Realistic STA-115 - Circuit Description; Protection Circuits

CIRCUIT
DESCRIPTION
PROTECTION
CIRCUITS
The output from
tone
amp
circuit to main
amp circuit
isswitched
on/off
with
two
FETsTR601
and
TR602.
When
pin
10 of 1C601 is high,
the FETs
are
on; when low,
FETs
are off.
The switching
will take
place
in
the following
three
instances.
1
. Tu
rn
ing the receiver
on
and off
(
F igure
1
)
When
the
power
is
switched
on, both
pins
1 and 2
of
1C601 are
high,
so
pin
3 also becomes
high
and
hence
pin
4
goes
high.
After the
delay time
set
by
the
time constant
R607
and
C608,
pin
10 of
1C601
becomes
high
and
signals
from the tone
amp
are fed
to
main
amp.
This time
constant R607
and C608 is
required
to
keep
the
speakers
off to
protect
them
from
the
switching
transients.
At
power
off, DC
voltage
will not
turn
off
immediately,
so the
AC
portion
is
taken
from the
secon-
dary
of
the
power
transformer
via
D606
to
pin
1 and
2.
When
this
goes
low,
pin
3
goes
low,
C608
is
discharged
by D602,
and
pin
10
goes
low.
lo
TONE
AMP
(L)
lo
MAIN
AMP
(
L)
A
!
I
I
lo
B+
to B-
D6O6
D6O2
Figure 1
2. Changing
the
selector
switch
position
(Figure
2)
This
prevents
the
switching
noise
from
going
to
the speakers.
The selector
switch is
a shorting type;
when
it is turned
from
one
position
to another,
the
switch
contact
will
make a momentary
short
circuit to
ground.
This low
level is
applied
to
pin
12
of
1C601. Pin 11
goes
low,
pin
10
goes
low
through the
time constant
circuit
C607, D603
and
R608.
The
time constant
is required
so
the
cut- off condition lasts for
about
0.3
to 0.6 seconds.
,o
MAIN
AMP
(R)
-12-