Circuit Description - Onkyo A-10 Service Manual

CIRCUIT DESCRIPTION
|. SPEAKER PROTECTION CIRCUIT
The speaker protection circuit is operated:
|) When the B circuit is unstable when the power is turned ON (approximately 5 seconds),
2) When the speaker terminals are shorted and abnormal current has flowed in the power amplifier thru this low im-
pedance.
3) When the center voltage has increased because of trouble at the differential amplifier, etc.
4) When the temperature of the heat sink has risen.
Rol Q17 Q703 Q704, Q705 Q706
CURRENT PROTECTION SCHMITT HOLD
DETECTOR TRIGGER CIRCUIT
of Power
- PROTECTION
level SPEAKER INDICATOR
RELAY SWITCH
NEGATIVE RL701
VOLTAGE
INDICATOR
from
Power
Amp.
POSITIVE
VOLTAGE
DETECTOR
Protection Circuit Block Diagram
When Q703 is turned on by voltage detection or current detection, Q704 is turned ON by the voltage drop across R709.
Q704, Q705 constitute a digitalized, fast response Schmitt trigger circuit. When Q704 is turned ON, Q705 is turned OFF.
Q705 is a relay drive transistor. When it is turned OFF, the relay is also turned OFF.
When the power switch is turned ON, charging current flows thru the loop R710 > C703 > R708-'R709 and Q704 is
turned ON by the voltage drop across R709. Consequently, Q705 and the relay are turned OFF until the charging
current drops below a certain value. When the power switch is turned OFF, the B voltage falls and C703 is quickly dis-
charged thru the loop C703 > R710 > D701. During normal operation, C703 is charged to almost the B voltage. But since
the saturation resistance of Q7Q3 is sufficiently low, when Q703 is turned ON, C704 is quickly discharged thru the loop
C703 > R710 > Q703 and the relay is also turned OFF. The relay is not turned ON again thereafter until C70 3is charged,
even if the set should return to normal and Q703 is turned OFF.
HOLD CIRCUIT
The reference voltage is produced by R716, R717, Q706 is operated as a comparator. When Q705 has been turned OFF,
the collector voltage of Q705 rises and C706 is charged. Therefore, when C706 is charged to above a certain voltage
relative to.the reference voltage at the junction of R716 and R717, Q706 is turned ON, Q703 is turned ON thru R728 and
the circuit is held.
CURRENT DETECTOR
Q17 is turned ON by the voltage detected from the collector circuit of the power amplifier. C24 prevents erroneous
Operation, R61 sets the current detection level and thermister R66 controls changes in the current detection level caused
by tempeature rise.
When the impedance is low at a certain frequency of the speaker, the protection circuit may be unexpectedly actuated each
time a large audio signal of that frequency has entered. However, when this occurs the relay is opened and the power ampli-
fier current returns to normal. The power amplifier current is also automatically returned to normal in a like manner when
the load has been inadvertently shorted momentarily.
When connected with the load shorted, the relay is repeatedly turned ON and OFF in load short > relay OFF (no load) >
automatic reset (load short current detection) > relay OFF order. Since the OFF time is sufficiently longer than the relay
ON time in this case, the voltage across C705 gradually increases until a voltage sufficient to turn Q706 is reached, at which
time the relay is held OFF, thus protecting the power transistor against damage by a continuous overcurrent.