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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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