Lab.gruppen SS 1300 Service Manual page 22

Lab.grupfen
1
NOV.
Si
Service in the
SS
1300
output-circuits.
Required
measurement
equipment:
-Audiogenerator
-AC
-voltmeter/
THD-meter
-DVM
-Two channel
oscilloscope for
audio
1)
a) Identify
which
channel,
A
or B,
which
is
broken, by checking
if
the
fuse(s)
F2
and/or
F3
are melt.
b)
Take away
the
F2 and
the
F3
on the other channel.
c)
Replace
F2 and F3
with two
resistors
47 0/
5W
on
the
broken
channel.
2)
Turn
the
TP3
on switchtransistor
board
PSUB
fully
counter clockwise
and
turn
on
the
main
switch
on
the
amplifier.
3)
Turn
TP3
slowly clockwise
until
the voltage across
C17 and C18
is
1-2V.
(TP3
is
the current limit
adjustment,
and
act
like
a variac
in this case.)
4)
If
the voltage across
C17
or
C18
won
t
reach
IV
without high heat
disipation in
the
47Q/
5W-resistors,
change
D22
or
D23.
5)
Measure
the outputvoltage refered to
ground
at R62.
If
the voltage
is
positive:
Someone
of the transistors
Q21,
Q23-Q28
is
broken.
If
the voltage
is
negative:
Someone
of
the transistors Q22,
Q32-Q37
is
broken.
If
the voltage
is
below +/-20mV:
The
output
circuits
are probably ok.
In that case
go
further to
item
7).
6)
To determine
which
output
transistor(s)
which
are
defect,
proceed as
follow:
a)
Connect
the
DVM
with the referens to
point
R62/
R63.
b)
Measure
the voltage across the emitter-resistors
R46-R50 and
R54-R58.
c)
Those
emitter-resistors
with the highiest voltage indicate
corrsponding
defect outputdevice.
d)
Desolder
and unscrew
the
defect transistor(s).
e)
Increase the
rail
voltage
by
turning
TP3
clockwise
and measure the output
voltage in
accordance
to
item
5).
7)
If
the railvoltage
can
be increased (with
TP3
)
without
any
high heatdisipation
in the
47£V
5W-resistors, the amplifier
can be connected
to
a
sinewawe
source
at the input.
The
output
will
be checked
by an
oscolloscope.
(
The
amplifier
can
run without
or
with reduced
load.)
If
there
still is
a
DC-
potential at
the output,
check the
drivertransistors
Q7-
Q9 and Q12 and
their
mica
insulations.
If
there
still is
DC,
go
to
item
9).
If
the
sinewawe
is
distorted,
check the DC-protection
triac
QUO,
if it still
distortion
go
to
item
9).