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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.
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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