JBL SG520 - Replacement Parts; Information Service

When
the
Graphic
Controller
is
first
turned
on,
a
control
signal
of
—-20
volts
appears
at
the
shield
of
the
channel
A
output.
This
operates
the
AC
power
relay
to
turn
on
the
Energizer
or
power
amplifier.
The
A-B
relay,
Ry2,
is
not
actuated
because
of
the
series
diode.
Tracing
the
connections
through
the
A-B
relay
contact,
indicates
that
both
loudspeakers
are
connected
in
the
normal
manner
exactly
as
if
the
relay
were
not
in
the
circuit
at
all.
When the Graphic Controller ‘‘test'’’ pushbutton is depressed, the polarity of the control signal
is
reversed
so
that
a
+20
volt
signal
appears
at
the
shield
of
channel
A.
The
AC
power
relay
re-
mains closed and the A—B relay is now actuated. If you will trace the connections through the
contacts
Fig.
3-3,
you
will
find
that
both
of
the
Energizer
black
output
terminals
have
been
dis-
connected and the speakers are now connected in series between the two red Energizer output
terminals.
This
effectively
causes
the
speakers
to
respond
only
to
an
A—B
or
difference
signal,
enabling the aural null feature to be used.
When
trouble-shooting
the
operation
of
the
F22,
remove
the
cover
and
see
if
the
two
relays
are
operating
as
described
Fig.
3-2.
In
other
words,
whenever
the
Graphic
Controller
is
turned
on,
the
armature
of
the
power
relay
should
be
pulled
in,
but the
A—-B
relay
should
not
be
affected.
When
the
test
button
is
depressed,.the
power
relay
should
remain
pulled
in
and
the
A—B
should
also
be
actuated.
If
the
two
relays
seem
to
be
operating
properly,
then
the
only
possible
source
of
trouble
in
the
system
is
in
the
connections
between
the
A—B
relay,
the
loudspeakers,
the
power
amplifier
or
Energizer.
On
the
other
hand,
if
the
relays
are
not
actuated
in
the
proper
sequence,
can
check
the
operating
voltage
and
polarity
with
an
ordinary
voltmeter.
NOTE:
That
it
is
vitally
important
that
the
shield
of
the
channel
cable
be
continuous,
and
that
it
not
be
connected
to
the
chassis.
Section 5
REPLACEMENT PARTS AND SCHEMATICS
5.1 REPLACEMENT PARTS.
Replacement parts for the SG520 Graphic Controller are listed in the following pages. It is rec-
ommended that only JBL replacement parts be used in this high-quality amplifier.
5.1.1
Resistors.—All
resistors
are
the
deposited
carbon
film
type.
Any
failures
should
be
replaced
with
the
same
type.
*Ordinary
molded
composition
carbon
resistors
must
not
be
used.
Replace-
ments
are
available
directly
from
JBL.
5.1.2 Capacitors.—Most of the electrolytic capacitors used in the SG520 are specially made. Replace-
ments
should
be
ordered
directly
from
JBL
by
part
number
or
reference
number.
Under
no
cir-
cumstances
should
inferior-rated
electrolytics
be
substituted.
Lower
capacities
in
any
circuit
will
impair performance. Lower working voltage ratings will cause premature failure. Capacitors have
been selected for low-noise specifications.
*Technicians
who
are
familiar
with
professional
or
military
dc
amplifiers
will
remember
that
car-
bon
resistors
can
be
more
noisy
and
have
a
greater
resistance
change
with
temperature.
5.1.3 Transistors.—All transistors are selected for current gain, low noise, and other important charac-
teristics.
Experience
has
shown
that
the
current
gain
(8)
from
one
transistor
to
the
next
in
any
production
batch
varies
so
much
that
sometimes
one
will
not
work
well
in
the
circuit.
For
this
reason,
all
transistors
carry
special
part
numbers
and
must
be
ordered
from
JBL.
Note
the
color code dot when ordering.
5.2 INFORMATION SERVICE.
Information
on
parts
and
service
tips
can
be
obtained
from
the
JBL
factory
Service
Department.
Write a letter explaining any unusual problem to:
JBL
Service Department
3249 Casitas Avenue
Los Angeles, California 90039
If
it
is
necessary
to
return
a
faulty
subassembly
or
the
complete
amplifier,
first
request
ship-
ping
permission.
Then
pack
the
equipment
in
an
over-size
carton,
completely
surround
it
in
cush-
ioning
material,
and
ship
by
Railway
Express
or
truck
express.
Be
sure
to
insure
the
shipment
and
prepay
the
shipping
charges.
Mark
the
carton:
Fragile
—
Delicate
Electronic
Equipment.
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