Philips FW2017 - Amplifier and Tuner Operations; Tuner Operation and Presets

Current
limitation:
If
the
emitter
current
through
TS7304
increases,
the
voltage
at
R3312
will
also
increase.
Via
the
potential
divider
R3316/3314
the
latter
also
reaches
the
Bcsssa
7316
base
of
TS7318.
Once
the
threshold
level
oo is reached, TS7318 switches through. |
BDTS638 Consequently, the base of TS7304 beco-
nee
mes
more
negative
than
the
emitter
and
blocks.
B8CS48B
33290
22arR
Fig. 2
Signal path:
The
circuit
shown
in
Fig.
3
corresponds
in
principle
with
the
circuit
in
the
manual,
but
components
which
are
not
|
absolutely
essential
to
its
basic
understanding
have
been
omitted.
The transistors 7310 and 7316 are to be
considered
as
switches
rather
than
am-
sce’
plifier
elements.
The
power
supply
is
switched on and off via the +STBY and
-STBY voltages respectively.
If
the
preamplifier
(IC7322)
is
modulated,
a
current
flows
across
its
load
resistor
(3340),
which
in
first
approximation
is
calculated
from:
Signai iin
4
SPEAKER
é u,*V,
ee eo tu
R,
aoT638
Vy=
voltage
amplification
of
1C7322
Ug=
voltage
between
pin
2
and
3
Ra=
R3340
3312
A186
This
current
must
be
supplied
from
the
-vecs
power
supply
and
causes
an
AC
voltage
drop
at
resistors
3324
and
3320
which
modulates
the
output-stage
transistors.
Fig.
3
Mute:
The
preamplifier
is
muted.
If
the
base
of
TS7320
is
HIGH,
it
switches
through
and
short-circuits
the
input
signal
of
the
preamplifier
(7322).
Muting
occurs
when:
a)
the
astable
monoflop
is
active
(to
avoid
plops
when
switching
off
the
output
stage).
b)
the
headphones
are
connected
(base
of
TS7284
goes
to
LOW
via
connector
1256.
TS7284
biocks.
The
collector
is
HIGH,
so
the
base
of
TS7320
is
also
HIGH.)
c)
During
the
various
switching
operations
(source
selector)
a
MUTE
signal
(LOW
active)
is
supplied
via
pin
8
of
1251,
so
that
the
base
of
TS7284
goes
to
LOW.
10 CS 57 058