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4 . 3 P R O T E C T I O N C I R C U I T
The main purpose of the protection circuit is to
protect the equipment and give an indication in
the event of malfunction, incorrect operation,
faulty connections, or other causes.
The block diagram of the protection circuit func-
tions is shown in Fig. 6. The following description
is in reference to the dia$am.
Relay and Lamp Drivers
q27 - Q30 form the relay drive circuit (see Fig. 7).
while Q31 drives the lamp. In normal operation,
reverse bias is applied to Q27, maintaining it in
the cut off state. When an abnormality is detected
by the DC balance or temperature detector, current
flows through R1. Q27 base potential falls rapidly
and it switches ON. This causes forward bias
voltage to be applied to Q28, turning Q28 ON.
Potential at point B drops, turning off Q29 and
Q30. With Q29 OFF, current to relay RL5 is cut
off, tuming Q31 (lamp drive) ON and lighting the
protection lamp. Q30 cuts off the current to RL4
and RL2 (when impedance switch is at 8Q; when
at 4Q, RL4, RL6 and RL3).
Because of RL4 off, output circuit of the power
amplifier becomes open, and the stages beyond the
predriver are stopped operation completely by
RL5.
RL2, RLg and RL6 are for changing the trans-
former secondary taps when the impedance switch
is selected and determining the value of the current
control circuit in the power amplifier. They are
thus not directly related to the protectiorr func-
tion.
Zener diodes D2 and DB fix the operating poten-
tials of Q29 and Q30, and the difference in their
starting points prevent misoperation.
Power Switch ON Muting
This circuit also includes a delaying function when
the power switch is set to ON. The delay time is
determined by R8 and C1 in the Q29 and Q30
base circuits.
Immediately after the power switch is turned ON,
Q27 and Q28 are reverse biased by
-B1 and -B2,
and switched OFF. Point B potential from'+B3
passes through R8 and rises as Cl charges. The
rise time is determined by the time constant of
C1 and R8. When the potential exceeds D3
(zener diode) zener voltage, Q30 is switched ON
and current flows in the relay circuit connected
to Q30.
In the same manner, when the potential at point
B exceeds D2 zener voltage, Q29 is switched ON
and current flows in RL5. Correct voltage is thus
applied to the power amplifier.
Fig. 6 Block Diagram of the Protection Circuit
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