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INSTRUCTION MANUAL
MAINTENANCE
Digital
Multlmeler
Models
172A.
173A
6-7.
TROUBLESHOOTING AND REPAIR.
The troubleshooting
and repair
information
in
this
section
is
intended
only
for
qualified
personnel
having
a basic
understanding
of
analog
and digital
principles
and components
used
in a precision
electronic
test
instrument.
Since
most
repairs
(or
replacement
of
parts)
will
require
that
the
unit
be recalibrated
or
its
performance
reverified,
a facility
having
calibration
test
equipment
may also
be
required.
If
the
required
personnel
or
facilities
are
not
available,
or
if
doubt
exists
the
Keithley
representative
in your
area
should
be contacted
in the
ewnt
repair
of
the
instrument
is
needed.
If
it
has been determined
that
you have
the
means
to properly
repair
and return
the
Model
172A/l73A
to service,
the
following
instructions
may prove
useful
to help
you
isolate
the
fault
or make the
repair.
It
is strongly
recommended
that
the
following
instructions
be read
carefully
and that
the
Theory
of
Operation
and
Calibration
Procedure
both
be reviewed,
before
any attempt
is made to
repair
the
Model
172A/173A.
NOTE
For
instruments
that
are
still
under
warranty,
(less
than
12 months
since
date
of
shipment),
if
the
instrument's
performance
is outside
of
specifications
at
any point
or
if
abnormal
operation
is
indicated,
contact
your
Keithley
representati
or
the
factory
before
attempting
any
troubleshooting
or
repair,
other
than
fuse
replacement.
a.
Disassembly.
If
there
is
a need
to
renwve
the
top
cover
and partially
disassemble
the
Model
172A/l73A
for
troubleshooting
or
to
replace
a part,
the
following
procedure
should
be followed:
WARNING
t
Disconnect
the
line
cord
before
removing
the
top
cover.
To discharge
voltage
on capacitors,
depress
the
LINE
pushbutton
after
disconnecting
the
line
cord.
I)
Turn
off
power
and disconnect
the
line
cord.
2)
Turn
the
instrument
over
so that
the
bottom
cover
is
facing
up,
loosen
the
four
screws
in
the
bottom
panel.
These
screws
are
held
captive
by rubber
O-rings.
3)
Holt
the
top
and bottom
covers
together
to
prevent
their
separation
and
turn
the
DMM over
to
normal
position.
4)
Carefully
lift
off
the
top
cover
and disconnect
the
connector.from
the
wires
going
to
the
rear
panel
(current
fuse).
5)
If
the
Model
I722
or
1723 Digital
Interface
is
installed,
disconnect
its
connectors
to
the
DMM motherboard
and
remove
interface
board(s)
along
with
the
instrument
top
cover.
(See
Figure
3-9).
61
If
the
Model
1728 Batterv
Pack
is
installed,
disconnect
its
connectors
as shown
_,
in
Figure
2-3
and
remove
the
bettery
pack
from
the
DMM.
NOTE
If
the
RF shield
on the
main
printed
circuit
board
is
removed
and
recalibration
of
the
AC voltage
circuits
may be required.
If
the
the
shield
is
disturbed
during
troubleshooting
or
repair,
perform
verification
check
to determine
whether
recalibration
is
needed.
7)
The main
printed
circuit
board
RF shield
is
held
in place
by 4
replaced.
position
of
the
AC voltage
spring
clips.
Remove the
shield
only
if
absolutely
necessary,
as its
removal
and replacement
may
necessitate
recalibrating
the
AC voltage
circuit.
6-
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