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MODELS 610B,
6lOBR ELECTROMETERS
MAINTENANCE
To check
these
resistors
it
is necessary
to use a bridge
capable
of better
thban 1% accuracy
up to 1011 oh&.
An accurate
megobm bridge,
such as the
Keithley
Instruments
Model
515 Megohm Bridge
which
is accurate
to 0.25%
for
these
ranges,
is
therefore
necessary.
If
such
equipment
is not
available,
two pro-
cedures
are
recommended
to check
out
the resistors:
1.
Return
the complete
instrument
to the
factory
for
resistor
calibration.
2.
Replace
the high-megohm
resistors
periodically
with
a certified
set
from
Keithley
Instruments
to assure
absolute
calibration
accuracy.
4-10.
CALIBRATION
OF Om
RANGES.
This
calibration
adjusts
the constant
power
supply
needed
for
resistance
measurements.
It
must be accurate
to
maintain
specified
resistance
measurement
accuracy.
Connect
the Model
662 to the
junction
of
resistors
RlL5
and R116
(Faigure
24) and the
conrmon lead
to the
+ terminal
of capacitor
C203
(Figure
23).
Adjust
the OHMS CAL potentiometer,
RlL5
(Figure
18),
for
10
volts
dc 20.5%.
b.
If
the
control
does not
have
enough
latitude,
check
the -21 volt
supply
(paragraph
4-4).
4-11.
METER ZERO CALIBRATION.
Check meter
zero
whenever
components
are
replaced
or other
adjustments
are made.
a.
Turn
the METER Switch
to METER OFF.
Set
the mechanical
Zero Meter
adjust-
ment for
zero
meter
reading
(top-scale
zero).
b.
Turn
the Model
610B on.
Zero
the meter
on the
.OOl-volt
range.
Then
switch
to the
LO-volt
range.
Set the
Multiplier
Switch
to 10; apply
10
volts
?0.05%
to the tide1
6108 INPUT Receptacle.
Adjust
the METER CAL
potentiometer,
R175 (Figure
18),
for
full-scale
meter
reading.
c.
Set the
center
zero
by first
zeroing
the meter
on the
.OOl-volt
range.
Then switch
to the
l-volt
range.
Set the METER Switch
to CENTER ZERO and
adjust
the CENTER ZERO CAL potentiometer,
R167 (Figure
181,
for
exact
center-scale
meter
zero.
4-12.
ACCURACY CHECK.
a.
Checking
the accuracy
is
the quickest
way to spot
improper
Electrometer
operation;
Perform
the
check
about
once a year,
if
components
are
replaced,
or if
other
adjustments
made.
If
accuracy
is
verified
over
all
ranges,
the Model
610B should
be able
to meet all
specifications.
If
the
accuracy
must be checked
often,
check
for
zero
drift.
b.
Voltaee.
Connect
the Model
610B to the Model
241 Power Supply.
First,
set
the Model
610B for
the
lo-volt
range.
Increase
the
input
voltage
in
l-
volt
steps.
The Model
610B should
indicate
the
input
voltage
to ?l% of
full
scale.
Check
the other
voltage
ranges
for
accuracy
at full
scale.
c.
Current.
Connect
the Model
610B to the Model 261 Picoampere
source.
Check the
full-scale
accuracy
of all
current
positions
on the Range Switch,
For the
1 to
lOA
ampere
ranges,
set
the
FEEDBACK switch
to XORMAL.
For
066iR
35
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