Pioneer SX-303L/HEZ Instrukcja serwisowa - Strona 7
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AM Tuner
This' uses
a
variable
capacitor
type
tuning
circuit
composed
of three
transistors,
a 2-tran-
sistor
front
end
and a
single transistor AM
IF
detector.
AF Section
The equalizer circuit has a low-noise operation
amp (2 channel) M5218P.
The power amp has a 45; watt output power IC
STK4141-2S (See Fig. 5-4).
|
The tone control circuit is placed in the negative
feedback loop of the power amp.
Protective Circuits
This unit has a circuit to detect DC voltages at
the power amp
outputs and a muting circuit that
operates when power is turned on and off.
Fig.
5-2
shows
the
construction
of the
DC
voltage
protection
circuit.
When
a DC
voltage
VOLUME
BALANCE
VOLUME
TO POWER
AMP
appears
at the power
amp
output,
either
Q13
(plus) or Q14 (minus) is activated depending on
the polarity
of the DC
voltage and a trigger is
applied to D15. This turns on D15, +B1 is shorted,
the fuse on the primary side is cut and the power
supply circuit is shut off.
Fig. 5-3 shows the power on/off muting circuit.
Time constant t, is longer than t, so that imme-
diately after the power is turned on, the emitter
potential of Q15 exceeds its base potential, Q15 is
turned on to activate Q16 and Q17 and ground the
signal. A few seconds later, the base potential of
@15 rises to turn off Q15 along with Q16 and Q17.
When
the power
is turned
off, the charge
of
C161 passes through D18; it is almost completely
discharged in an instant to make the base potential
of Q15 zero. However, because the charge of C160
remains, Q15 is turned on and muting is activated
in the same manner as when power is turned on.
TO
SPEAKER
SWITCHES
+B3
tr,t2
*
time
constant
Fig. 5-3.
Power ON/OFF muting circuit