Hitachi HA-7700 Serviceanleitung Seite 10

HITACHI HA-7700
If this operation makes the relay activate, this indicates
that the speaker protection circuit is operating normally.
Moreover, pay attention during this operation check so
that nearby parts are not short-circuited.
Method of checking the POWER MOS FET =
Remove the FET.
In the case of an N channel FET, it is normal : When touch-
ing the black test probe of the tester (ohm range X100) to
the drain of the FET, and the red test probe to the source,
after applying the black test probe to the gate and the red
test probe to the source at the same time, a current flows.
And next, when brushing both the gate and source simulta-
neously with your hand, the current stops.
In the case of a P channel, the red and black test probes of
the tester are applied in reverse.
Lt
Fig. 12
@ Phenomena and remedy when the protection circuit operates
Phenomenon when the
otecti ircuit : pie:
Type oF protccbo incl ient protection circuit operates
Remedy
Muting circuit No sound comes out about Normal
4-7 sec, after the power
switch is turned ON.
Protection circuit of the . No sound comes out. Short circuit of Turn off the power switch, check
POWER MOS FET . When the power switch speaker output whether the speaker terminal is
(ASO protection circuit) is turned ON, the relay terminal short circuited or not, and turn
becomes ON, then OFF ON again.
when the signal is applied.
3| Speaker protection circuit . No sound comes out. DC leakage of control! 1. In case the input select switch
(DC voltage detection/ 2. Neutral point voltage is amp., power amplifier has been set to DIRECT
protection circuit) more than +2V. faulty, etc. COUPLING : If the fault is
. After the power switch corrected by setting it to CON-
is turned ON, relay DENSER COUPLING, control
doesn't turn ON. amp. DC is leaking so repair
the control amplifier.
. Incase the input select switch
has been set to CONDENSER
COUPLING : Repair fault parts
in power amplifier.
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