P R O T E C T I O N C I R C U I T - Pioneer SX-1250 Service Manual

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connection. The output center point is at 0V from
the balanced plus and minus power supply, while
LOOT> DC NFB is applied to Q2 through R3.
Because of this, circuit DC gain becomes OdB and
DC voltage of the output center point is deter-
mined by Ql base voltage. VR1 is a semi-fixed
resistor for fine adjustment of the center point
voltage to compensate for irregularities in circuit
elements.
SPEAKERS Switches
The SX-1250 is provided with 3 sets of speaker
terminals (A, B, C). Since the power amplifier can
become overloaded if all three sets are used simul-
taneously, when all 3 SPEAKERS switches are set
to ON, the circuit functions as if all switches were
OFF and the indicator lamps extinguish.
4 . 3 P R O T E C T I O N C I R C U I T
This circuit protects the power transistors in case
of overload, the speakers in case of power amplifier
malfunction, and also performs a muting function
when the power supply is turned ON or OFF. The
protection circuit is composed of three sections
(Fig. 18).
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Fig. 18. Block diagram of protection circuit
1. Relay Driver Circuit (Fig. 19)
The relay which connects the output circuits is
driven by this circuit. It also performs a muting
function to prevent unpleasant noise during ON-
OFF operation of the power supply as well as
opening the output circuit on command from the
detector circuits,
Muting Operation
When the power supply is turned ON, Q6 base is
reverse biased through D6 and R19-R21, turning
Q6 OFF. Q7 base potential rises as C4 charges
through R22 & R23, and Q7 turns ON several
seconds later. The collector current of Q? then
six-1 eE'c'
flows through the relay coil, operating the relay to
turn on the power amplifier output circuit. The
reverse bias of Q6 base from D6 & R19-R21 dis-
appears when the power supply is set from ON to
OFF. Q6 remains ON however, due to the residual
power supply voltage. C4 very rapidly discharges,
Q7 base potential drops and Q7 turns OFF. The
relay releases and the power amplifier output
circuit tums OFF.
Note:
Q5 is normally OFF due to bdse bias and does not
participate in the muting operotion.
Opetation by Detector Circuit Comrnand
Command from the detector circuits pass through
one of D3, D4 or D5 and are applied in the form of
a cutrent flow. Q5 is normally reverse biased
through R14, but when a large current flows
through on of these diodes, Q5 base potential
declines according to the voltage drop at R14.
Q5 then becomes ON, Q6 base potential rises and
QG becomes ON. C4 rapidly discharges and Q7
base potential drops, turning Q7 OFF. The relay
releases and the power amplifier output circuit
becomes cut off.
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Fig. 19. Schematic diagram of relay driver circuit
2, Overload Detector Circuit
Shorting of the power amplifier load or a load
impedance below the specified value causes a
command to be sent to the relay drive circuit.
This is illustrated in Fig. 20.
With the output stage in class B operation, when
Qa is operating in the positive half cycle, Qb
becomes cut off and the signal current flows as
indicated by the solid arrows in Fig. 20. Point D
potential at this time is the point A potential
divided by Rl and R3. Also, point C potential is
the point A potential divided by RE1 and RL
(load). Point D is connected to Q1 base and point
C to Ql emitter through R2 and RE2. When RL is
extremely small, the point C potential becomes
considerably lower than point D. This potential
difference forward biases Q1. Q1 tums ON and
current flows in D3.
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