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SERVICE
MANUAL
FOR
SX300A AMPLIFIER
symmetrical. Consequently, there are
two
differ-
ential pairs
(Q201-Q203 and Q202-Q204). The
differential
pairs
take
as
input
the
signal
from
the
input
circuit
board
and
the
NFB
(negative
feed
back)
signal.
They
significantly
reduce
the
hcir-
monic
distortion of
the
overall amplifier
as
well
as
maintain
an
automatic zero
calibration of
the
DC
voltage
on
the
amplifier
output
so
that
it
may
be
directly
connected
to
the
speaker.
The
differential
pairs
are followed
by a
standard
VAS
(voltage amplifier stage) transistor
(Q205
Eind
Q206).
To
linearize
the
VAS
at
high frequen-
cies
a
local
NFB
shunt
is
used.
The
signal
is
then
feed
into
the
MOSFET
driver
stage.
The
power
amplifier
circuit
is
what
is
commonly
called
a
"simple
quasicomplementary"
output con-
figuration
running
in
class
B
mode
using
MOSFETs
as
the
drivers.
The
main power
tran-
sistors
are
BJTs
rather
than
MOSFETs
because
they
produce
significantly
less
distortion.
The
power
transistor bias
voltage
is
provided by
a
standard
setup.
Note,
if
the
speaker
limiting circuits
are
properly
adjusted,
the
amplifier
should
never
clip
the
out-
put,
and
consequently, the
distortion
should
al-
ways
remain
at
very low values.
POWER
SUPPLY
TypiccUly,
the
components
associated
with the
main
amplifier section
have
numbers
in
the
300
range
(e.g.
ZD301
,
C302,
Q303,...).
The
power
supply
consists
of
a
toroidial
transformer
fused
in
the primary
and
the
secondary.
The
nomi-
nal
output
of
54
Volts of
the
secondary
is
rectified
with
a
diode
bridge.
The
rectified
output then feeds
three
separate
DC
supplies.
The
first is
the high
voltage unregulated
rails
supply
of
±68V
(see
below
for
details).
This
is
the
main
supply
to
the
power
transistors
and
is
heavily
filtered.
Tfie
second
out-
put
is
a
high voltage regulated
supply
of
used
for
the
voltage
amplification
and
driver
stages
of
the
main
amplifier.
The
third
supply
is
a
low
voltage regulated
supply
of
+1
8V
used on
the
input
circuit
board
for
the
intergrated
circuit
chips
and
in
the
differential
amplifier of
the
main
amplifier.
Measured
at
fuse FI
,
the transformer
secondary
at
no
load
should
provide
approximately 54.5
V
with the
primary
voltage
at
228.2.
At
200
Watts
output
into
a 4-ohms
resistor,
the transformer
secondary
voltage should
be
approximately 45.2
V
with
a
primary
voltage
of
224.2
V.
The
unregulated
power
amplifier
rails
voltage
marked ±68
V
on
the
schematic should read
ap-
proximately 73.7
volts with
no
load,
and
53.7
V
at
200
Watts
output
into
a
4-ohm
resistor.
The
regulated
rails
voltage
marked ±67
V
on
the
schematic should
read approximately 72.65
V
no
load,
and
52.8
V
at
200
Watts
output
into
a 4-ohm
resistor.
EQUIPMENT NEEDS FOR
TESTING
In
order to
make
the
driver
protection adjust-
ments,
you
will
need:
•
a
sine
wave
generator
•
6in
AC
voltmeter
•
an 8
ohm
resistive
load
capable
of
dissi
pating
200
Watts
(average) of
power
In
order
to
adjust the
quiescent bias
voltage,
you
will
need:
•
a
DC
voltmeter
capable
of
reading 5
m
V.
•
an
8-ohm
resistive
load
capable
of
dissi
pating
200
Watts
(average) of
power
Additional useful
equipment:
•
an
oscilloscope
•
a spectrum
analyzer
INTERNAL
AMPLIFIER
ADJUSTMENTS
The P300SX
has
three
variable resistors
mounted
on
the
circuit
board
that
are
used
to
make
the
in-
ternal
adjustments
of
the
amplifier.
Two
of
the
variable resistors
are
used
to
adjust the
driver
protection
circuits
in
the
P300Sx
to
prevent
over-
load of the tweeter
and
the
woofer
drivers.
The
third varieible
resistor
is
used
to
set the
quiescent
bias
voltage
on
the
power
output transformers.
Transducer Overload
Protection
Adjustment
Procedure
VR102
is
a
variable
resistor
that
sets the
maxi-
mum
power
delivered
to
the
woofer on
the
P300SX.
•
Connect
the output
of
the
amplifier
directly to
a
4-ohm
resistive
load
capable
of dissipating at
least
200
Watts
of
power.
The
crossover
circuit
should
not
be connected
(or
should
be
bypassed).
•
Put
an
AC
voltage
meter across
the
amplifier
output
capable
of
measuring
up
to
30
V.
•
Ensure
that
the
driver
sensing wire
(2)
is
con
nected
to
the
output.
•
Put
the
volume
control
to
minimum.
•
For
the
P300Sx,
supply
a
sine
wave
input of
4
V
at
1
kHz.
•
Slowly increase
the
volume
control while
ad
justing
VR102
to
make
the
amplifier
output
be
come
28.3
V
when
the
volume
control
is
at
maximum.
•
Once
you have
properly set
VR102,
we
recom
mend
that
you
apply
a
small
amount
of
electri
cal
paint to
the outside
of
the shaft
to
prevent
it
from
turning
from
acoustical vibrations.
NOTE;
If
VR102
does
not
change
the output
volt-
P«9«*
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