Werbung
HITACHI HMA.6500
CHECKING
THE
OPERATION
OF
When
the output circuit is
repaired
by
replacing
the
power MOS
FET,
etc., perform
an operation check
on
the ASO (Area
of
Safe
Operation) detection
circuit
and
the
speaker
protection
circuit.
1. Operation check
of
the
ASO
detection
circuit
for
the
power
MOS FET
Connect
the audio
oscillator
to
the INPUT
terminals
with
the
speaker
terminals unloaded (speaker
:
dis-
connect). Set
the
frequency
of
the
audio oscillator
at
lkHz
and adjust the level
of
the input
signal so
that
the
voltage
at the
speaker
terminals
is
approx.
5V
rms.
Under
these
conditions,
short-circuit the
speaker
termi-
nals
of
the
channel
to
which the
input
signal
is
applied
using
a
lead
wire, etc.
If
this short-circuit
makes the ASO
detection
circuit
operate,
no
output
appears
at
the
speaker
terminals even
if
the
lead
wire
used
for
short-
circuiting
is
removed.
Next,
turn off the
power switch
and,
after
approx.
10
sec.,
turn the
power
switch on
again.
When
out-
put
comes
out of the
speaker
terminals,
this
indicates
that the ASO
detection
circuit is
operating normally.
2.
Operation
check
of the
speaker
protection
circuit
Make
sure
that the
relay
operates(a
click
sound
is
heard)
approx.
4
seconds
after
the power
switch is
turned
on
with the
speaker
terminals
unloaded
(speaker:
discon-
nect).
ACvolîmeler
0scilloscope
Fig.
8
o
Phenomena and remedy when
the proîection circuit
operates
THE
PROTECTION
CIRCUIT
Next,
when
a resistor
of
approx.
l0
kohms
and2 dry
cells
(1.5V)
are
connected
in
series
to
R902
on
the
audio printed
wiring
board
and the
jumper
wire
@
,
the relay turns
off within
I
sec. When
the dry
cells
are
taken
away,
the
relay
operates again.
Next,
change
the
polarities
of
the dry cell
and
carry
out
the
above-
mentioned operation to
check
the
operation
of
the
relay.
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.
of
checking the
POWER
MOS FET
Remove
the FET.
In
the
case
of
an
N
channel
FET,
it
is
normal
:
When
touching the black test
probe
of
the tester (ohm
range
)
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
simultaneously
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.
î
î
î
^
TVpe
of protection circuil
Phenomenon
when
the
protecîion circuit
operates
Cause
Remedy
l
Muting circuil
No
sound comes
out
about
4
rec. after
the
power
switch is
turned
on
Normal
2
Protection circuit of
the
power
l\4OS
FET
(ASO
protection circuit)
I
No
sound comes
out
2.
When the power
switch
is
turned ON the
relay
becomes
ON,
then
OFF
when the signal
is
applied
Short circuiî
of
speaker
output
teminal
Turn
off
the
power switch,
check
whether
the speaker
teminal
is short
circuited
or not,
and
tum
ON
again.
3
Speaker
protection
circuit
(DC voltage
detection
/
protection
circuit)
No
sound comes
out.
Neutral point
voltage
is
more
thil i
2V
After
the power
switch
is
tumed
ON,
rclay
doesn't
tum
ON
DC
leakage
of control
amp
,
power amp.
faulty,
etc
I
ln
care
the
input
select
switch
has
been set
to
DIRECT COUPLING:
If
the fault
is corrected
by
setting
it
to
coNDEN-
SER
COUPLING, control
amp. DC
is
leaking
so
repair the control
mp
2
In
case
the
input
s€lect
vitch
has
been
set
to
CONDENSER COUP.
LINC:
Repair
faulty
parts
in
power amp
-6-
î
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