Sharp SM-510H - Protection Circuit

4. PROTECTION CIRCUIT
As seen from the item(1) “MAIN AMPLIFIER”, the protection circuit incorporated in the 1¢301 prevents the set from
being affected by the overload signal due to the shortcircuit of speaker terminal, but the speaker happens to be burn
out due to the employment of OCL circuit with which the DC current may run directly into the speaker if a big DC
voltage is applied to between the earth K323 and output terminal K313 or K314 in the worst case. In an ordinary state,
it scarcely occurs that DC voltage is applied to between the K323 and K313 or K314 because the IC-301 or 1C302
incorporates the circuit to balance DC. DC is applied thereto only when the 1C301 (or IC-302), Q401 and Q403 (or
Q402 and Q404) are accidentally destroyed.
However, to further counteract the rare case above, the protection circuit formed by the Q301 ~ Q304 and relay is
additionally provided to protect the speaker. When the plus DC voltage appears in between the K313 and K323, it is
applied to the base of Q304 via the R329.
As the result the current runs in the Q304 and its collector voltage drops more abundantly than in the case of
non-signal because of the R319. The Q302 and Q301 are PNP transistors.
The PNP transistor gets conductive when the base voltage becomes minus with respect to the emitter voltage, and
therefore the current runs in the Q304 and also in the Q302, to cut off the Q301, then the relay stops to operate.
In a normal condition, as soon as the power switch is turned on, the relay starts to operate to connect the amplifier
with the speaker.
In an abnormal condition, however, the relay is cut off to protect the speaker from possible damage. When the voltage
between the speaker terminals is plus, the Q303 does not function but when it is minus, the Q303 functions to cut off
the relay as in the case the voltage is plus.
The R329, C315 and C316 form low pass filter to prevent the relay being worked by audio signal. The relay is the
inductive one and will generates reaction voltage if it is abruptly turned on or off. This voltage is absorbed by the D301,
however. The minimum nubalanced DC voltage to make the relay operate is depended upon the value of R319 or
R327.
Not operate
Fig. 8 Principle Protection Circuit