drive circuits. The detec-
tor circuit in the HA1137 is not employed in this
set however a separate ratio detector circuit is
employed instead,resultingin improved S.Nratio.
1 tran-
The IC
Fig.1 Block Diagram
of HA1
138
Fig.2 Block
Diagram
of HA1137
13

Multiplex Decoder
Demodulation is performed by switching detection.
A singleIC (HA1196) composes
the circuit, which
isdivided into 3 sections.
Fig. 3 shows
the HA1196
block diagtam (see
circuit diagram on page65).
1. Switching signalgenerator
A PPL (phase
locked loop) system
isemployed.
76kHz isgenerated
by a VCO (voltagecontrolled
oscillator: oscillator in which the frequency is
varied by a control voltage) and converted into
19kHz by a frequency divider. This signal and
the pilot (19kHz) of the received signal are
applied to a phasecomparator, which converts
the phasedifferences of the two signalsinto a
voltage.The voltage
isthen fed back to the VCO.
The oscillator signal phase becomeslocked to
the pilot signalby this loop (PLL) and a 38kHz
signal
synchronised
to the pilot signal
is obtained
andemployed as
the switchingsignal.
2. Automatic stereodetector
With the PLL locked to the pilot signal,
the pilot
signal and a 19kHz signal of the same phaseare
produced. A voltage is then obtained at the
phase comparator that is proportional to the
pilot signalamplitude. As it increases,
the lamp
lights and the switch becomeson. The switching
signal
isappliedto the demodulator.
3. Demodulator
This is a switching circuit employing two dif-
ferential amplifiers (Fig. a). Q1 and Q2 are
altemately switched on and off by the switch-
Com0osit
siFel
ing signal. The composite signal is amplified at
Q3, switched and demodulated. Q6 and QB are
Ioosely coupled at their emitters by R1 - R3.
QG is driven in reversephase to Q3. This is
switched at Q4 and Q5, and by composing
with
Q1 and Q2 at the collector, crosstalk becomes
cancelled.
Adequate current flow is required to QB and
Q6 to improve distortion figures at this point.
However, if the basebias voltage is raised,the
voltage component at the collector becomes
reduced and clipping occurs (power supply
voltage is limited by IC voltage endurance).
For this reason, current from an external source
is inserted at Q3 and Q6 collectors to become
11 and Ir. The same current amounts are ob-
tained as 13 and Ia from the emitters. Q3 and
Q6 therefore operate with adequate current,
and distortion at this stage becomes remark-
ablly improved. A feedback amplifi.er amplifies
the demodulatedoutput.
Ycc
Switching
signal(38tl|r)-*
Com0osite
signal
Fig.
4 Equivalent
Circuitof Demodulator
Af
am!.
Phasc
conlaratol
14
Fig.
3 BlockDiagram
of HA1196

5.3 FM
MUTING
CIRCUIT
At time of detuning (more than t 70kHz) and with
an antennainput of less
than 10dB (OdB
= 1pV), a
DC voltage
is produced
at pin 12 of IC, (HAl137).
Thisvoltage
is employed as
the muting trigger.
With the MUTING switch ON, Q1 becomesON
and Q2 OFF in the Q1-{2 Schmitt circuit asthe
muting trigger is produced. Q3 becomesON when
Q2 is OFF, and Q4, Q5 and Q6 alsobecome ON.
With q4 & Qb ON, the FM output becomes
grounded, while IC, output is grounded by Q6 to
apply muting.
Fig.
5 FMMutingCircuit
5.4 PHONO
EOUALIZER
AMPLIFIER
IC (TA7136P1) is used
with independentleft and
right channels. Grade G styrole capacitors and
grade F metal film resistors comprise the equal-
izer elements, leading to an RIAA deviation
within 0.2dBfrom 30Hzto 15kHz.
sx-75Cl
5.5 MICROPHONE
CIRCUIT
A 2 stage transistor amplifier (monophonic) is
provided in addition to the phono equalizer
ampli-
fier. A selector switch cuts the AUX jack input
when a plug is inserted into the MIC jack. The
amplified microphone signal is then supplied to
both the left and right channels.The FUNCTION
switch is set to the AUX position when using a
microphone.
Function
SW . B
Lch
AUX
Fis.
7
5.6 TONECONTROL
CR type tone controls are connected to an ex-
tremely low output impedance flat applifier
(IC = T47136, 31dB gain).Bass
can bevaried
in
the rangeof +343 - -7dB (100H2)and treblein
the range of +943 - - 7dB (10kHz). A TONE
switch also allows the tone controls to be switch
ON/OFF.
R1 and R2 arc designedto provide the sameloss
when the TONE switch is OFF as obtained with
the TONE switch ON and the BASSand TREBLE
controls at center positions (flat). Frequency
response thus becomes flat when the TONE
switchisset
to OFF.
R3, and Cl form a 6kHz 6dB/octave HIGH CUT
filter which is connected following the TONE
switch.
Fig.
6
Fis.8
4E
FurLction
5V
TA''t136P-r
TA'r
|
36

5.7 PROTECTION
CIRCUIT
This protection circuit functions to protect the
speakers and the power amplifiers from damage
due to short-circuit of the load, etc.,and performs
a muting operation to cut noise and distortion
which occur when switchingthe power on and off.
The circuit is shown in Fig. 15, and consists
of a
bridge type over-current and overload detector, a
differential amplifier DC voltage detector, and a
power switch ON/OFF detector section.
Relay
Driving
Circuit
Q4-Q6, in Fig. 9, comprise the relay driving
circuit.
In the normal condition reversebias is applied to
the base of Q4, and Q4 is in a off state. When
one of the above mentioned detection circuits
goes or, current flows through R11, the base
potential falls and Q4 is turned on. Consequently
Qb comeson and Q6 goes
off. When Q6 goes off,
the current of the relay circuit is cut, to release
the switch of the output circuit.
When the power switch is turned on, a delay
operation occursin this circuit. R17 and C3,in the
base
circuit of Q6, arethe time constant elements
which determine the delay time. When the power
switch is tumed on, C3 charges
to a potential of
+60 volts through R17 and R18, and Q6 iskept in
the off state during this time. When the power
sourceis switched off, the muting operation of Qb
preventsshock noise. In the normal condition, the
potentials of +33 volts and - 5.1 volts are applied
to Qb through RL4 and R15. The resultant poten-
tial at the base of Q5 is -1 volt in the cutout
condition. When the power supply is turned off,
PoY|er
amplifier 0Yer-current
and
overload
detectioncircuit
of - 5.1 volts disappears
immediately due to the
small time constant of the power circuit. Thus a
positive basepotential remains, switching Qb on,
which in turn switchesoff Q6 andhence
the relay.
Detection of DC Voltage
This is a differential amplifier consisting of Q2
and Q3, as shown in Fig. 10. The basesof Q2
and Q3 are connected to the center points of the
right and the left power amplifiers. When the
DC balance of the power stage is lost for some
r€ason, a potential difference is produced in the
input signal to the differential amplifier, and the
collector currents of Q2 and Q3 are put out of
balance.Ttrus, the relay driving circuit functions,
and the relay switch is tumed off.
tr
-------l
+B
Ich
Ru R-n
-fiRelaydriring
ci
rcuit
Rch n 0,
Iv- It' 0"
\"
R
ro
B
L----_ --l
Fig.
10 DC
Voltage
DetectionCircuit
RelaY
-arl
ftl
Power
amplilier
IlC
vultagedetectioll
circuit
*Y"l
I
I
Retaydrivingcircuit
o,
I R6
c")
Fig.
9 Protection
Circuit

f,
Over-currentand OverloadDetection
The equivalent circuit of this detector section is
shown in Fig. 11, and Fig. L2-a showsthe equiva-
lent circuit at the time of a positive half cycle.
When this equivalent circuit is overloaded, the
balance of the bridge, formed by RE1, R1, Rg
and RL, is disturbed, and a potential is produced
between b and a in such a direction that Q1 is
turned on. When Ql is turned on, the collector
current increases,
the relay driving circuit functions
and the relay switch of the output circuit is
turned off.
After the cause of the overload is removed, the
biasof Ql isreducedand the relay switch turns on
to automatically restore
normal operation,Fig.18-b
shows the equivalent circuit at the time of a
negative half cycle. In this circuit, a potential is
produced between b and e as above, and Q1 is
tumed on.
Powsr
amplilier
+ Vcc
Fig.
11 Over-currentandOverloadDetection
Circuit
FiS.
12 EquivalentCircuit
of
sx-75cl
5.8 POWER
AMPLIFIERCIRCUIT
Composed of differential first stage, all stages
direct coupled pure complementary OCL circuit.
Open gain at lkHz is approximately 80dB and
NFB amount is approximately 50dB. R3 and R4
are provided with this circuit in order to obtain
adequate stability evenwith the NFB disconnected.
Ql form a differential amplifier LOOVod.c. feed-
back is applied from the junction point of the
power stageto the baseof Q1 so the potential of
the junction point is always maintained at the
samelevel.
Fis.
13
5.9 POWERSUPPLY
Two windings are provided in the power trans-
former secondary and each is separately bridge
rectified. One of theseis sent as148 VDC to the
voltage stabilizer circuit to become 13V, 33V
and -37V regulated voltages for supply to each
assembly.
The other secondaryvoltage is bridgerectified and
becomes t 4IV or supply to the power amplifier
predriver stage. Extremely low power supply
impedance is maintained by a 15,000pF electro-
lytic capacitor.
17

6. ADJUSTMENTS
6.1. AM SECTTON
1. Setfunction switch to AM.
2. Connect AM signal generatorthrough lk-ohm
resistor to AM antenna terminal.
3. Set DUPLICATE switch to OFF and connect
anAC voltmeter to TAPE 1 RECjacks.
4. Set AM SG for 4O0Hz30%modulation 74dB
output.
5. Set SX-750 dial indication and AM SG fre-
quency for 600kHz.
6. Adjust T8 core for maximum reading on AC
voltmeter.
7. Set SX-750 dial indication and AM SG fre-
quency for 1,400kH2.
8. Adjust TCz for maximum reading on AC
voltmeter.
9. SetAM SGfor 30dB output.
10. Set SX-750 dial indication and AM SG fre-
quency for 600kHz.
11. Adjust T8 and bar antenna core for maximum
readingon AC voltmeter.
12. Set SX-750 dial indication and AM SG fre-
quency
for 1,400kH2.
13. Adjust TCz, TC4 for maximum readingon AC
voltmeter.
14. Repeat steps 10-13 to eliminate variations in
AC voltmeter readings.
AM signal
generator
Fig.
14
1El

I6.2 FMSECTION
FM Trac(ing
1. Connect measuring equipment as shown in
Fig.15.
2. Set FM SG to lO|Vo modulation (x7lkHz
deviation) at 4OOHz
and 100dB output.
3. On SX-750 front panel,set FM switch to ON,
FM muting to OFF and VOLUME control to
minimum position.
4. Set
TC6 to center of turning range.
5. T\rne FM SG and SX-Z5O to dial readings
of
9OMHz.
6. Adjust T4 core for maximum indication on
Signalmeter.
7. Adjust T6 core for center of scale
indication on
Tuning meter.
8. SetFM SGoutput to 8- 10dB and adjust cores
of T1, T2, and T3 for maximum indication on
Signalmeter.
9. Ttrne FM SG and SX-250 to dial readings
of
106MHz.
10. Set FM SG output to 100dB and adjust TC6
for muimum indication on Signalmeter.
11. Set FM SG
output to 8-10dB andadjust
TC1,
TC3. TC5 and TC6 for maximum indication on
Signal
meter.
12. Repeat above adjustment steps 5- 11 and
adjustfor optimum conditions.
13. Tune FM SG and SX-750 to dial readingsof
90MHz.
14. Adjust T5 core for maximum indication on
Signalmeter.
L5.Detune SX-750 (to noise
only).
16. Adjust T6 for center of scale indication on
Tuning meter.
17. Tune FM SG and SX-750 to dial readings
of
98MHz.
18. Set FM SG output to 60dB and adjust upper
core of T7 for maximum readingon AC volt-
meter.
19. Adjust lower core of T7 for minimum audio
frequency output distortion.
20. Set FM SG for 100dB output and adjust VR1
sothat Signalmeter indicates 5 of the scale.
ACvoltmeter Distortion
meter
FM.
SG
FMsignal
generator
Fig.15

6.3 MPXAdjustment
1. Oonnect measuring equipment as shown in
Fig.16.
2. T\rne FM SG and SX-750 to dial readingsof
98MHz.
3. SetFM SGfor 60dB unmodulated output.
4. Connect the output signal(19kHz) of MPX SG
PILOT OUT terminal to the horizontal input of
anoscilloscope, andpin 20 of the tuner assembly
(AWE-073) via a probe to the oscilloscope
vertical input.
5.Adjust VR2 so that lissajous
pattern displayed
on oscilloscopebecomesstationary
(Fig.
16).
6. Set MPX SG to 67.5kHz deviationat lkHz for
left and right channels, and to 7
.1kHz deviation
for 19kHz pilot signal.
7. Adjust T5 core for minimurn audio frequency
distortion. Take care to turn core only within
t180'.
8. Adju.st
VR3 for minimum signalleakage from R
channel to L channel, and from L channel to
R channel.
OscilloscoPe
Lissajous
AC voltmeter Distortionmeter
"[Em ,/ |
1V
Fis.
16

6.4 POWER
AMPLIFIERSECTION
ldleCurrentAdjustment
1. ConnectDC voltmeter asshown in Fig. 17.
2. Do not connect load to speakerterminals. Set
VOLUME control to minimum (fully counter-
clockwise).
3. Turn VRl and VR2 (shown in Fig. 17) fully
counter-clockwise, then set POWER switch to
oN.
4.1- 2 minutes after tuming on the power, adjust
VR3 (L channel) and VR4 (R channel) for
50mV indication on DC voltmeter.
5. 20 minutes after turning on the power, again
adjust VR1 and VR2 for 30mV indication on
DC voltmeter.
Eix-7E'cl
Fig.17
21