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Onkyo TX-4500 - Protection Circuit Explanation; Steady function; Detection of abnormal voltage; Detection of abnormal current

Onkyo TX-4500
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5.
The
Explanation
of
Protection
Circuit
1.
Steady
function
While
the
B
voltage
is
supplied
to
the
protection
circuit
as
the
power
source
switch
is
turned
on,
it
is
so
designed
as
to
allow
a
lag
of
almost
4
seconds
in
the
time
required
for
the
relay
to
be
turned
ON,
by
means
of
the
time-constant
circuit
set
to
motion
by
R
716
and
706
so
that
a
drop
in
the
output
may
be
prevented
of
the
main
circuit
in
the
unsteady
state.
In
the
steady
state,
transistors
of
Q
701
to
Q
705
are
cut
off,
so
much
so
that
the
voltage
at
both
ends
of
C
706
is
caused
to
register
a
value
of
6
by
the
function
of
R
720
and
D
709,
causing
the
transistor
of
Q
706
and
the
delay
to
be
turned
ON,
bringing
forth
the
output.
Again,
D
709
is
designed
so
as
to
keep
both
ends
of
the
relay
coil
from
being
brought
under
the
pressure
of
abnormal
voltage.
In
addition,
1S
1554
between
the
base
collectors
for
respective
transistors
is
designed
to
thwart
inverse
current.
2.
Detection
of
abnormal
voltage
As
the
equivalent
of
2
or
more
is
generated
at
the
center
line
of
the
main
amplifier,
there
is
a
mixing
between
the
right
side
and
the
left
side
at
R
701
and
702,
with
the
alternate
current
portion
being
cut
off
at
C
701
and
702.
In
the
case
that
the
direct
current
generated
here
is
negative,
the
current,
passing
through
D
701,
is
broken
up
into
partial
pressure,
acting
on
the
base
of
the
transistor
at
Q
702,
which,
in
turn,
causes
Q
702
to
open
up
allowing
the
current
to
pass
through
in
the
sequence
of
D
706-—
R
707
——
R
708.
As
a
result,
the
portion
of
voltage
generated
by
R
708
is
caused
to
be
reinforced
to
Q
705,
rendering
the
transistor
to
be
turned
ON.
This,
in
turn,
causes
the
voltage
of
the
collector
at
Q
705
to
fall,
thus
making
the
base
potential
of
Q
706
go
deeper
into
the
negative
side
and
turning
the
Q
706
OFF.
This
in
turn
causes
the
relay
to
be
turned
OFF,
and
also,
the
output
circuit
to
be
cut
off.
In
the
event
the
base
potential
is
on
the
positive
side,
the
similar
function
takes
place
within
the
loop
of
Q
721
——
D
202
—R
707——
R
708.
By
eliminating
a
cause
or
causes
for
DC
generation
at
the
center
line,
the
relay
is
caused
to
turn
ON
by
the
function
staged
in
reverse
order
of
the
description
given,
causing
the
proper
function
to
start
automatically.
3.
Detection
of
abnormal
current
When
the
abnormal
current
is
generated
on
the
driver
stage
(power
stage)
of
the
main
amplifier,
the
voltage
is
detected
by
the
collector
resistance
and
added
subsequently
to
semi-solid
resistance
R
537
and
637
which
are
designed
for
current
detection
in
the
protection
circuit.
(The
semi-solid
resistance
is
set
to
function
with
the
adequate
flow
of
current
causing
the
protection
circuit
to
start
operating.
Refer
to
the
Chapter
dealing
with
Adjustment.)
By
the
voltage
reaching
Q
704
by
way
of
D
536
and
D
636,
Q
704
is
caused
to
be
turned
ON
while
R
705
witnesses
a
voltage
drop,
resulting
in
Q
703
being
turned
ON.
As
a
result,
the
current
flows
in
the
sequence
of
Q
703-—R
711—D
707-—R
708,
and
by
the
voltage
generated
at
R
708,
@
705
is
turned
ON
while
Q
706
is
turned
OFF,
causing
the
relay,
in
turn,
to
be
turned
OFF,
thereby
cutting
off
the
output
circuit.
By
the
voltage
added
to
the
base
of
Q
704
through
R
713,
on
the
other
hand,
Q
704
is
turned
ON
and
on
account
of
this,
the
lock
circuit
is
set
to
work
by
Q
704
and
Q
703,
causing
the
realy
to
be
kept
in
the
OFF
state.
In
the
event
a
large
current
flows
into
the
transistor
of
the
main
circuit,
the
said
circuit
is
caused
to
start
functioning,
locking
up
the
amplifier
output
in
the
OFF
state.
For
this
reason,
it
is
necessary
for
the
power
source
to
be
cut
off
before
setting
out
to
remove
the
cause
for
abnormal
current.
Turning
the
power
source
back
on
will
set
off
the
normal
function.
It
should
be
noted
that
turning
the
lock
circuit
on
immediately
after
the
power
source
was
turned
OFF
will
not
lead
to
a
spontaneous
release
of
the
lock-up,
and
therefore,
it
must
be
assured
that
the
lock
circuit
is
turned
ON
a
few
seconds
after
the
power
source
has
been
turned
OFF.

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