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and C; time
constant.
Consequently,
Q, emitter
voltage
will rise by increase
of the base voltage.
Therefore,
the
muting
action
when
the
power
switch
is turned
on
is achieved
by delaying the
supply of the +B,
to the equalizer amplifier and
the first stage of the power amplifier.
When the power switch is turned off, the —B, is
discharged through
the route R,;—> C,, and since
the residual
voltage
of C, is then applied to the
base of Q, through R, and R;, Q; is immediately
turned on. Therefore, the base voltage of the Q,
drops to OV, and the +B, supply to the equalizer
amplifier and the first stage of the power amplifier
are interrupted.
Indicator Circuit
The SA-5800
output power
and function
indi-
cators
feature
fluorescent
indicator
tube
(FL
tube).
In this
tube,
thermionic emissions from
the cathode
are
accelerated
into the fluorescent
substance of the segmental anodes, resulting in the
emission of light.
This
tube
is used to indicate
numerals, letters, and other symbols.
IC (TA73IGP)
172
SIGNAL
INO
FLUORESCENT
FILAMENT
SA-5800
An outline of the FL tube drive circuit is shown
in Fig. 4-2. The output circuit signal is applied to
pin no.
6 (4) of the IC (TA7318P-A).
The IC
contains
a detector
circuit,
compressor
(40dB),
and peak hold circuit for both left and right chan-
nels.
The
dynamic
range
of the
signal
is thus
contracted
by 40dB
to obtain a ''peak held" DC
voltage.
The output power indicator segments of the FL
tube
are
driven
by the HA12010
ICs (one for
each channel) equipped
with 12 pairs of differen-
tial amplifiers. These
amplifiers are biased at in-
creasing
levels, so each
amplifier
will commence
to operate separately as the input level increases.
And
since these
amplifiers
apply the voltages to
the
output
power
indicator
segments,
each
suc-
cessive segment will light up in turn as the input
level rises.
The function indicators are lit up as a result of a
voltage
being applied
to the corresponding func-
tion indicator
segment
according
to the selected
positions of the FUNCTION
switch.
INDICATOR
TUBE
Fig. 4-2
Indicator circuit
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