Pioneer SX-1280 User manual

XRABi3

Typo Volr.rp R.mark!
KU 120Vonly U.S.A.rnodel
KC 120Vonly Canads model
110v,120v.22OVand240V(Switchsble) Gen6ralexDortmodel
s/G 1'10V,120V,220V.nd 240V
{$i,itchsbl6l U.S.Milit8rymodel
six-1 eEto
MODEL
SX-1280
COMESINFOURVERSIONS
DTSTINGUISHED
AS FOLLOWS:
NOTICE:
r This
service
manual
is
applicableto the KUand
KC
types
. Asto theSandS/G
types,
please
refer
to page 81.
CONTENTS
1. SPECIFTCAT|ONS. 3
2, CONNECTIONDIAGRAM 5
3. FRONT PANEL
FACILITIES 7
4. LEVELDTAGRAM
.... ....... . 10
5. BLOCKDIAGRAM
.... ....... 11
6. CIRCUIT
DESCR]PTIONS
6.1 Tuner
Section ..........13
6.2 AF
Section ......14
6.3 Power
Supply
Circuit
. .....16
6.4 Proteqtion
Circuit.. .......11
7, DTSASSEMBLY ........ 18
8, PARTS
LOCATION
8.1 FrontPanel
View... ......20
8.2 Front
ViewwithPanel
Removed ...........21
8.3 Top
View .......22
8.4 Bottom
View ....29
8.5 RearPanel
View
.. . .. .... 24
9. DIAL
CORD
STRINGING ....
.. 25

IO. ADJUSTMENTS
10.1 FM
Tuner
1O.2 AMTuner
10.3 Power Amplifier
10.4 MeterAmplifier
11. EXPLODEDVIEWS
11.1 ExternalPart . , .
11,2 lnternalPart
12. PACKING
13. SCHEMATICDIAGRAMS,P.C.
BOARDPATTERNS
AND PARTS
LIST
13.1 SchematicDiagramand
MiscellaneousParts
. .
13.2 TunerAssembly
(AWE-092)
13.3 Microphone
JackAssembly
(GWM-I05)
13.4 Cartridge
LoadAssembly
(cWS-134)
13.5 Function
SwitchAssembly
(GWS-I33)
13.6 EOAmplifier
Assembly
(AWF-031)
. .. ..
13.7 Flat
AmplifierAssembly
(AWG-0601
13.8 Tone
AmplifierAssembly
(AWG-059)
. . ..
13.9 Filter
Amplif
ierAssembly
(AWM-119)
13.10 PowerAmplifier
Assembly
(AWH.074)
13.11 ResistorAssembly
(AWX-I27)
13.12 Meter
AmplifierAssembly
(AWM-I211
13,13 Power
SupplyAssembly
{AWR-I57) . . . .
13.14 'Fuse
Assembly
(AWX-I261
or (AWX-I391
13.15 Protection
CircuitAssembly
(AWM-I23)
ADDITIONAL
SERVICE
MANUAL
27
29
30
30
31
33
40
41
45
52
53
54
56
58
61
64
67
70
72
74
77
78
1. SPEC|F|CAT|ONS
... . ... ..... 82
2. CONTRAST
OF MISCELLANEOUS PARTS ,.... 82
3. EXPLODEDVIEW . ... . 84
4. SCHEMATIC
DIAGRAMS,
P.C.
BOARDPATTERNS
AND PARTS
LIST
SchernaticDiagram .. ...... gs
5PConnector
Assembly
(AWX-062)
... .......... g7
Fuse
Assembly
(AWX-134) .......88
NO?E: * .F'nseossembly
(AWX-I26) isapplicable
to theKII type,andAWX-tB9 isfor theKC type.
4.1
4.2
4.3
e

1.SPECIFICATIONS
Semiconductors
FETS... 5
lCs..... 6
Transistors ......61
Diodes.. ......50
Power
Amplifier Section
Continuous
power
output
of 185watts.
per
channel,
min.,at8 ohms
from20
Hertzto
20,000
Hertz
withn0morethan0.03%total
harmonicdistortion.
Total
HarmonicDistortion
(20Hertzto 20,000Hertz)
ContinuousRated
PowerOutput . .Nomorethan 0.03%
90watts
per
channel
power
output,Sohms . . Nomorethan 0.01%
I watt
per
channel
Power
output,
Sohms . . No morethan0.01%
IntermodulationDistortion
(50
Hertz:
7,000Hertz
=
4:1)
ContinuousRatedPowerOutput . .
Nomorethan
0.01%
90watts
per
channel
power
output,
8 ohms No morethan 0.006%
1wattper channel
power
output,8
ohms Nomore than0.005%
Frequency
Response
. . ... 5 Hertzto IO,OOOHertzl!aA
lnputSensitivity/lm
pedance
POWERAMPIN
..... .. lV/5Okilohms
Output
SPEAKERS . . . . . A,B,
C,A+8,B+C,C+A
Damping
Factor
(20
Hertzto
20,000
Hertz,8
ohms)
........... 30
Hum
and
Noise
(lHF,
short-circuited,
A Network). . 105d8
PreamplifierSection
Input
(sens
itivity/lm
pedance)
PHONO
I .
Cartridgeload
(capacitance)
. . .
PHONO2
t\4tc....
...2.5mV/50kilohms
'100,
200,
300,400pF
. . .2.5mV/50kilohms
. . .7.5mV/50kilohms
AUX... .. 'l50mV/50 kilohms
TAPE
PLAY'1
...... 150mV/50kilohms
TAPEPLAY2...... 150mVi50kilohms
PHONO
Overload
Level
(1kHz;T.H.D:
0.03%)
PHONOl,2...... 300mV
Output
Level/lmpedance
TAPERECl...... ..150mV
TAPEREC2...... 150mV
PREOUT ..... 1V/1kilohms
Total
HarmonicDistortion
(20H2
to20,000H2,1Voutput).. .Nomorethan0.01%
Frequency
Response
PHONO(RIAAequalization).20Hz
to20,000H2
l0.2dB
ToneControl
BASS...
TREBLE.
Filter
LOW
...
sx-,| 480
. ,LlOdB
(l00Hz)
main
control
a5dB
(50H2)
subcontrol
. r10dB (1okHz)
main
control
e5dB (20kHz)
subcontrol
HIGH...
LoudnessContour
(Volume
control
set
. 15Hz
(12dBloct.)
. 8kHz
(l2dB/oct.)
at
-40dB
position)
. . .. +6dB
(100H2),
+3Og
1'l6kli.1
Hum
and
Noise
(lH
F,
short-circuited,A Network)
PHONO .
AUX,
TAPEPLAY, . .
Muting..
FMTunerSection
UsableSensitivity
t\40No
..
50dB
Quieting
Sensitivity
MONO..
STEREO.
Signal-to-Noise
Ratio
(at
75dBf)
. . .STEREO.
(at
65dBf)
...MONO..
STEREO.
Distortion
(at
65dBf)
'f00Hz
MoNo/STEREO
.. ..o.1%1o.2%
lkHz MONO/STEREO ... O.1o/o10.'1570
6kHz I\4ONO/STEREO . ... 0.1YolO.2%
FrequencyResponse.. .... .. 3OHzto15,O0OHzl33dB
CaptureRatio ..........1.0dB
AlternateChannelSelectivity .........80d8
SpuriousResponseRatio. .. ........100d8
lmageResponse
Ratio
... ..........90d8
lFResponseRatio... ....110d8
AMSuppressionRatio... ....55d8
Muting
Threshold . . 19.2dBf(5gV)
StereoSeparation. . . 50dB(lkHz),35dB(30H2
- 15kHz)
SubcarrierProductRatio ...........65d8
SCARejectionRatio... ...........65d8
Antenna
Input 300
ohmsbalanced
75ohmsunbalanced
...80d8
. . .95dB
. .-20d8
AUX,TAPE
PLAY . . . 5HztoS0,OOOHZI?dB

AMTunerSection
Sensitivity
(lHF,Ferriteantenna)
... .......
(lHF,
Ext.
antenna).
Selectivity
Signal-to-Noise
Ratio... . . . . .
lmageResponseRatio.. . . . . .
lFResponseRatio.........
Antenna. .. Built-in
300!V/m
.. 15pV
. . .30dB
...50d8
. . .40dB
......40dB
FerriteLoopstick
Antenna
Misdellaneous
PowerRequirements...... .... 120V
60Hz
PowerConsumption..... 430W(UL),1000VA(CSA)
1100W
(max.)
Dimensions .. 556(W)x i86(H)x461(D)mm
21-11/l6(w)
x6-1sll6(H)x17-5l16(D)
in
WeightWithout
PackaSe.
. 28.7kg
(631b
3oz)
With
Package
FurnishedParts .32.1kg
(701b
10oz)
FMT-typeAntenna ..........1
OperatingInstruclions ........1
Hex.Wrench 1
*Meacurcd pursudnt to the Federal Trade Commission's
Trade Regulation rule on Pouer Output Claifis for
Amplifierc.
NO?E:
Specificotionsdnd thedesign
subjectto possible modifico-
tion uithout aoticedue to improuements.
HEX.WRENCH
The accessory
hexagonalwrench is provided for removing
the TUNING knob and VOLUME knob or tightening
their setscrewsshould they becomeloose.
If required, loosen the set screw by inserting the wrench
into the hole on the side.of the knob and tuming the
wrench counter-clockwise. Be particularly careful not to
scratchthe front panel when employing the wrench.

2. CONNECTION
DIAGRAM
Turntable
I (PHONO
1)
AM outdoor antenna
Cartridge
type
player
(AUX)
-'--'t
& Grc ou €^L E!
-
-
-
j-,
-
--
-
-
-
-
Turntable
2 (PHONO
2)
300ohm
feeder
T.tvpc FM anicnna FM outdoor antenna
5

six-1 eElo
Connectthe power plug to the wall outlet because
this
receiver
requires
alotof
Tape deck 1
(TAPE 1) Tape
deck 2
(TAPE
2}
Speakersystems
A
(SPEAKERS
A}
Dolby*NR adaptor
Right
ru
onm
ax
ial
0re
L-_,
,fi.
/i\
TI
Ac12ov
m
60Hz
only
Switched Unswitched
SWITCHED:
(50W
max.)
Speakersystems B
(SPEAKERS
B}
Speakersystems
C
(SPEAKERS
C}
A frequently used component
(turntable,
tape deck,etc.)can be
plugged into this outlet. By leav-
ing the power switch of that
component in the ON position,
power supply to the component
will be coupled with the receiver
switchoperation.
UNSWITCHED: Less
frequently usedcomponents,
(Total lOOW which do not require couplec
max'' power, can be plugged
into either
of these outlets.
Lef
t Right
* Tlre
word "Dolby" is a trademarkof Dolby LaboratoriesInc.

3. FRONT PANEL FACILITIES
POWER
SWITCH
Flipthis
switchtothe
ONposition tosupply
power
tothe
stereoreceiver.
Therewill bea shortdelaywhenit i; set
to ON, becausethe mutingcircuit hasbeen
actuatedto
suppressthe unpleasantnoise
thatissometimes
generated
when
the
power
isswitched
onandoff.
PHONES
JACK
Plugthe headphonesinto this jack when you want to
listen
through
yourstereoheadphones.
Release
the allSPEAKERSbuttonsif you want to listen
to thesoundthrough
your headphones
only (This means
that
allthreebuttons
will bereleased).
SPEAKERS
BUTT
Press
the button corresponding
to thespeakers
connerrted
to theSPEAKERSterminals
(A, B,C) on therear
panel.
You can presrtwo of thesebuttonsto listen
to sound
from two pairs
of speaker systemsat thesametime.
NOTE:
No ound utill beheardthrough the speahers if all lhree
buttou are
p?esed
ot lhe same
time.
BALANCE
CONTROL
Usethir control to balancethe volume
of the left and
right chennels.First, however,
setthe MODE switchto
MONO, and adiust
so that the soundappears
to c,)me
from somewhere
exactly
betweenthetwo speakers.ll the
sound
appears
to be louder
on the right,
it means
that
the
volumeof therightchannel
is
higher.TurntheBALAIICE
control to the left and adiust.
Conversely,if the sound
appears
to be louder on the left, it meansthat the
volume
of the left channelishigher.
Therefore,
turn the
BALANCE control to the right
andadiust.After adjust-
ing,returntheMODE
switchto STEREO.
POWER
METERS
Thesepower meters allow you to read out the rated
powerlevelwhen speakers
with a nominalimpedanceof
8 ohmsareconnectedto the receiver'sspeakerterminals.
NOTE:
These ualuesare related to the impedanceof the
speahersand they wry accprding to the frequency.
In order to find out the exactoutput leuel,connectan
8-ohmdummy loadinsteadof thespeahers.
FUNCTION
INDICATOR
MEMORYMARKERS
These
arevery
convenientfor frequenttuning in to the
same
broadcastingstation.
MUTINGSWITCH
Set this switch to the -20d8 position
to attenuatethe
audiooutputindicated
by the VOLUME
controlby 20d8.
There is no needto adiust
the VOLUMEcontrolif you
usethis switch when turningdown the audiooutput
temporarily and when changing
over records or tapes.
VOLUME
CONTROL
Usethiscontrolto adlusttheoutputlevelto thespeakers
andheadphones.Turn it clockwiseto increasetheoutput
level.No sound
will beheardif you set it to-. The scale
is graduated in dB which indicate
the attenuation
when
themaximum
outputlevel
is0dB.
ti
t"
tl
III
II
Irt
Iti
il
l,l
I
_rEeLE_
tw ,oi! ---- +" -..,E;l'"ia'- o^.',r E
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o osss$":f^fl#"
OErEn

_TONE
SWITCH
Set this switchto ON when adjusting
the BASS
and
TREBLE controls.
In the OFF position,
it causes
the
amplifierto operate
withaflat frequency
response.
FUNCTION
BUTTONS
Press
the functionbutton whichcorresponds
to the pro-
gram source.
Turn the VOLUME control down first
before selectinga different function button while the
soundfrom one
program
sourceis
beingreproduced.
FM. . . . . Press
this
buttonfor FMbroadcasts.
The FM STEREO
indicator
tights
up
when
the
receiveris
tunedin
toanFM
stereo
broadcast.
The soundis auto-
matically received monophonically
during
FM
monophonic
broadcasts.
TWIN
BASS
AND
Use these
controlsTREBLE
CONTR
to adjust the bass
and the treble.
TAPE
DUPLICATE
SWITCH
Setthisswitch
to ON when
youwant
to duplicate
oredit
a pre-recorded
tape using two tape decks.
TAPE
MONTTOR
SWTTCHES
(1,2)
SetswitchI to ON with a tape
deck
which isconnected
to theTAPE 1 jacks
(REC
andPLAY)when youwanr
to
monitor
theplayback
or recording
of atape.
Thetape
on
a deck
whichisconnected
to theTApE 2 jacks
(REC
and
PLAY) can
besimilarly
monitored
by setting
switch
2to
oN.
NOTE:
Set theseswitches to the upper (offl position when
Iistening
to records
or a broadcast.
AM..
.
AUX..
PHONO
2IMIC
. .
Press
this button for AM broadcasts.
Press
this
buttonwhenlistening
to an
audio component cornected to the
AUXinput
jacks.
Press
this button when playing a
record
on the turntable
connected
to
the PHONO
2 iacks, or when
using
a
microphone
which you have plugged
intothe
MIC
iack.
Press
this button when playinga
record
on the turntable
connected
to
thePHONO
i jacks.
PHONO
1
... ..
NOTES:
1. Unplug the microphone from the MIC jach when you
do not intend to use the microphone otherwise you
will not beable to use
thePHONO 2 jachs.
2. Only one function button should be pressed at a
time.
Plug
your
microphone
into
this
lack.
The microphone
signals
arereproduced
in monothrough
the
leftand
right
speakers.
a
ti Ir'
Itl
tl
Lt
tt
U'
ttl
II
t'l
.,
t:
I
EEE! ffi g@@t@ trED
----

TUNING
METER
Whentuning
in to an FM station,
theoptimumreception
position
is indicated
whenthe metei'
pointer
deflects
to
deadcenter.
Checkthat theSIGNAL meter
pointer
has
deflected
asfarto theright as
possible.
SIGNALMETER
When
tuningin to an AM or FM station,
theoptimum
reception
position
is indicated
by the maximum
deflec.
tionof the
meter
pointer
to theright.
FM
MULTIPATH
BUTTON
Usethis
switchto detectmultipath
soundwhen
installing
the FM antenna
ina position
which
yields
theminimum
multipath interference.
STEREOINDICATOR
FILTER
15Hz BUTTONS
. When thisbuttonispressed,
a 12dB/oct
atten-
uation can be provided
for frequencies
below
l5Hz. Thismeans
that
youcancancel
out noise
in the ultra-lowfrequencies
whichis
generated
by low-pitched
rumblefrom a turntable
and
other forms of distortion.
Although
thisnoise
cannot
be heard,
it can
generate
intermodula-
tiondistortion.
8kHz . . Press
this button to provide
a l2dBloct atten-
uation
at frequencies
above
8kHz.
Setit to this
positionwhen you find high-frequency
noise,
such
asthat from scratched
records,unpleasant.
FM
25ps
BUTTON
Press
this
buttonwhen
listeningto a *Dolby FM broad-
cast;
otherwisekeep
thisbuttonat thereleased
position.
sx-1 eao
PHONO1 CARTRIDGE
LOADSWITCH
Usethis knob to selectthe input capacitance
(pF) in
accordance
with the sp.ecifiedload capacitance
of the
moving
magnet
(MM)cartridgeconnected
to the
PHONO
1jacks.
TUNING
KNOB
Usethis
to tune
into broadcasting
stations.
Selectthe stationand tune for optimum reception
by
observing
the SIGNAL meter
for AM stations
andboth
the
SIGNAL
andTUNING
metersfor FM
stations.
MODE
SWITCH
Usethis
switchfor selectingmonoor stereo
performances.
STEREO: Setto thisposition for normal
stereoopera-
tion.
MONO: Whenset
to thisposition,
the left andright
channel
signals
will bemixed
andreproduced
monophonically
from both speakersystems.
FM
MUTING
BUTTON
ON (released
position)
. . . Release
this button to sup-
press
unpleasant
inter-station
noise
whentuningin to FM
stations.
OFF
(depressed
position)
. Depress
this.
button to pick
upweal(
stattons.
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ill
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tl
It$ "'5"-,6t*
OFOTEn s]E@ FrcErvr Sx-1eAO

5. BLOCK DIAGRAM
ANTENNA
t-
FM
AM
L
AUX
PHONO
I
PHONO
2
Mlc
R
PHONO
2
PHONO
I
AUX
L#
GND
-+7
16 9.? Ortver
Of pOWer Omp.
' ' i c,rc
'le' gmpl
tf
ier
r A: 3-i'r,
rer
r^-e "anlrnl
Ver q. 51!lrf rQr
-... <oaac a{ ^n
.vWCr O
mP
-,r"^. ^F f rrrrrrl
, ..::e. 0mDlrf ier
Progrom
indico
tor
FT AX AUX PH-z
TU
NER
l---- ---
I
rc t-!
.r3 e_r rJ e_
I
li
S6
).tOq
q
H
ll
I EQUALIZER
iAMPLIFIER i
L--------J
I
,'Locol0SC
AM
TUNER
r,.'-- ---l
TUN
ING
meter STEREO
indicotor
SIGNAL
meter
+68V
-58V
I
ts2
I
-#
I I sz"P-*a
POWER
SUPPLY
CIRCUIT
I
Diol
scole
illu
minolion
,E: .q- H
:*a s2*e ;€l
E d H d 3-=
I
T:4 **d ?g_,
LHJ LHJ F
flFE
VF
VF
VF
VF
VF
VF
l."u
,l-3'V
]".
To L chonnel
Power
stoge
of oower
amptifrer
To
R chonnel
Power
stoge
of power amplrfier
t-r-r
+76v
+J T_..' 5
=m
_?crr- | _
.ruu-J-E1
-T6v
*--J- t -

six-1 aao
ER
AMPLIFIER METER
AMPLIFIER
P0w
r- ffi -f,+
KERS
l
F
r€-
ot
,2.
+.'
| "h.
I
I
I
I-s,, 1t"
l'Sts
VRI
: BALANCE
VRz:
VOLUME
VRr
:
BASS
(l00Hz)
VRo:TREBLE(lOkHz)
VR5:
9155
(
50
Hz
)
VRe:IREBLE
(
20kHz)
I
I
FLAT
AMPLI.
wziiFIER TONE
CONTROL
FILTER
AMPLIFIER POWER
AMPLIFIER
6
Sta'
f,) -
+i
loy
)ti
ltl
Re
PROTECTION
C
IRCU
IT
SI8
:
HIGH
FILTER
Sts:
LOW
FILTER
5zr
:
SPEAKER
A
SzZ
:
SPEAKER
B
SzT:
SPEAKERC
SPEAKERS
A
POI{ER
meter
aE-
I
LIIiIV iL:
\oJ n
q----o -
PRE
OUT
POWERAMP
IN
SW
ITCHES
:
51-55:
FUNCTION
Sr:FM(on-off)
52:AM
(on-off)
$3:AUX(on-off)
Sc'PH0N0
2r
on-
off)
Ss:PH0N0
I(on-off)
( I kHz-off
) Szs
: POWER (on-off)
(15Hz-off
)
(on-off)
(on
-off)
(on
-off)
----
---B
-_-
I 4 IrI\ )
,76v+J h L?.-i
:"t vr q' t
.z6v--JJf --T---
f:-] rT--?
.76v-| L ql 3.-E
= tD 'v--L--=---J
-l6V >--J_
Ss
: CARTRIDGE
LOAD
(
lCO
, 200,30C,
400
)
pF
g7
: FM
MUTING
(
on
- off
)
Se
: MULTI PATH
(
on
- off t
Ss: 0E
-
EMPHASIS
{ llrs - 75ps
)
Sro
:
0UPLICATE
(
on
-
off )
Srr
: MONlT0R1 (
on
-off )
512:
MONlT0R
2 (on-gl!
)
Srs:
ADAPTOR ton-q{l
Stc:
M00E ( Stereo
- Mono
)
Sts:MUTING
(
-20d8 - oft )
Sre
: LOUDNESS
(
on
- orl )
Srr:
T0NE (
on
- off )

6.CIRCUIT
DESCRIPTIONS
Refer to the block diagramon page11 and the
schematic on page 43 for the circuit composition
of this unit.
6.1 TUNER
SECTION
AM Tuner
The AM tuner employs a 2-gangvariable capaci-
tor, one IC (HA1197) and one AM ceramic filter.
Its block diagram is given in Fig. 1.
HA1L97 is an IC containing an RF amplifier,
converter, 2-stage
IF amplifier, detector, and AGC
circuit, and features excellent AF frequency re-
sponse
and distortion. Q1of the output circuit isa
specialAM muting circuit. This circuit is operated
until the AM tuner stabilizes
immediately after the
FUNCTION AM switch has been set to ON. The
instant the AM switch is set to ON, +3 is supplied
to Rr, thru Ce, and the baseof Q, is forward
biased.
Consequently, Q, is tumed ON, ancl the AM
output signalis shorted to ground during the time
constant
of Ce,
R6.
FM Front End
The FM front end employs a 4-gang variable
capacitor in the tuning circuit, dual gate MOS
FETs at the RF amplifier and mixer, and a local
osoillator with J-FET buffer.
The FM front-end input is 75St unbalanced
single-tuned circuit. The RF amplifier is a dual
gate MOS FET. An M-coupled double-tuned cir-
cuit is inserted between stages.The dual gate FET
is an amplifying element suitable for RF circuits,
and features extremely stable amplification be-
cause
of its low NF (Noise
Figure),
highPG(Power
Gain) and low feedbackcapacitance.
The mixer is also a dual gate MOS FET. The
reception signal is applied to gate 1 and the loeal
oscillator signal is applied to gate 2. This circuit
Ceromic
frlter
ANT
has low local oscillator signalinjection power and
the strong reception signalhaslittle effect on the
local oscillator.
The local oscillator is a modified Clapp circuit.
Its output signal is injected into the mixer thru a
J-FET buffer.
lF Amplifier
Four dual elementceramicfilters areused
as
the
selection
elements,
andoneIC (TA7302P)contain-
ing two differential amplifiers and one FM IF IC
(PA3001-4) areused
asthe amplification elements.
The TA7302P compensates
for the filter inser-
tion loss,and also limits the amplitude of the l'M
signal.
The PA3001-A performs IF amplification,
amplitude limiting and FM detection. It also
drives the TUNING and SIGNAL meters and con-
trols muting.
The block diagram of the PA3001-A is givenin
Fig.2.
FM Multiplex Decoder
The FM multiplex decoder is a PLL (Phase-
Locked-Loop) MPX IC (PA1001-A). Its block
diagram is given in Fig. 3.
FUNCTION
(AM
switch
)
+B
OUT
SIGNALmeter
Fig.1 Block dragram
ofAMtuner
Fig.
2 Block
diagram
of pA3001-A
HAll97
Mixer Deleclor
AGC
line AGCAmp
13

PA1001-A contains
a PLL VCO circuit, double-
balance type differential demodulator with NFB
amplifier, and pilot auto-cancelcircuit to improve
the distortion charact,:ristics,
frequency response,
andS/N.
The nonlinear distortion produced at the de-
modulator has beenimproved by adding the NFB
amplifier to the double-baiance type differential
demodulator.
The pilot auto-cancel circuit eliminates carrier
leakage(19kHz), without a loss of demodulated
signal
frequency response.
Moreover, sincethe cancel
signal
leveltracks the
input pilot signal level, the rejection ratio does
not drop evenif the input pilot signal
levelchanges.
FM Output Amplifier
The FM output amp is an IC (PA1002-A).
PA1002-A containsan AF amp. muting gate
circuit
and power ON/OFF muting control circuit. Its
block diagram is given in Fig. 4.
The AF amp. is a differentially coupled NFB
amp. featuring excellent dynamic range, S/N and
distortion characteristics. A de-emphasis charac-
teristic is obtained by providing a frequency
selection characteristic at the NFB loop of this
amp.
The muting circuit electronically grounds the
signal circuit when a control voltage of 1.4V or
greaterhas beenappliedto pin 8 of the PA1002-A.
This control voltage is applied from pin 12 of the
PA3001-A thru the MUTING switch. A 1.4V DC
voltage is generated at pin 72 of.the PA3001-A
when de-tuned more than 170kHz and at weak
inputs (antennainput conversionSprV
or less).
6.2 AF SECTION
MIC InputCircuit
PHONO 2 is switched over to MIC when the
microphone plug is inserted in to the MIC jack.
This input circuit is equipped with an inverse
RIAA curve designed to counteract the phono
equalizeramplifier circuit.
PHONO 1 Input Capacitance
Switching Circuit
(Fig.
5)
The PHONO 1 input circuit includes an input
capacitance
switching stagewhich permits the user
to select the optimum load capacitance for the
phono cartridgebeingemployed.
When C is excluded, the input capacitance of
the circuit shown in Fig. 5 is 100pF. The capaci-
tance of C may be set to either 0, 100, 200, or
300pF, thereby making the input load capacitance
100,200,300,
or 400pF.
Fig.5 PHONO
1input
capacitance
switchingcircuit
Equalizer
Amplifier
This is a 3-stagedirect-coupled equalizer ampli-
fier with a first-stage differential amplifier and
final-stage
pure complementary SEPP.
The role of the differential amplifier is to ampli-
fy the signal, and to stabilize the DC balance of
the output neutral point. The bootstrap circuit in
the second stage ensuressufficient gain to drive
the final stagecomposed of apure complementary
SEPP circuit for a wide dynamic range and low
output impedance. The RIAA equalization curve
is produced by an NFB circuit employing LVo-
error metal film resistors and 2Voaftor polypro-
pylene capacitors,resulting in a equalization devia-
tion of 10.2d8(2OHz- 20kHz).
STERE0
ind
Fig.
3 Blockdiagram
of PA1001-A
Co+Co'=100pF
Fig.
4 Block
diagram
of PA1002-,4

FlatAmplifier
The first stage of this 2-stage direct-coupled
flat amplifier is equipped with a FET for higher
input impedance. Ample NFB also permits an
lower output impedance.As a result, there islittle
change
in noise level when the VOLUME control
resistance is altered, and there is also sufficient
power to drive the NFB-type tone control circuits
connected
to the following stage:
NOTE:
Since the input impedance of the NFB tone control
circuits is relatiuely low, there will bea certain amount of
change
when the uariableresistor is rotated.
Tone
Controls
The SX-1280 featurestwo pairsof tone control
circuits (twin tone controls), eachwith a different
turnover frequency. The main tone controls are
similar to other regular tone controls, while the
subtone controls permit variation of the frequency
responseat te very high and very low ends.Both
main and sub tone controls apply C-Bfeedbackto
a singletransistor amplifier, with frequency selec-
tion provided within the feedback loop, and the
frequency responsealtered when these
frequencies
arechanged.
All of thesecircuits may beby-passed by simply
putting the TONE switch into the OFF position.
sx-1 eao
FilterCircuits
The filters employed in this receiverare-12dB/
octave NFB-type active filters. By coupling the
emitter-follower to RC passive
filters, peaks have
been obtained in the region of the cut-off fre-
quencies, thereby sharpening the "knee charac-
teristics" of the RC passive
filters.
Power
Amplifier(Fig.
6)
The first stageconsists of a PNP dual transistor
differential amplifier (Qz
) which loads the current
mirror circuit (Qs) to obtain high stablegain even
in the high frequency range. The predriver stage
(Qa
) operates with the constant current circuit
(Qr) asthe load, amplifying the voltage to the re-
quired level.With a 3-stageDarlington connection
in the power stage,and a parallel push-pull circuit
in the final stage,
a power output level of 185W +
185W/89 (20H2- 20kHz, T.H.D. less
than O.OgVo\
has
beenobtained.
The purpose of Qt is to equalize temperature
changesin Qr. By keeping
the input base
potential
of Qz at a constant 0V, the Qz output neutral
point potential will remain steady despitechanges
in temperature. Sincethis power amplifier is a DC
amplifier, any DC voltages appearing at the Q, in-
put base will be amplified and passed to the
output. In order to avoid this, the drop in poten-
tial acrossR1due to the Qz input base
current, is
designed to be absorbed by the Qr basecurrent,
thereby maintaining the Qu input basepotential
constantat 0V.
OUT
To protection
circuit
+Br
+Bz Constant
current
load
circuit
Current
mirror
circuit
Fig.
6 Poweramplifiercircuit

Q,a to Q,, constitutes the power limiter stage.
This stage measures the current flowing through
the power transistors by means of the voltage
produced by the power transistor emitter resist-
ance. If the load impedancedrops below 4 ohms,
or the output level exceeds180 watts, Qrc
- Qrz
are activated, thereby limiting the drive voltage.
Qre and Q15
operate for positive half-cycles, and
Qre
andQtt for negative half-cycles.
D,, Dr, D5, and D6 limit the drive voltageto
prevent drive voltages in excess of a fixed level
from being applied to the power stage, thereby
avoiding clipping in the power stage, and the
generation of oscillations.
NOTE:
If an excessiuely high driue voltage is applied to the power
stage, the Ds and D6 anode peah voltage will exceed the
power stagepower supply uoltage, thereby activating D5
and D6, and limiting the driue uoltage.
MeterAmplifier(Figs
7,8)
In order to indicate the 0.01W- 400W range
with one meter without range switching, a loga-
rithmic indication type meter must be employed
and the input signal must be logarithmically com-
pressed. The meter amplifier circuit is shown in
Fig. 7. This circuit consists
of a logarithmic com-
pression circuit and a meter drive circuit. The
output signal of the power amplifier is applied to
the logarithmic compression circuit, and its dynam-
ic range is compressed. The principles of the
logarithmic compression circuit are given in Fig. 8.
The output voltage of this circuit is the value
divided by Rr and Z. The attenuation at low
signal input is reduced and the attenuation at large
signal input is increased, by using the rise of the
diode current-voltage characteristic at Z.
The compressed signal is shaped by D5 and
applied to Q, of the meter drive circuit. Q, current
amplifies the DC voltage from D, to drive the
power meter.
c:=+
- INPUT
V0LTAGE
Fig.
8 Schematic
diagramof logarithmic
compressor
6.3 POWER
SUPPLYCIRCUIT
The L and R channels in the power stage are
both equipped with independent coils, bridge
rectifiers, and a pair of 15000pF capacitors for
independent
supplyof 176V.
The class A amplifier circuits (equalizer amp,
flat amp, tone control circuit, filter amp, first
stageof the power amp, and the pre-driver stage)
are supplied with l68V and t32V via a coil, bridge
rectifier, and voltage regulator stage independent
of the porverstage.
The tuner sectionis suppliedwith +13.5V via.
lamp coils, voltage multiplier rectifier, and voltage
regulator stage.
SurgeCurrent
Suppressor
Circuit (Fag. 9)
The power supply stagein this equipment em-
ploys toroidal transformers of low internal resist-
ance,and 4 high capacitance
(150001F) electrolyt-
ic capacitors for smoothing purposes. Therefore
without any effective countermeasuresi a sudden
Iargesurgeof cunent would flow once the POWER
switch was turned on, resulting in possible
damage
to fuses, the POWER switch itself. and other
components. In order to prevent this, a sur3e
cur-
rent suppressor
circuit hasbeenincorporated.
When the POWER switch is tumed ON. the AC
cunent is applied to T (power transformer) via R,
and the micro temp (thermal fuse).The secondary
coil output in then rectified by diode D. resulting
in capacitor C1 being charged up, and relay Rr.
being activated. So until Rr is turned on, the volt-
age applied to the primary coil of T will be the
voltage divided by R, and the T primary coil im-
pedance,which meansthat the cunent flowing.in
when the POWER switch is first turned ON will be
very small. The time required for R; to beturned
on will depend on the D and C, time constant
Micro temp is a protection fuse designed
to blow
U
J
F
F
I
OUT
z
I
+S
- INPUT
VOTTAGE
Fig.
7 Schematicdiagramof meteramplifier
16

upon detection of heat
should fail to turn on,
fail to operatenormally.
generated in R1 if RL
or the T secondary coil
Fig.
9 Surge
current
suppressor
circuit
6.4 PROTECTION
clRCUlT (Fig.10)
The protection circuit featured in this equip-
ment employs an IC (PA3004) recently developed
by Pioneer.This IC includes DC voltage detection
(a), +B voltage level detection (b), -B voltage
level detection (c), AC voltage detection (d), Ct
discharge switching (e), relay drive trigger switch-
ing (f), and constant current circuitry (g & h).
This protection circuit is responsible for muting
when the POWER switch is turned ON and OFF,
speaker protection in case of an abnormal power
amplifier neutral point potential, power amplifier
protection in case of overloading, and speaker
disconnection in case of protection circuit failure
due to abnormal voltage in the power supply to
the protection circuit IC.
Over
load
Detector
Power
amplifier
sx-1 Elacl
POWER
SwitchMuting
The delayingaction employed when the POWER
switch is turned ON is determined by the time con-
stants of the timing capacitor (C1) and the con-
stant current circuit (g). When the POWER switch
istumed ON, +3 ispassed
via (g) to charge
up CT.
When fully charged, switch (f) wiil tum on, and a
base current will flow from circuit (h) to Q3,
thereby tuming the relay on.
When the POWER switch is turned OFF again,
the AC voltage disappears immediately, resulting
in switch (e) beingturned on by the AC voltage
de-
tector (d), thereby dischargingCT. Consequently
switch (f), Qr, andthe relay will all be turned off.
AbnormalPower
Amplifier NeutralPointPotential
The occurrence
of any abnormality in the power
amplifier neutral point potential will be detected at
(a),resultingin switch(e)being
turned on. C1 will
thus be chargedup, and switch(f),Q3,
&rd the relay
aII turned off.
Protection AgainstOverload
If the bridge type detector circuit (Q,) detects
the presence of an overload condition, Q, will be
chargedon, resulting in aDC voltagebeingapplied
to detector (a). Switch (e) will thus tum on, and
switch (f), Q., and the relay will all tum off.
Abnormal Protector Circuit PowerVoltage
The power supply voltage level is detected by
detectors (b) and (d). If the prescribedlevelisnot
attained, switch (e) will turn on, and switch (f),
Qr, andthe relay all tum off.
tBz SPAEKER
terminal
PA3004
Fig.10 Protectioncircuit
17

7. DISASSEMBLY
S
TUNING
knob
@
VOLUME
knob
f
1El
Fis.
11

BonnetCover
Remove the bonnet cover by undoing the 4
screws
O - O.
WoodenCover
The wooden cover can only be removed after
the bonnet cover has been removed. Undo the
second
set
of 4 screws
O- @.
Bottom Plate
Undo 13 screws(p- (D to remove the bottom
plate.
@q
six-1 aBo
Front Panel
To remove the front panel, first loosen the
hexagonal screws in the VOLUME control and
TUNING knobs, and then gently pull them off.
All other front panel knobs are pulled off direct-
ly. Then undo the screws
(D and (D, and the pair
of nuts andwashers
@ and(0.
Power
Amplifier
and
Heat
Sinks
(Fig.
12)
Undo the 4 screws
@ - @ to remove the power
amplifier andheat sinks.
Heat
sink
,P*
Fis.
12
@

8. PARTS LOCATION
8.1 FRONT
PANELVIEW
Knob
(F
I
LTE
R
)
AAD.140
Knob
lSPEAKE
RS)
AAD,140
Front
panel
assembly
AN8-554
Knob (FNI)
AAD.I40
Knob
{FM,
AM,AUX,
PHONO
2/MlC.
PHONO1)
AAD.140
Knob
(TUNING)
AAA-047
Knob
(VOLUM
E)
AAB-,I74
Knob
(BALANCE.
PHONO1
CARTR
IDGELOAD}
AAB.176
Knob
(TAPE,
ADAPTOR,MODE.
LOUDNESS)
AAD.138
{MUTING)
138
Knob
(BASS)
AAB-175
Knob
(TON
E)
AAD.138
Knob
(TREBLE
AAB.175
al
;
-"-i
t
I
I
I
I
I
Knob
AAD.
eo