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5)
Troubleshooting IC
Opamps
IC
opamps
are usually
operated
so
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
IC
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
IC
is
sitting at
-14
volts,
this
almost
certainly
indicates that
it
is
bad.
The
same
holds true
if
the
above
polarities
are reversed.
Because
the characteristics of the
273A
are essentially
independent
of
opamp
AC
characteristics,
an
opamp
can
usually
be replaced without
need
for
recalibration.
However, some
of the control circuitry
is
sensitive to
opamp
DC
characteristics,
like
bias
current and
offset voltage.
Because
of
this,
high-performance
LF412
dual
opamps
are used
in
several sockets.
These
devices
must
be replaced by exact
replacements
;
TL072's
or
LF353's
are not satisfactory.
NOTE
IC12
contains
most
of
the
Noise
Reduction
circuitry.
If
it
is
replaced,
recalibration of
R132
and
R134
according
to instructions
in
4)
Component
Replacement
(above)
is
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.
4.
FACTORY
Please
refer
to
the
terms
of
your
Orban
Associates Limited One-Year
Standard
SERVICE
Warranty
,
which extends
to
the
first
end-user. This
warranty
was packed
with the
273A,
but
is
not
bound
with
this
manual. After
expiration
of the
warranty,
a
reasonable charge
will
be
made
for parts, labor,
and packing
if
you choose
to
use the
factory
service
facility.
Repaired
units
will
be
returned
C.O.D.
In
all
cases,
transportation
charges (which
are usually quite
nominal)
shall
be
borne
by the
customer.
You
must
get
a
Return
Authorization
number
from Orban
Customer
Service before
returning
this
unit
for
service.
After
a
formal Return
Authorization
number
is
obtained,
ship
the
unit to
CUSTOMER
SERVICE
at
the address
on
the
title
page
of
this
manual.
4-4
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