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
To Next Page
Loading...