causing
high-resistance
short-circuits and erratic
opera-
tion. The
front
panel should
be periodically
cleaned;
use
of
a
strong
household
detergent will
usually do an
adequate
job
without
danger
of
damaging
paint,
screen-
ing, or
plastic
parts.
In
time,
operation
of the pots
and/or rotary
switch
may become
erratic
because
of wear, corrosion,
or dirt
build-up. These
units are
not
hermetically sealed,
and
may
be cleaned
with commercial
spray-type
contact
cleaner.
Avoid
letting excess
cleaner drip
onto parts
other than
those
being serviced.
The
electronics
are stable
indefinitely,
and require
no
periodic
alignment.
Alignment
instructions
have
been
included as
Appendix
A
primarily
for reference. Align-
ment should
not
be
attempted
unless the
service facility
has the necessary
specified
test
equipment, and
the tech-
nician
is highly
skilled and
experienced
in the
mainte-
nance of
equipment
employing
IC's.
Corrective
Maintenance
—
A
General
Note:
The
418 employs
advanced
and
sophisticated
circuit
techniques based
largely
upon linear
integrated
circuit
technology.
In case
of
failure,
it is
highly advised
that
repairs
be
performed
at the
factory,
in order to take ad-
vantage
of the
factory
technicians'
experience
and stock
of correct spare
parts.
Field
repairs
should only
be at-
tempted if the
technician is
highly experienced
and
com-
petent in
the field
of linear
integrated
circuits,
and is
skilled
in
the
fine art
of working
on
double-sided
PC
boards with
plated-through
holes (see
following section).
In
most cases,
failed
1C
opamps
or
comparators
may
be
replaced
without
realignment.
Flowever,
other parts
will
often affect
alignment.
This is
particularly
true
of the
quad-FET
arrays
and
associated
components.
In
addition,
a
number
of
tight -tolerance
parts are
used.
These
must
be replaced
by exact
equivalents,
or
circuit
performance
will suffer.
SERVICE
NOTE: It is
usually more
economic to
return
the
418
to
the factory
for repair, unless your house
technicians are
extremely familiar
with the unit, and an
adequate stock of spare
parts is
on
hand.
Replacement of Components
in Printed
Circuit Boards:
Most
circuit boards used
in the 418 are
of the double-
sided
plated-through
variety. This
means
that there
are
traces
on both sides
of the board and that
the
through-
holes contain
a
metallic plating
in order to
conduct
cur-
rent through
the board. Because
of the
plated-through
holes, solder
often creeps
1/16"
up into
the
hole requir-
ing
a
more
sophisticated technique
for component
remov-
al
in
order
to prevent
serious damage to
the traces on
the
board. In
particular,
excessive
heating
of
a
point
on
the board
will almost always
cause
damage.
If the user is
not
thoroughly familiar
with elegant
techniques of removing
components
from double-sided
boards, it is
wiser to cut
each
of
the
leads
of an
offend-
ing component
from the body while
the
leads
are still
soldered
into the board. The
body is then discarded
and
each
of the leads is heated
independently and pulled
out
of the hole
with
a
pair
of
long nose
pliers. Each hole
may then
be cleared
of
solder
by carefully heating
with
a
low wattage
iron and sucking
the residual
solder with
a
spring activated solder
vacuum
tool
.
The
new
component should be
installed in the usual
way
and soldered
from the
bottom side
of the board.
If
no damage
has been done
to the
plated-through hole,
soldering of the topside pad
is not
necessary. However,
if
the
removal procedure
did
not
progress
smoothly
it
would
be prudent to solder
each lead
carefully at the
topside as
well
in
order to avoid
potential
intermittent
problems.
After
soldering, the
residual flux should
be removed
with
a
cotton swab
moistened with
a
solvent
such
as
1,1,1
trichloroethane,
naphtha,
or
99%
isopropyl alco-
hol.
These
first two
solvents are
often available in
su-
permarkets
marketed under
the brand name "Energine"
fire-proof
spot remover and
regular spot
remover, respec-
tively.
The
alcohol, which is
less
effective, is usually
available
in drug stores. Note:
Rubbing
alcohol is high-
ly diluted
with water and is not
effective. There are
al-
so other
solvents marketed under
various trade names
which contain Freon
.
These are
often
available in
electronic
supply houses and
are also useful.
It
is good
policy to
make sure
that this defluxing
oper-
ation has
actually
removed the
flux and not just smeared
it
about so that
it is less
visible.
While rosin flux is
not
corrosive
normally,
it
can absorb
moisture and become
conductive
enough to cause severe
deterioration
in spec-
ifications
over time.
Comments on
1C
Opamps:
1C
opamps are
operated
in such a way
that their trans-
fer characteristics
are
essentially
independent
of
1C
char-
acteristics and
dependent only
on external
feedback com-
ponents. The
feedback
forces the voltage
of the
(-)
input
terminal to
be very close
to the
voltage at
the
(+)
input
terminal.
Therefore,
if
the technician measures
more
than
a
few
millivolts
difference between
these terminals,
the
1C
is
probably bad.
Exceptions are IC's
used
open-loop as
comparators,
and IC's
whose
outputs
have been
saturated in
one direc-
tion
or
the other due
to abnormal
inputs.
However,
if
the
technician
measures that
the
(t)
input
is
more posi-
tive than the
(-)
input,
yet the output
of the
1C
is sitting
at
-14
volts, this
almost surely
indicates
1C
failure. If
all the above
polarities are
reversed,
the same
thing
holds
.
Because
the
characteristics of the
418
are
essentially
independent
of
1C
opamp
characteristics, an
1C
opamp
can
usually
be replaced
with no change in
performance.
A
defective
opamp
may appear to
work, yet have ex-
treme
temperature
sensitivity
of
DC
characteristics. If
parameters
appear
to drift excessively
with temperature.
4
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