Protection Circuit 1 - Pioneer SX-750 Service Manual

5 . 7 P R O T E C T I O N C I R C U I T
This protection circuit functions to protect the
speakers and the power amplifiers from damage
due to short-circuit of the load, etc., and performs
a muting operation to cut noise and distortion
which occur when switching the power on and off.
The circuit is shown in Fig. 15, and consists of a
bridge type over-current and overload detector, a
differential amplifier DC voltage detector, and a
power switch ON/OFF detector section.
Relay Driving Circuit
Q4-Q6,
in Fig. 9, comprise the relay driving
circuit.
In the normal condition reverse bias is applied to
the base of Q4, and Q4 is in a off state. When
one of the above mentioned detection circuits
goes or, current flows through R11, the base
potential falls and Q4 is turned on. Consequently
Qb comes on and Q6 goes off. When Q6 goes off,
the current of the relay circuit is cut, to release
the switch of the output circuit.
When the power switch is turned on, a delay
operation occurs in this circuit. R17 and C3, in the
base circuit of Q6, are the time constant elements
which determine the delay time. When the power
switch is tumed on, C3 charges to a potential of
+60 volts through R17 and R18, and Q6 is kept in
the off state during this time. When the power
source is switched off, the muting operation of Qb
prevents shock noise. In the normal condition, the
potentials of +33 volts and - 5.1 volts are applied
to Qb through RL4 and R15. The resultant poten-
tial at the base of Q5 is -1 volt in the cutout
condition. When the power supply is turned off,
PoY|er amplifier
0Yer-current
and overload
detection circuit
of - 5.1 volts disappears immediately due to the
small time constant of the power circuit. Thus a
positive base potential remains, switching Qb on,
which in turn switches off Q6 and hence the relay.
Detection of DC Voltage
This is a differential amplifier consisting of Q2
and Q3, as shown in Fig. 10. The bases of Q2
and Q3 are connected to the center points of the
right and the left power amplifiers. When the
DC balance of the power stage is lost for some
r€ason, a potential difference is produced in the
input signal to the differential amplifier, and the
collector currents of Q2 and Q3 are put out of
balance. Ttrus, the relay driving circuit functions,
and the relay switch is tumed off.
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Fig. 9 Protection Circuit