Kenwood KR-4070 Service Manual page 11

CIRCUIT DESCRIPTION
ASO PROTECTION CIRCUIT
The basic function of the ASO protection circuit is such that it
detects an overcurrent flowing into the emitter resistor on the
positive side of the power transistor, and leads the positive
signal and +Vcc of the differential amplifier to the GND side,
thus protecting the power transistor from overload.
The transistor Qm19 is the detecting transistor. If an
overcurrent flows, the voltage across the emitter resistor
Rm47 is divided by the resistors Rm69 and 71, so that the
voltage appears across the base and emitter of Qm19.
When this voltage exceeds 0.6V and Qm19 is ON, the
voltage at the base of Qm21 which is connected to the
collector of Qm19 via a resistor decreases. Since Qm21
turns to ON, the voltage at the collector of Qm21 reaches
+Vec, and Dm7 connected to the base of Qm22 is reverse
biased, Qm22 turns to OFF. This increases the base voltage
of Qm24. towards negative and thus Qm24 turns to ON, in
turn, the base voltage of Qm23 increases. Qm23 is ON.
Since the positive signal and +Vcc of the differential
amplifier are connected to the collector of Qm23, no signal is
fed to the complementary stage and, hence, the power
transistor is protected from overload. (Fig. 11)
Without Dm17
LEVEL
, With Dm17
Power SW Power SW ——
OFF ON
Fig. 8 SHOCK NOISE CURVE
Qm19 Qm21
Excessive [ON] Ei]
Qm22
OF
The capacitor Cm33 is charged and discharged repeatedly
through Rm73 and Dm8 when AC voltage at the output side
turns to positive and negative so that Qm19 is not energized
by a large input level. (Fig. 9)
Fig. 9 Cm33 FUNCTION
Cm38 and Rm79 are used to eliminate shock noise which
may be generated when the power SW is set to ON.
(Fig. 10)
When the power SW is turned to ON, Cm38 is charged
gradually. Qm22 remains OFF until Cm38 is fully charged
because Dm7 is reverse biased and leads the signal and
+Vcc of the differential amplifier to the GND side to
eliminate the shock noise. When the power SW is turned to
OFF, the voltage +Vcc is discharged gradually. Note that a
shock noise will be generated if the power SW is turned ON
again before the voltage is fully discharged. However, the
discharge time is shortened by the use of Dm17 at the input
of the differential amplifier. (Fig. 8)
+Vec
Rush current
Cm38
Qm22
~Vec
Fig. 10 SHOCK NOISE ELIMINATION CIRCU IT
+Vec
Differential
amp.
of differential amp
short to GND.
Fig. 11 ASO PROTECTION FUNCTION
11
KR-4070