Circuit Description; Protection Circuit; Muting Circuit - Sony TA-F3A Service Manual

1-2. CIRCUIT DESCRIPTION

PROTECTION CIRCUIT

Two kinds of protection circuit are employed in
this power amplifier. One is a power-transistor
protection circuit and the other is a speaker pro-
tection circuit.
1. Power-transistor Protection Circuit
To protect overloaded power transistors from
destruction, a protection circuit is employed. The
fusible resistors are inserted in each of Q704,
4705 (L-CH) and 4754, 4755 (R-CH) collector
and emitter circuits. When the excessive current
flows into the power transistors, these fusible
resistors break faster than the transistors and thus
protects the transistors from destruction.
Speaker Protection Circuit (See Fig. l-l and Fig. 1-2)
the excessive current flows into the power
When
transistors, B+ voltage decreases. Therefore, 4301
turns on, biasing the base of Q302, and Q302
conducts. This removes the bias voltage at the
base of Q306 and Q306 turns off. The relay is
not activated by off state of Q306. According-
ly, the speaker is cut off from the amplifier
circuit.
When the positive dc component presents at the
speaker output terminal, C310 is charged up as
shown in Fig. l-2. Q303 turns on, Q306 turns off
and the relay RY301 is not activated (4304 re-
mains in off condition). Also, when the negative
dc component presents, C3 11 is charged up. This
turns 9304 on, Q306 off and RY301 is not
activated (9303 remains in off condition).
Fig. l-1.
3. Muting Circuit (See Fig. 1-3 and Fig. 1-4)
This muting circuit prevents the loud "pop" (due
to initial current flow) or click noises produced
just after turning the POWER switch to on or off.
These transients might
fidelity speaker system.
When POWER
.
Switch turned on:
C308 commerces to charge through R309. At the
same time, the base voltage of Q306 gradually
rises in accordance with charging voltage of C308.
It takes about 3 seconds until Q306 is turned on.
Accordingly, the speaker output terminal is cut off
by the relay RY301 for about 3 seconds after the
POWER switch is turned on. This protects the
speaker from the destruction caused by the "pop"
noises.
. When POWER
Switch turned off:
The voltage (about - 1.7 V) that turns off
reverse
Q306 immediately reduces, and the positive poten-
tial is applied to the base of Q305. Q305 turns on
and Q306 turns off, cutting off the operation of
RY301. Therefore, no pop noise presents on the
speaker output.
POWER Switch :ON
-4-
damage a delicate high-
Fig. 1-2.
Fig. 1-3.