Pioneer ARP-230-0 Manual de servicio - Página 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 25 watt output power IC
STK4141-2S (See Fig. 5-a).
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
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 Dl5, +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 t1 is longer than t2 so that imme-
diately after the power is turned on, the emitter
potential of Q15 exceeds its base potential, QL5 is
turned on to activate Q16 and Q17 and ground the
signal. A few seconds later, the base potential of
Q15 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
S P E A K E R
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F i g . 5 - 2
D C V o l t a g e p r o t e c t i o n c i r c u i t
POWER AMP ( L ch )
M A I N
V O L U M E
B A L A N C E
V O L U M E
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D t 7 D r 6 R t S l
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- J
i
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Fi g. 5 -3
P o w e r ON /OF F mut ing ci rcui t