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Technics SA-203

Technics SA-203
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_SA-203
|
M@
TECHNICAL
GUIDE
The
power
amplifier
!C
(SV!STK2028)
is
provided
with
a
protection
circuit.
1.
Muting
(shock
noise
prevention)
with
power
switch
ON.
(1)
When
power
is
supplied,
negative
voltage
is
applied
to
the
emitter
of
switching
transistor
(Q703).
(2)
Positive
voltage
flows
from
the
regulator
(Q707)
to
R709
and
R708
to
turn
D705
and
Q703
ON.
(It
takes
a
few
seconds
until
0703
turns
ON
due
to
time
constant
of
C706).
(3)
When
Q703
turns
ON,
negative
voltage
is
applied
to
the
pin
©)
of
input
amplifier
1C
(AN7O60)
causing
the
input
amplifier
to
operate.
*
The
signal
is
applied
to
the
power
amplifier
a
few
seconds
later
so
that
shock
noise
can
be
prevented.
2.
Muting
(shock
noise
prevention)
with
power
switch
OF
F.
(1)
When
power
supply
is
cut
off,
the
voltage
applied
to
Q703
drops
at
the
base
faster
than
the
negative
voltage
at
the
emitter
connected
to
the
large
capacity
condenser,
thus
turning
off
D705
and
Q703.
(2)
Input
amplifier
is
off
with
708
off.
*
The
signal
is
cut
off
on
the
input
side
while
the
operation
voltage
of
power
amplifier
still
remains,
thus
preventing
shock
noise.
3.
Operation
of
the
protection
circuit
with
speaker
terminal
short-circuited.
(1)
Transistor
Q7
in
the
power
amplifier
|C
(SViISTK2028)
is
usually
OFF,
but
when
the
speaker
terminal
is
short-circuit-
ed
to
the
ground,
a
large
amount
of
current
flows
to
Q6,
causing
a
voltage
to
be
generated
at
R5.
(2)
The
voltage
causes
Q7
to
turn
ON,
then
(Ii)
flows
to
raise
the
base
potential
of
702.
And
Q702
turns
ON.
(3)
As
Q702
is
ON,
®
flows
to
generate
a
voltage
at
R707,
and
then
0701
turns
ON.
(4)
G)
flows
with
Q701
turned
ON,
®
flows
to
hold
Q702
ON.
(5)
The
current
flowing
in
R709
increases
due
to
),
and
the
positive
voltage
from
regulator
(0707)
drops,
causing
D705
to
turn
off.
(6)
tn
other
words,
Q703
turns
OFF
and
the
negative
power
supply
to
the
input
amplifier
is
discontinued.
*
This
condition
continues
until
power
supply
switch
is
set
OFF.
A
few
minutes
after
the
power
switch
is
set
to
ON,
if
no
sound
is
produced
while
the
operation
is
normal,
cut
off
the
power
supply
and
check
the
speaker
connections
and
the
input
equipment.
(C601
©.
{C603
(SVISTK2028)
Power
Amp.
1C601,
602
AN7060
Differential
Amp.
1C602
Ck
a
eS.
ZO:
(}
oe
Se
oe
oS
ee
eee
Ive
ee
Nel
|
ERE
From
regulator
(0707)
Ov

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