11~.L
06283-3
FAST OFF CIRCUIT (Mutes
the
channel)
The
circuit
consisting of Q109, Rll..5,
Rll7, Rll8,
0107,
0108
and
0109
tum
the
current
off
at
the
input
stage,
(Q10l) when
the
AC
power is
turned
off.
Under
normal
operating
conditions Q109 is biased off and
the
-33V power supply is up and
approximately
7V
DC
is
on pin
10
of
J 101. This
7V
DC forward biases
0109
and
allows
current
to
flow through
Rll8
to
the
negative
power supply. This
reverse
biases
0108
removing any forward bias
to
Q109 and when
AC
power is
turned
off
the
DC
voltage
on pin
10
of
J10l
goes
to
zero.
This allows
current
to
flow through
R117,
0108
and
Rl18.
This turns on Q109, which turns
off
the
current
source (-
33V
power supply) for
QlOl
and muting
the
amplifier.
SPEAKER PROTECTION DC
circuit
This
circuit
shunts
the
loudspeaker load
at
frequencies
of 10Hz
or
lower
at
the
amplifiers full
output
swing, or if
the
amplifier fails. (Either
of
the
above
conditions looks like DC
at
the
loudspeaker terminals). A signal
appearing
at
the
output
terminal
passes through R163 and is
intergrated
by C122, if
this
signal level
exceeds approximately 8 volts,
the
bidirectional switch Q 129
conducts
and triggers
the
triac
Q 128 shunting
the
amplifier
output.
If
this
signal is
transient
in
nature,
the
amplifier
output
voltage
reverses thus,
communtating
the
triac.
If
the
signal is
DC
in
nature,
the
triac
Q128 stays
turned
on until power supply or
the
amplifier
is
turned
off.
The
"B"
Channel works in
the
same
manner as
the
"A" Channel described, but,
reference
numbers used
are
200
series.
BRIDGED
MODE
When
the
Mono/Stereo switch
S..502
is
set
to
Mono,
it
converts
the
"B" Channel
amplifier
to
a unity gain inverting power
amplifier.
S..502
Mono/Stereo switch grounds
the
normal input
to
"B" channel and
connects
the
output
of
the
channel "A" amplifier
to
drive
the
"B"
channel summing point R219.
DISPLAY METER
The l2V
AC
input
to
J404
&.
J40..5
is full wave
rectified
by
0408-0411
and
integrated
by C401
to
give approximately
17V
DC.
0407
provides 7 Volts DC
to
J403
the
fast
off
circuits
on both modules.
POWER ON LEDls
0403,
D406,
R4l9
and R420
are
connected
in
series
across
the
l7V DC
to
light both
green "power on" LED's
0403
and
0406.
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