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5.3.
INDICATOR
CIRCUIT
The SA-8800 output power and function indica-
tors feature fluorescent indicator tube (FL tube).
In
this
tube,
thermionic
emissions
from
the
cathode are accelerated
into the fluorescent
sub-
stance
of the segmental
anodes, resulting in the
emission
of light. This
tube is used to indicate
numerals, letters, and other symbols.
An outline of the FL tube drive circuit is shown
in Fig. 5-10. The output circuit signal is first passed
through a low-pass filter and a compressor circuit
before
being applied to pin no.6
(4) of the IC
(TA7318P-A).
The
compressor
circuit makes use
of the non-linearity
of the rising portion of the
diode's
Vd-—Id
characteristics
to
contract
the
signal dynamic range by 20dB. The IC contains a
detector
circuit,
compressor
(40dB),
and
peak
COMPRESSOR
SIGNAL
IN
LPF
(20dB8)
SA-8800
hold circuit for both left and right channels. The
dynamic range of the signal is thus contracted by
60dB 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 differential
amplifiers. These amplifiers are biased at increasing
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 successive 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 and TAPE MONITOR
switches.
IC
(TA73I8P-A)
1/2
COMPRESSOR!
HOLD &
)
(7)
3H
IC
(HAI2ZO10 )
O
FLUORESCENT
O
FUNCTION
FILAMENT
eo
O~O
©
TAPE
MONITOR
Fig. 5-10
tNDICATOR
TUBE
Indicator Circuit
13
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