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8.7 PROTECTION CIRCUIT
The purpose of this circuit is to protecty both
the speakers and the power amplifiers. The relay in
the output circuit is automatically opened in any
of the following cases:
1. During the "transient operations" when the
power supply is turned on and off.
2. Upon detection of an overload, caused by a
short circuit in the load.
3. Upon detection of a DC voltage in the output
circuit, caused by component failure or ac¬
cident.
An outline of this circuit is shown in Fig. 8-1.
The relay-activating transistor (Qr) is controlled by
the IC {PA3004).
Muting Operation When Power Supply Is turned
Off and On (Fig. 8-1)
When the power supply is first turned on, the
voltages on pins 1, 7, and
6
of PA3004 will exceed
a prescribed level. If there is no input (DC) on pin
no.4, S
2
will be off, and a charging current will
commence to flow to the timing capacitor (Ct)
connected to pin no.
8
. Once Ct has been charged
up to a level where the voltage on pin no
.8
exceeds
a prescribed level, S, will turn on, thereby applying
a bias current from pin no.3 to the relay driving
transistor (Qr). Consequently Qr will turn on, and
current will flow through the relay coil to activate
the relay, and close the connection in the output
circuit. The time required for this connection to
close after the power supply is first turned on is
several seconds. During this period, any unwanted
transient noises will be therefore muted.
When the power supply is turned off, the input
(AC) applied to pin no.7 ceases immediately,
resulting in Sj turning on, Ct discharging rapidly,
and Si and Qr both turning off. The relay is thus
opened, disconnecting the output circuit.
DC Voltage Detector(Fig. 8-1)
The output circuit is connected to pin no.4
via a low-pass filter (Rg and Cj). Any DC voltages
appearing in the output circuit will also be applied
to pin no.4, turning S
2
on. Ct will thus discharge
rapidly, turning S, and Qr off, thereby releasing
the relay, and disconnecting the output circuit
from the load.
Overload Detection
The overload detector circuit incorporates the
load (RL) in one side of a Wheatstone bridge
(see Fig. 8-2). The base and emitter of a sensing
transistor (Qi) are connected to the opposite
corners of the bridge, so if RL decreases, Q| will
become forward biased. If RL falls below a pre¬
scribed value, Qi will turn on, thereby passing a
current through Rs, D
3
and R^. Due to the voltage
difference generated across R^,, Q
2
will become
forward biased, and consequently turn on. A DC
voltage will then be applied to pin no.4, turning S
2
on, and resulting in the rapid discharge of Ct, and
S, and Qr both turning off. The relay will again be
released to disconnect the output circuit.
8.8 POWER SUPPLY
The power amplifier and power stage plus and
minus supply voltages (±62V) are obtained by
means of a bridge full-wave rectification system.
22000/71 X
2 electrolytic capacitors are used.
Plus and minus voltages are supplied to the small
signal circuit of the AF Section thru a constant
voltage circuit by full-wave rectification from a
winding separate from the power stage supply.
Tuner section, lamp circuit and protection circuit
power is supplied thru transistors Darlington con¬
nected ripple tilter, after full-wave rectification.
S3
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