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JVC M-7050 - Page 11

JVC M-7050
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Warning
:
When
replacing
the
parts
marked
with
A\
,
be
sure
to
designated
parts
to
ensure
safety.
Service
Manual
PRECAUTION
IN
SERVICING
PRECAUTIONS
FOR
SERVICING
AND
REPAIRING
1.
Removal
of
power
transformer
(replacement
of
transistor
and
parts
on
circuit
board)
Remove
the
power
transformer
before
replacing
the
transistor
mounted
on
a
heat
sink
and
parts
mounted
on
the
circuit
board
(TAP-279).
Follow
the
procedure
below.
1)
Remove
four
screws
(1)
and
then
cover
(2).
(See
Fig.
16.)
2)
Remove
three
screws
(3)
from
the
back.
3)
Remove
three
screws
(4)
from
the
top.
4)
Remove
four
screws
(5)
from
each
side.
5)
Disconnect
all
connectors
of
the
power
transformer
wiring.
(See
Fig.
8
for
how
to
disconnect
the
con-
nector
of
the
power
switch
wiring.)
6)
Remove
the
power
transformer
from
the
chassis
carefully
since
it
is
very
heavy.
(Do
not
let
it
bump
on
the
circuit
board
underneath.)
Disconnect
with
pushing
here.
Fig.
8.
Disassembly
of
front
panel
1)
Disconnect
all
connectors
of
the
front
panel
wiring
from
the
circuit
board.
(See
Fig.
8
.
for
how
to
dis-
connect
the
connector
of
the
power
switch-trans-
former
wiring.
2)
Remove
four
screws
(6).
Screws
(7)
need
not
be
removed.
3)
Remove
four
screws
(7)
to
disassemble
the
front
panel
assembly
into
the
meter
and
the
glass
panel.
4)
Holding
the
meter
vertically,
adjust
its
zero
point.
5)
Replace
the
wire
plugs
into
the
circuit
board
sockets,
paying
attention
to
their
direction.
Follow
the
indication
on
direction
marked
on
the
circuit
board.
(See
Fig.
9.)
Insert
the
sockets
to
direction
Ba
on
the
circuit
board.
Fig.
9.
(For
only
qualified
service
personnel)
Show
this
manual
to
service
personnel
when
servicing
SUPER-A
block
The
SUPER-A
bias
circuit
is
made
into
a
single
block
to
maintain
performance
including
temperature
characteris-
tics.
Trouble
occurring
in
the
SUPER-A
block
is
rarely
independent
of
other
stages.
Defects
of
a
stage
usually
affects
the
preceding
stage
adversely
and
finally
the
SUPER-A
block
suffers.
Starting
from
the
final
stage,
replace
defective
transistors.
When
‘the
stages
work
well,
check
the
SUPER-A
block
by
the
following
procedure.
(Refer
to
‘4.
Check
after
repair’’
for
the
sequence
of
turning
on
power.)
1)
Measure
the
voltage
developing
between
terminals
701
and
709
(X013’s
collector
and
test
point
F)
or
terminals
705
and
709
(X013’s
emitter
and
test
point
F)
using
a
multimeter
set
in
the
“1V
AC”
range.
Normally
it
reads
OV
when
no
signal
is
delivered
and
about
0.6V
for
the
1kHz
rated
output
(into
an
8-ohm
load).
The
SUPER-A
block
is
in
trouble
if
the
reading
is
greater
than
1V.
Check
the
R-channe!
in
the
same
way.
(In
the
R-channel,
check
voltage
between
X014
and
test
point
A.)
2)
Setting
the
multimeter
in
a
DC
range,
check
voltage
between
terminals
703
and
707
(X805's
emitter
and
X807's
emitter).
Normally
the
reading
varies
from
3.8V
to
4.3V
when
the
level
of
1kHz
input
signal
is
varied
from
zero
to
the
rated
output
(into
8-ohm
load).
The
SUPER-A
block
is
in
trouble
if
the
read-
ing
does
not
vary.
Replace
the
SUPER-A
block
as
a
unit
in
that
case.
Check
the
R-Channel
in
the
same
way.
(In
the
R-channel,
check
voltage
between
X806
and
X808.)
Check
after
repair
1)
Unplug
the
red,
white,
and
black
wires
of
TPS-
226.
(The
wires
connect
to
a
large
electrolytic
capacitor.)
Now
the
final
stage
is
not
supplied
with
power.
2)
Apply
1-kHz
signal
of
approximately
-—30dB
{30mV)
to
the
input
terminal.
Connect
an
oscillo-
scope
to
test
points
A
and
F
to
monitor
waveform.
3)
Start
supplying
power.
(If
a
varying-voltage
power
transformer
is
available,
you
can
raise
the
voltage
gradually
while
watching
the
waveform
on
the
oscilloscope
to
secure
safety.)
4)
If
you
see
the
waveform
shown
in
Fig.
19,
the
stages
up
to
the
driver
work
well.
(At
this
time
no
voltage
is
applied
to
the
final
stage
and
the
power
transistor
is
working
as
a
diode.)
Up
to
now
we
assume
that
the
final-stage
power
transistor
is
intact.
Confirm
in
advance
that
the
power
transistor
is
not
defective
after
disconnecting
it
from
the
circuit
board.
If
the
power
transistor
is
shorted
between
its
base
and
emitter,
the
drive
stage
will
be
shorted
and
damaged
(you
will
see
no
wave-
form).
Caution:
Never
connect
a
dummy
!oad
up
to
step
4:
the
drive
stage
will
be
overloaded
and
damaged.

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