Sharp OPTONICA SM-1616H - Circuit Description - Muting and Thermal; Muting Circuit Functionality

= MUTING CIRCUIT (See Figure 8)
tS+5 55
MAIN AMP.
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49401
R404
PTHEO1 t 33K
POWER TRANS. {T904)
TONE AMP. (305)
TONE AMP. (0306)
+
CSOT
100/50V
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RS
ry
7. 6e
R524 56K
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C502 Nace card kK Dez bs cai
3.3/50V 15
IC-50:
Q504 =a
Figure 8
The electronic muting circuit is composed of transistors
Q401 to Q404, diodes D401 to D403 and other resistors
and capacitors and it works to prevent an occurrence of
noises when the power switch SW901 is turned on or off,
and to eliminate an abnormal thermal increase of the heat
sink.
When the power switch is turned “ON”
The muting circuit works as follows;
A current runs in the bases of Q403 and Q404 through R401
and thus inputs to the main amplifier are shorted so that the
signals of the tone amplifier won’t enter the main amplifier.
At the same time, a current also runs in R402 and C401
and so the base potential of Q401 increases until the Q401
is eventually turned on, so that a collector current will be
produced. As a result, a voltage drop of R401 becomes drastic
so that there will be no current caused at the bases of Q403
and
Q404.
Thus
it
is
allowed
for
the
main
amplifier
to
receive signals.
When the power switch is turned “OFF”
The muting circuit works as follows;
The negative potential which has been charged in C402
becomes zeronized instantly and so the emitter potential
of Q401 is increased to turn off the Q401. As a result, a
current runs in the bases of Q403 and Q404 so that these
transistors are turned on; this results in that no signal will
enter the main amplifier.
Q402 is to discharge the C401 which has been once charged
up when the power switch was turned off.
When temperature of the heat sink abnormally increases
The muting circuit works as follows;
If the heat sink of IC501 and [C502 are cver-heated,
the resistance of PTH601 is suddenly increased. As a result,
the current of Q401 is reduced while its collector potential
is increased so that the transistors Q403 and Q404 are turned
on; this results in that no signals will enter the main amplifier
so that an abnormal thermal increase of the heat sink can be
eliminated.