Orban 424A Operating Manual page 34

3)
Remove
all residual flux
with
a
cotton swab moistened
with a solvent
like
1,1,1
trichloroethane, naptha, or
99%
isopropyl alcohol.
The
first
two
solvents
are usually available
in
supermarkets under
the brand
name
"Energine" fire-
proof spot remover
and regular spot
remover,
respectively.
The
alcohol,
which
is
less
effective,
is
usually available
in
drug stores.
Rubbing
alcohol
is
highly
diluted
with water and
is
ineffective.
It is
good
policy
to
make
sure
that this defluxing
operation has
actually
removed
the flux
and
has not
just
smeared
it
so that
it is
less visible.
While
most
rosin fluxes
are not corrosive, they can slowly absorb moisture and
become
sufficiently
conductive
to
cause
progressive deterioration
of
performance.
Troubleshooting 1C
Opamps
1C
opamps
are usually operated such that the characteristics
of their
associated
circuits are essentially
independent
of 1C
characteristics
and dependent
only
on
external feedback
components. The feedback
forces
the voltage
at
the
(-)
input
terminal
to
be
extremely
close
to
the voltage
at the
(+)
input
terminal. Therefore,
if
the technician
measures more
than a
few
millivolts
between
these
two
terminals,
the 1C
is
probably bad.
Exceptions are
IC's
used without feedback
(as
comparators)
and
IC's
whose
outputs
have been
saturated due
to
excessive
input voltage
because
of
a defect
in
an
earlier stage. Also, be
sure that
the voltmeter
is
not interacting
with these
sensitive points and affecting the
measured
voltage. However,
if
an
IC's
(+)
input
is
more
positive
than
its (-)
input,
yet the output
of the 1C
is
sitting at
-14
volts,
this
almost certainly
indicates that
it
is
bad.
The same
holds
if
the
above
polarities
are reversed.
Because
the characteristics
of
the
424A
are essentially
independent
of
opamp
AC
characteristics,
an opamp can
usually
be replaced without
need
for
recalibration.
However, most
of the circuitry
in
the
compressor/
limiter
control
loop
is
sensitive
to
opamp
DC
characteristics,
like
bias
current and
offset voltage.
Because
of
this,
high-performance
dual
opamps are used
in
many
sockets.
These
devices
must be
replaced by exact replacements
.
NOTE
The
dual current-controlled gain block
IC5 used
in
the
VCA
is
not an
opamp
.
If it is
replaced, recalibration according
to
the instructions
in
Part
I
of
this
MAINTENANCE
section
is
absolutely
necessary
.
A
defective
opamp
may
appear
to
work, yet
it
may
have extreme temperature
sensitivity.
If
parameters appear
to drift
excessively, freeze-spray
may
aid
in
diagnosing the problem. Freeze-spray
is
also invaluable
in
tracking
down
intermittent
problems. But, use sparingly
,
because
it
can
cause
resistive
short
circuits
due
to
moisture condensation on
cold surfaces.
30