Philips FW2017 - Advanced Features and Connections; Timer, Phones, and Tuner Panel; Cassette & CD Function Buttons

POWER BOARD
The
output
stage
is
switched
on
via
the
voltages
+STBY
and
-STBY.
These
voltages
are
derived
from
the
voltages
+VCC1
and
-VCC1,
respectively,
and
can
be
switched
off
via
the
astable
monoflop
(TS7287,
7290).
This
astable
monoflop
has
the
task
of
switching
off
the
output
stage
in
the
event
of
a
fault
(short-circuit
or
DC
at
the
output).
if
a
brief
fault
occurs,
then
the
output
stage
will
be
switched
off
for
around
4-5
seconds
(charging
time
of
C2276
via
R3288).
In
the
case
of
prolonged
DC
voltage
at
the
output,
TS7287
will
operate
as
a
switch
(see
faults
+/-
DC
at
the
output)
and
switch
off
the
output
stage
for
the
duration
of
the
fault.
Without
fault
is
the
state
of
the
monoflop
whereby
TS7290
is
conducting
and
is
blocking
transistor
7287
via
R3291.
As
a
result,
the
collector
of
TS7287
is
HIGH
(approx.
+11V).
Via
R3286
the
base
of
TS7280
also
becomes
HIGH
and
switches
the
latter
through.
This
causes
TS7279
and
7282
to
block.
Transistors
7278
and
7283
are
conducting
and
switch
through
the
voltages
+STBY
and
-STBY
respectively.
Faults:
+DC at the output: Via D6287 a positive voltage arrives at the base of TS7287, causing the collector of TS7287
to become LOW. TS7280 blocks. Via the potential divider R3279/R3281 a voltage of approx.
1V presents itself at R3281. The result is that a voltage of approx. 0.7V lies on the base of
7S7279 and the emitter of TS7282 respectively. The transistors switch through and thus
block TS7278 and 7283. The voltages +STBY and -STBY are not switched through and the
output stage is not operative.
-DC
at
the
output:
Via
D6290
a
negative
voltage
arrives
at
the
base
of
TS7290,
causing
it
to
block.
At
the
collector of
TS7290
there
now
reside
around
+20V.
TS7287
is
switched
through via
R3291,
causing
the
collector
of TS7287
to
become
LOW.
TS7280
blocks and
switches
the
unit
to
standby.
+/-DC
pulse:
Normally,
there
are
around
3V
DC
at
C2276.
If
TS7287
is
now
brought
into
conductance
by
a
pulse
(+DC
across
D6287,
-DC across
D6290/TS7290),
then
the
end
of the
elco,
which
had
previously
been
charged
to
+11V,
is
suddenly
on earth. As
the
other
end
of
the
elco
was around
3V more
negative,
a
negative
voltage
now
lies
at
the
base
of TS7290
and
blocks
it.
Consequently,
around
+20V reside at
the
collector
of TS7290.
The
+voltage
arrives
at the
base
of
TS7287
via
the
potential
divider R38291/
R3274+3249
and
keeps
it
in
conductance.
Meanwhile
C2276
is
charged
via
R3288.
After
4-5 seconds
the
voltage
at
C2276
has risen
so
far
that
transistor
7290
is
switched
through
again
via
R3289,
causing
transistor
7287 to be
blocked again via
R3291.
Short-circult at the output:
The principle on which the circuit operates is explained only on the basis of the right-hand channel and is of
course also valid for the left-hand channel and the bass channel.
in normal operation, with every positive half
wave via the feedback the base of TS7308
becomes more positive than the emitter. The
transistor remains blocked. D6302 prevents
switching through during the negative haif wa-
ve.
If during the short-circuit at the output the si-
gnal being fed back falls away, in the case of
the positive half wave the emitter becomes
more positive than the base. TS7308 is con-
ducting. D6308 is for a positive pulse in the
conducting direction. Via R3274 this positive
pulse reaches the base of TS7287 and swit-
ches the latter through. The astable monoflop switches off the output stage, as described under fault +/-DC pulse,
for 4-5 seconds. Due to threshold voltages (7308, 6308, 7287), this short-circuit interruption will only operate from
around 1.4Veff at the output of IC7322.
7322
Feedback
Fig. 1
9 Gs)