Kenwood KR-7400 Service Manual page 9

CIRCUIT DESCRIPTION
PROTECTION CIRCUIT
Current limiter protection operates out of transistor's SO A
(Safe Operating Area). This protection circuit is accom¬
plished by detecting the ic of power transistor. Safeguard
against overcurrent is decreased by the bias on the
complementary stage. For DC drifts of the center voltage
level, a relay is employed to cut the speaker line out of
service when the center level drifts more than ±7 volts.
This protective action, as well as the SOA protection
mentioned above, is self-return. All these protective
schemes operate free from the influence of speaker load
impedance. Confirm the current limiter protection to
operate. The following is the method: connect the dummy
resistor to both speaker terminals, and the oscilloscope
across the dummy resistor of the left channel.
And then feed the signal (1 kHz) to AUX jack of the
receiver. Next short-circuit the right speaker terminal, and
the left output is increase. Other channel in the same.
L-ch waveform
L-ch waveform
When R-ch is shorted
MUTING CIRCUIT
When the power switch is on the protection relay does no^
operate to eliminate the shock noise from speaker. And
then the relay operates after power switch is on.
▲ MAIN AMP (X07-1270-10)
SOA protection DC drift protection
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