Pioneer SX-1080 User manual

AM/FM STEREO RECEIVER
SX-IOSO
SERVICE MANUAL

sx-noso
MODEL SX-1080 COMES IN FIVE VERSIONS DISTINGUISHED AS FOLLOWS:
Type Voltage
Remarks
KU
120V only
U.S.A. model
KC 120V only Canada model
HG 220V and 240V (Switchablel Europe or Oceania model
S 110V. 120V, 220V and 240V (Switchable) General export model
S/G
110V. 120V, 220V and 240V (Switchable) U.S. Military model
NOTICE:
• This service manual is applicable to the SX-1080/KU.
• The SX'1080/HG, S/G, S, KC service manual is issued as an appendix.
CONTENTS
1. SPECIFICATIONS. 3
2. CONNECTION DIAGRAM. 5
3. FRONT PANEL FACILITIES. 7
4. DISASSEMBLY. 10
5. PARTS LOCATION
5.1 Front Panel View. 11
5.2 Front View with Front Panel Removed. 12
5.3 Top View with Bonnet Removed. 13
5.4 Bottom View with Bottom Plate Removed. 14
5.5 Rear Panel View. 15
6. LEVEL DIAGRAM. 16
7. BLOCK DIAGRAM . 17
8. CIRCUIT DESCRIPTIONS
8.1 Tuner .. 19
8.2 Phono, Mic Input Circuit. 20
8.3 Flat Amplifier. 20
8.4 Tone Control. 20
8.5 Power Amplifier. 20
' 8.6 Meter Amplifier... 21
8.7 Protection Circuit.-. 22
8.8 Power Supply. 22
1

9. ADJUSTMENT
9.1 AM Tuner. 23
9.2 FM Tuner. 24
9.3 FM MPX. 25
9.4 Power Amplifier. 26
9.5 Meter. 26
10. DIAL CORD STRINGING. 27
11. EXPLODED VIEWS. 28
11.1 Exterior. 29
11.2 Component of Panel Stay. 31
11.3 Component of Chassis. 33
11.4 Rear Panel. 35
12. SCHEMATIC DIAGRAMS, P.C. BOARD PATTERNS AND PARTS LIST
12.1 Miscellaneous Parts List. 36
12.2 Schematic Diagram. 39
12.3 Tuner Assembly (AWE-092). 41
12.4 Power Transistor Connector Assembly (AWX-125). 47
12.5 Tone Control Assembly (AWG-056). 48
12.6 Flat Amplifier Assembly (GWG-112). 51
12.7 Power Amplifier Assembly (AWH-071). 54
12.8 Speaker Selector Assembly (GWS-124). 57
12.9 Function Equalizer Assembly (GWS-125). 60
12.10 Filter Assembly (AWT-001). 64
12.11 Power Supply Assembly (AWR-152). 66
12.12 Terminal Assembly (GWS-126). 70
12.13 Terminal Assembly (GWS-127). 70
12.14 Speaker Terminal Assembly (GWS-128). 71
12.15 Protection Assembly (AWM-120). 72
13. PACKING. 75

SX-1 080
1. SPECIFICATIONS
Semiconductors
FETs. 5
ICs. 8
Transistors . 36
Diodes. 42
Power Amplifier Section
Continuous power output of 120 watts* per
channel, min. at 8 ohms or 150 watts* per
channel at 4 ohms from 20 Hertz to 20,000
Hertz with no more than 0.05% total harmonic
distortion.
Total Harmonic Distortion (20 Hertz to 20,000 Hertz)
Continuous Rated Power Output . , No more than 0.05%
60 watts per channel power
output, 8 ohms . No more than 0.02%
1 watt per channel power
output, 8 ohms . No more than 0.02%
Intermodulation Distortion (50 Hertz: 7,000 Hertz = 4:1)
Continuous Rated Power Output . . No more than 0.05%
60 watts per channel power
output, 8 ohms , . . . No more than 0.02%
1 watt per channel power
output, 8 ohms . No more than 0.02%
Frequency Response .... 5 Hertz to 100,000 Hertz^^^^dB
Input Sensitivity/ Impedance
POWER AMP IN.lV/50kilohms
Output
Speaker . A, B, A+B
Damping Factor
(20 Hertz to 20,000 Hertz, 8 ohms). 30
Hum and Noise (iHF, short-circuited, A Network). . lOOdB
Preamplifier Section
Input (Sensitivity/Impedance)
PHONO 1,2 .2.5mV/50 kilohms
MIC.7.5mV/50 kilohms
AUX .150mV/50 kilohms
TAPE PLAY 1.150mV/50 kilohms
TAPE PLAY 2.150mV/50 kilohms
PHONO Overload Level (IkHzjT.H.D; 0.05%)
PHONO 1,2 . 200mV
Output Level/Impedance
TAPE REC 1. 150mV
TAPE REC 2.150mV
PRE OUT.lV/1 kilohms
Total Harmonic Distortion
(20Hz to 20,000Hz IV output) . . .No more than 0.05%
Frequency Response
PHONO(RIAA equalization) .20Hz to 20,000Hz ±0.2dB
AUX.TAPEPLAY. 5Hz to 80,000Hz l?dB
Tone Control
BASS.±7dB/+10d8 (lOOHz)
Turnover Frequency 200Hz/400Hz
TREBLE.±7dB/+10dB (lOkHz)
Turnover Frequency 5kH2/2.5kHz
Filter
LOW .15Hz(6dB/oct.)
HIGH.6kHz {6dB/oct.)
Loudness Contour (Volume control set
at-40dB position).+6dB (100Hz)+3dB (10kHz)
Hum and Noise
(IHF, short-circuited, A Network)
PHONO .76dB
AUX, TAPE PLAY.90dB
Muting.-20dB
FM Section
Usable Sensitivity
MONO.9.8dBf {1.7/iV)
50dB Quieting Sensitivity
MONO.14.2dBf {2.8/iV)
STEREO.37dBf (39AiV)
Signal to Noise Ratio
(at75dBf) . . .STEREO .74dB
(at65dBf) . . .MONO.80dB
STEREO .71dB
Distortion (at 65dBf)
100Hz MONO/STEREO.0.1%/0.2%
IkHz MONO/STEREO.0.1%/0.15%
6kH2 MONO/STEREO.0.1%/0.2%
Frequency Response.30Hz to 15,000Hz ±0.5dB
Capture Ratio .I.OdB
Alternate Channel Selectivity.80dB
Spurious Response Ratio.lOOdB
Image Response Ratio.90dB
tF Response Ratio .lOOdB
AM Suppression Ratio.55dB
Muting Threshold. 19.2dBf (S/zV)
Stereo Separation .. 50dB (1 kHz), 35dB (30Hz - ISkHz)
Subcarrier Product Ratio.65dB
SCA Rejection Ratio.65dB
Antenna Input. 300 ohms balanced
75 ohms unbalanced
3

AM Section
Sensitivity (iHF, Ferrite antenna). 300/uV/m
(iHF, Ext. antenna) . 15 /jV
Selectivity.30dB
Signal-to-Noise Ratio.50dB
Image Response Ratio.40dB
IF Response Ratio .40dB
Antenna.Built-in Ferrite Loopstick Antenna
Miscellaneous ^
Power Requirements.120V 60Hz
Power Consumption. 350W (UL), 800W (CSA)
1100W (max.)
Dimensions. 526(W) x 176(H) x 440(D) mm
21-11 /16(W) X 6-15/16(H) x 17-5/16(D) in
Weight Without Package.21.3kg (471b)
With Package. 24.2kg (531b 6oz)
Furnished Parts
FM T-type Antenna . 1
Operating Instructions. 1
Hex. Wrench. 1
*Measured pursuant to Federal Trade Commission's Trade
Regulation rule on Power Output Claims for Amplifiers.
NOTE:
Specifications and the design subject to possible modifica¬
tion without notice due to improvements.

2. CONNECTION DIAGRAM
FM outdoor antenna
T-type FM antenna
O’O'
SX-i 080
Tape deck 2
ITAPE 21
tSBOO*;
AC 120 V
60Hz
Left Right
Speaker system
(SPEAKERS B>
B

3. FRONT PANEL FACILITIES
POWER METERS-
These power meters allow you to read out the rated
power level when speakers with a nominal impedance of
8 ohms are connected to the receiver’s speaker terminals.
NOTE:
These values are related to the impedance of the
speakers and they vary according to the frequency.
In order to find out the exact output level, connect an
S-ohm dummy load instead of the speakers.
PHONES JACK-
Plug the headphones into this jack when you want to
listen through your stereo headphones.
Release the all SPEAKERS buttons if you want to listen
to the sound through your headphones only. (This means
that all two buttons will be released).
-FILTER BUTTONS
15Hz When this button Is pressed, a 6dB/oct atteiiua-
tion can be provided for frequencies b ow
15Hz. This means that you can cancel out r >ise
in the ultra-low frequencies which is gener ted
by low-pitched rumble from a turntable md
other forms of distortion. Although this r jise
cannot be heard, it can generate intermo<ula-
tion distortion and damage the speakers.
6kHz , . Press this button to provide a 6dB/oct atlc ua-
tion at frequencies above 6kHz. Set it to this
position when you find high-frequency n lise,
such as that from scratched records, unpIca; jnl.
I— FUNCTION BUTTONS
Press the function button which corresponds to the pro¬
gram source. Turn the VOLUME control down first
before selecting a different function button while the
sound from one program source is being reproduced.
FM.Press this button for FM broadcasts.
The FM STEREO indicators light
up when the receiver is tuned into an
FM stereo broadcast. The sound is
automatically received monophonical-
ly during FM monophonic broadcasts.
AM.Press this button for AM broadcasts.
AUX.Press this button when listening to an
audio component connected to the
AUX input jacks.
POWER SWITCH^-
Flip this switch to the ON position to supply power to the
stereo receiver. There will be a short delay when it is set
to ON, because the muting circuit has been actuated to
suppress the unpleasant noise that is sometimes generated
when the power is switched on and off.
SPEAKERS BUTTONS
Press the button corresponding to the speakers connected
to the SPEAKERS terminals (A, B) on the rear panel. You
can press two of these buttons to listen to sound from
two pairs of speaker systems at the same time,
BASS TURNOVER SWITCH
Use this switch to change over the frequency in which the
sound adjustment with the BASS control is starting to
take effect. Select 200Hz or 400Hz in accordance with
the characteristics of your listening room and of your
speakers, and with your general preference.
TONE SWITCH-
Set this switch to ON when adjusting the BASS and
TREBLE controls. In the OFF position, it causes the
amplifier to operate with a flat frequency response.
BASS AND TREBLE CONTROLS
Use these controls to adjust the bass and the trlble.
If you set the TONE switch to ON and turn the B
control to the right from its center position, you wi
able to emphasize the sound in a frequency range whilti is
lower than that selected by the BASS TURNOt
switch. Conversely, turning this control from the ctfiter
position to the left will attenuate the sound.
You can use the TREBLE control to adjust the sound
frequency higher than that selected by the TREBLE
TURNOVER switch.
hSS
be
n a
LOUDNESS SWITCH
Set this switch to ON when listening at a low volume.
The frequency response of the human ear varies according
to the listening volume, and setting this switch to the ON
position compensates for hearing response by emphasizing
the bass and treble.
TREBLE TURNOVER SWITCH
Use this switch to change over the frequency in which the
sound adjustment with the TREBLE control is starting to
take effect. Select 2.5kHz or 5kHz in accordance with
the characteristics of your listening room and of your
speakers, and with your general preference.
PHONO 2/MIC.. Press this button when playing a
record on the turntable connected to
the PHONO 2 jacks, or when using a
microphone which you have plugged
into the MIC jack.
PHONO 1.Press this button when playing a
record on the turntable connected to
the PHONO 1 jacks.
NOTES:
1. Unplug the microphone from the MIC jack when you
do not intend to use the microphone otherwise you
will not be able to use the PHONO 2 jacks.
2. Only one function button should be pressed at a
time.
MIC JACK
Plug your microphone into this jack.
The microphone signals are reproduced in mono through
the left and right speakers.
NOTE:
A high impedance (approx. 50 kilohms) dynamic type
microphone with a standard plug can be connected to
this jack.
VOLUME CONTROL
Use this control to adjust the output level to the speakers
and headphones. Turn it clockwise to increase the output
level. No sound will be heard if you set it to The scale
is graduated in dB which indicate the attenuation when
the maximum output level is OdB.
MUTING SWITCH
Set this switch to the -20dB position to attenuate the
audio output indicated by the VOLUME control by 20dB.
There is no need to adjust'the VOLUME control if you
use this switch when turning down the audio output
temporarily and when changing over records or tapes.
BALANCE CONTROL
Use this control to balance the volume of the left and
right channels. First, however, set the MODE switch to
MONO, and adjust so that the sound appears to come
from somewhere exactly between the two speakers. If the
sound appears to be louder on the right, it means that the
volume of the right channel is higher. Turn the BALANCE
control to the left and adjust. Conversely, if the sound
appears to be louder on the left, it means that the
volume of the left channel is higher. Therefore, turn the
BALANCE control to the right and adjust. After adjust¬
ing, return the MODE switch to STEREO.
y

SX-i OBO
FM MUTING BUTTON-
ON (released position) . . . .Release this button to sup¬
press unpleasant inter-station
noise when tuning in to FM
stations.
OFF (depressed position) . Depress this button to pick
up weak stations.
FM 25t^S BUTTON -
Press this button when listening to a Dolby* FM broad¬
cast; otherwise keep this button at the released position.
|—TUNING KNOB
Use this to tune in to broadcasting stations.
Select the station and tune for optimum reception by
observing the SIGNAL meter for AM stations and both
the SIGNAL and TUNING meters for FM stations.
pSlGNAL METER
When tuning in to an AM or FM station, the optimum
reception position is indicated by the maximum deflec¬
tion of the meter pointer to the right.
FM MULTIPATH BUTTON-
Use this switch to detect multipath sound when installing
the FM antenna in a position which yields the minimum
multipath interference.
rTUNING METER
When tuning in an FM station, the optimum reception
position is indicated when the meter pointer deflects to
dead center. Check that the SIGNAL meter pointer has
deflected as far to the right as possible.
MEMORY MARKERS
These are very convenient for frequent tuning in to the
same broadcasting station.
STEREO INDICATOR
This indicator lights up when the receiver is tuned to
receive a stereo broadcast.
TAPE MONITOR SWITCHES (1,2)
Set switch 1 to ON with a tape deck which is connected
to the TAPE 1 jacks (REC and PLAY) when you want to
monitor the playback or recording of a tape. The tape on
a deck which is connected to the TAPE 2 jacks (REC and
PLAY) can be similarly monitored by setting switch 2 to
ON.
NOTE:
Set these switches to the upper (off) position when
listening to records or the broadcast.
MODE SWITCH
Use this switch for selecting mono or stereo performances.
Set to this position for normal stereo opera¬
tion.
When set to this position, the left and right
channel signals will be mixed and reproduced
monophonically from both speaker systems.
TAPE DUPLICATE SWITCH
Set this switch to ON when you want to duplicate or edit
a pre-recorded tape using two tape decks.
♦ The word "Dolby" is a trademark of Dolby Laboratories Inc.
9

4. DISASSEMBLY
1. Remove screws O ~ O remove the bon¬
net.
2. Remove screws 0-0 , all the knobs, and
remove the two nuts and two washers at the
front panel. (However, loosen the set screws
with the accessory alien wrench before at¬
tempting to remove the TUNING and VOL¬
UME knobs.)
3. Remove screws © ~ 0 and remove the bot-
1 o

SX-1 080
5. PARTS LOCATION
5.1 FRONT PANEL VIEW
Front panel assembly
ANB-543
Push knob (SPEAKERS. FILTER. FM)
AAD-140
Lever knob assembly (POWER}
AAD-139
Push knob {FM, AM, AUX, PHONO 2
MIC, PHONO 1)
rKnob (TUNING}
AAA047
Lever knob (MUTING)
Knob (BASS)
AAB-175
Knob (VOLUME)
AAB-174
Lever knob (TONE
AAD-138
Knob (TREBLE)
AAB-175
Knob (BALANCE)
AAB-176
Lever knob (DUPLICATE, MONITOR 1,2,
MODE, LOUDNESS)
AAD-138

SX-1080
5.3 TOP VIEW WITH BONNET REMOVED
Capacitor
ACH-085
Power transformer
ATT-448
Power amplifier assembly
AWH-071
Power amplifier assembly
AWH-071
— Tuner assembly
AWE-092
Power supply assembly
AWR-152
Aclylic board
ANR-020

5X-1 080
5.5 REAR PANEL VIEW
Antenna terminal
AKA002
4P pin-jack
AKB-035
Bar antenna
ATB-505
Bar antenna holder
AXB-012 I
AC socket
AKP-005
Power cord
ADG-013
Terminal (GNDI
AKE-031
Speaker terminal assembly
GWS-128
Input terminal assembly
GWS-127
— Jumper plug
AKM-004
Input terminal assembly
GWS-126

6. LEVEL DIAGRAM

SX-1 080
7. BLOCK DIAGRAM
ANTENNA
p75n -
POWER AMP
METER AMP
POWER AMP
IN
PRE OUT
TONE CONTROL
■ n K
v_ _y
SWITCHES
1-T4PE
S 1
POWER
M —
OFF
S 16
FM multipath ON
-m
S t
RELAY ON —
sn
Sit
LOW Filter
l5Hi
— OFF
Sj
TAPE DUPLICATE ON —
SIl
S 18
HIGH F .'ER 6I(H2
-fiEL
S4 TAPE MONITOR 1
ON —
m
Si>
BASS T'JRN OVER
20OMi
— 4D0Mi
Ss
TAPE MONITOR 2 ON —
m
Sto
treble turn over
SkHi
— 2.5liHt
S A
MODE
STEREO —
MONO Sti TONE
ON
S :
MUTING -2098 —
fi£F Sii SPEAKERS A
ON
Si LOUDNESS
ON —
Q££
Si3 SPEAKERS B
ON
-0£i
Sy
AM
ON —
QE£
Sio
FM Qtf —
OFF
Sti
PHONO 1
ON — OFF
PHONO 2 •'MIC
ON —
afi
Sn
AUX
ON —
m
Sl4
FM DE-EMPHASiS
75>iS — 25yiS
Sn
FM MUTING
OFF
RCC PLAr REC PLAY
J Ltapf 9J
JAMP
PROGRAM I NO
FM AH AUK PKONO
POWER AMP IN
PRE OUT
POWER SUPPLY
POWER AMP
PROTECTION Assy
Sioy S» y Sis ^ 5i2 ? Sii 9
METER AMP ]3V -
TONE CONTROL 33V
METER AMP -13V
EQUALIZER aPOWER AMP 23V
EOUALIZERa PO»ER AMP -23V
' POWER AMP 60V
PROTECTION a POWER AMP -60V
! PROTECTION Assy
POWER AMP 62V
POWER AMP -62V
DIAL SCALE LAMP
IB

8. CIRCUIT DESCRIPTIONS
Refer to the block diagram on page 17 and the
schematic on page 3 7 for the circuit composition
of this unit.
8.1 FM TUNER
FM Front end
The FM front end employs a 4-stage variable
capacitor in the tuning circuit, dual gate MOS
FETs at the RF amplifier and mixer, and a local
oscillator with J-FET buffer.
The FM front-end input is 75f2 unbalanced
single-tuned circuit. The RF amplifier is a dual
gate MOS FET. An M-coupled double-timed circuit
is inserted between stages. The dual gate FET is an
amplifying element suitable for RF circuits, and
features extremely stable amplification because of
its low NF (Noise Figure), high PG (Power Gain)
and low feedback capacitance.
The mixer is also a dual gate MOS FET. The re¬
ception signal is applied to gate 1 and the local
oscillator signal is applied to gate 2. This ciurcuit
has low local oscillator signal injection power and
the strong reception signal has little 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.
IF Amplifier
Four dual element ceramic filters are used as the
selection elements, and one IC (TA7302P) contain¬
ing two differential amplifiers £md one FM IF IC
(PA3001-A) are used as the amplification elements.
The TA7302P compensates for the filter inser¬
tion loss, and also limits the amplitude of the FM
signal.
The PA3001-A performs IF amplification,
amplitude limiting and FM detection. It also drives
the TUNING and SIGNAL meters and controls
muting.
The block diagram of the PA3001 is given in B'ig. 1.
Fig, 1 Block diagram of PA3001 A
FM Multiplex Decoder
The FM multiplex decoder is a PLL (Phase-
Locked-Loop) MPX IC (PAIOOI-A). Its block
diagram is given in Fig. 2.
PAIOOI-A contains a PLL VCO circuit, double¬
balance type differential demodulator with NFB
amplifier, and pilot auto-cancel circuit to improve
the distortion characteristics, frequency response,
and S/N.
The nonlinear distortion produced at the de¬
modulator has been improved by adding the NFB
amplifier to the double-balance type differential
demodulator. '
The pilot auto-cancel circuit eliminates carrier
leakage (19kHz), without a loss of demodulated
signal frequency response.
Moreover, since the cancel signal level tracks the
inpup pilot signal level, the rejection ratio does
not drop even if the input pilot signal level changes.
IN
STEREO inS
Fig, 2 Block diagram of PAIOOI-A
FM Output Amplifier
The FM output amp is an IC (PA1002-A).
PA1002-A contains an AF amp, muting gate circuit
and power ON/OFF muting control circuit. Its
block diagram is given in Fig. 3.
The AF amp is a differentially coupled NFB
amp featuring excellent dynamic range, S/N
and distortion chEuracteristics. A de-emphasis
characteristic 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
greater has been applied to 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 12 of the PA3001-A
when de-tuned more than ± 70kHz and at weak
inputs (antenna input conversion b/iV or less).
19

Fig. 3 Block diagram of PA1002-A
AM Tuner
The AM tuner employs a 2-stage variable capaci¬
tor, one IC (HA1197) and one AM ceramic filter.
Its block diagram is given in Fig. 4.
HA1197 is an IC containing an RF amplifier,
converter, 2-stage IF amplifier, detector, and AGC
circuit, and features excellent AF frequency
response and distortion. Qj of the output circuit
is a special AM 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, +B
is supplied to Rr, thru C 9 , and the base of Q, is
forward biased. Consequently, Q, is turned ON,
and the AM output signal is shorted to ground
during the time constant of C 9 , Rg.
8.2 P HONO, MIC INPUT CIRCUIT
PHONO 1, PHONO 2/MIC input switching is
performed by the switch and the MIC jack.
When a microphone plug is inserted into the
MIC jack, the input is switched to MIC at both the
L and R channels. Since the IC (HA1457) is used
as both a microphone amp and equalizer amp, a
circuit having a reverse RIAA curve is provided at
the MIC input circuit to obtain a flat frequency
response.
8.3 FLAT AMPLIFIER
The power amplifier is an all-stage direc1>coupled
circuit. It amplifies the signal to the required level
and sends a low impedance signal to the tone
control circuit.
8.4 TONE CONTROL
The tone control circuit is an NFB type inserted
in front of the power amp.
Bass turnover (200Hz 400Hz) and treble
turnover (2.5kHz <— ► 5kHz) switches and a tone
defeat circuit which bypasses the tone control
circuit to obtain a flat characteristic are provided,
in addition to variable resistor which adjusts the
rise and fall.
8.5 POWER AMPLIFIER
The power amplifier is an all-stage direct-coupled
pure complementary OCL circuit having a differen¬
tial amplifier at the first stage, current mirror cir¬
cuit at the predriver stage and a parallel push-pull
amplifier at the final stage (Fig.5).
The first stage (Q,) is a PNP type dual transistor
differential amplifier that amplifies the input signal
and stablizes the center voltage of the power stage.
Q 3 and Q 4 are driven by the opposite phases of the
output of Qi. The output of Q 4 is applied to the
current mirror circuit consisting of D 2 and Qs and
phase inverted. Consequently, Q 3 and Qs are in-
phase signals, and are operated as a push-pull pre¬
driver. Low distortion and improved rise charac¬
teristic at high amplitudes have been realized by
making the predriver stage push-pull.
Overcurrent is detected and destruction of the
power transistor prevented by Qg. The final stage
is a parallel connected power amplifier.

SX-1 080
Q 2 is a Qi temperature compensation transistor
which prevents changes in the center voltage due
to the temperature characteristic of Qi. Since the
output center voltage is applied to the feedback
side base of Qt by R 2 and R 3 , the input side base
potential of Qi becomes the standard. Therefore,
the input side base potential of Q, must be main¬
tained constant at OV. The voltage generated
across Ri by the input side base current of Q, is
cancelled by the base current of Q 2 to maintain
the input side base potential of Qj constant at OV.
8.6 METER AMPLIFIER
In order to indicate the 0.01W-120W range
with one meter without range switching, a loga¬
rithmic indication type meter must be employed
and the input signal must be logarithmically
compressed. The meter amplifier circuit is shown
in Fig. 6 .
This circuit consists of a logarithmic compres¬
sion circuit and a meter drive circuit. The output
signal of the power amplifier is applied to the
logarithmic compression circuit, and its dyna¬
mic range is compressed.
The principles of the logarithmic compression
circuit are given in Fig. 7. The output voltage of
this circuit is the value divided by R 9 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 Dg and
applied to Qi of the meter drive circuit. Qi
current amplifies the DC voltage from Dg to drive
the power meter.
Fig. 7 Schematic diagram of logarithmic compressor
21

8.7 PROTECTION CIRCUIT
The purpose of this circuit is to protecty both
the speakers and the power amplifiers. The relay in
the output circuit is automatically opened in any
of the following cases:
1. During the “transient operations” when the
power supply is turned on and off.
2. Upon detection of an overload, caused by a
short circuit in the load.
3. Upon detection of a DC voltage in the output
circuit, caused by component failure or ac¬
cident.
An outline of this circuit is shown in Fig. 8-1.
The relay-activating transistor (Qr) is controlled by
the IC {PA3004).
Muting Operation When Power Supply Is turned
Off and On (Fig. 8-1)
When the power supply is first turned on, the
voltages on pins 1, 7, and 6 of PA3004 will exceed
a prescribed level. If there is no input (DC) on pin
no.4, S 2 will be off, and a charging current will
commence to flow to the timing capacitor (Ct)
connected to pin no. 8 . Once Ct has been charged
up to a level where the voltage on pin no .8 exceeds
a prescribed level, S, will turn on, thereby applying
a bias current from pin no.3 to the relay driving
transistor (Qr). Consequently Qr will turn on, and
current will flow through the relay coil to activate
the relay, and close the connection in the output
circuit. The time required for this connection to
close after the power supply is first turned on is
several seconds. During this period, any unwanted
transient noises will be therefore muted.
When the power supply is turned off, the input
(AC) applied to pin no.7 ceases immediately,
resulting in Sj turning on, Ct discharging rapidly,
and Si and Qr both turning off. The relay is thus
opened, disconnecting the output circuit.
DC Voltage Detector(Fig. 8-1)
The output circuit is connected to pin no.4
via a low-pass filter (Rg and Cj). Any DC voltages
appearing in the output circuit will also be applied
to pin no.4, turning S 2 on. Ct will thus discharge
rapidly, turning S, and Qr off, thereby releasing
the relay, and disconnecting the output circuit
from the load.
Overload Detection
The overload detector circuit incorporates the
load (RL) in one side of a Wheatstone bridge
(see Fig. 8-2). The base and emitter of a sensing
transistor (Qi) are connected to the opposite
corners of the bridge, so if RL decreases, Q| will
become forward biased. If RL falls below a pre¬
scribed value, Qi will turn on, thereby passing a
current through Rs, D 3 and R^. Due to the voltage
difference generated across R^,, Q 2 will become
forward biased, and consequently turn on. A DC
voltage will then be applied to pin no.4, turning S 2
on, and resulting in the rapid discharge of Ct, and
S, and Qr both turning off. The relay will again be
released to disconnect the output circuit.
8.8 POWER SUPPLY
The power amplifier and power stage plus and
minus supply voltages (±62V) are obtained by
means of a bridge full-wave rectification system.
22000/71 X 2 electrolytic capacitors are used.
Plus and minus voltages are supplied to the small
signal circuit of the AF Section thru a constant
voltage circuit by full-wave rectification from a
winding separate from the power stage supply.
Tuner section, lamp circuit and protection circuit
power is supplied thru transistors Darlington con¬
nected ripple tilter, after full-wave rectification.
S3