MAINTENANCE
1. With the power off, place the end of a flat-blade screw-
driver into the slot in the rear panel LINE FUSE holder.
Push in gently and rotate the fuse carrier one-quarter
turn counterclockwise. Release pressure on the holder
and its internal spring will push the fuse and carrier out
of the holder.
2. Remove the fuse and replace it with the type recom-
mended in Table 8-l.
CAUTION
Do not use a fuse with a higher current rating
than specified, or instrument damage may oc-
cur. If the instrument repeatedly blows fuses,
locate and correct the cause of the trouble
before replacing the fuse.
3. Install the new fuse and the fuse carrier into the holder
by reversing the above procedure.
Table 8-l. Line Fuse Selection
Line Keithley
Voltage
Fuse Type
Part No.
I
I
90-l25v ‘/aA, 25OV, 3AG, Slo Blo FU-17
180-250V ‘isA, 25OV, 3AG, Slo Blo FU-20
8.4 CALIBRATION
Except for one manual adjustment, the entire calibration
procedure is digital in nature and can be done from the
front panel or over the IEEE-488 bus.
Calibration should be performed every 90 days or I2
months (depending on accuracy requirements), or if the
performance verification procedures in Section 6 show that
the instrument is out of specification. If any of the calibra-
tion procedures cannot be performed properly, refer to the
troubleshooting information in this section.
8.4.1 Calibration Overview
The Model 263 is calibrated twice. The first calibration is
performed when the instrument is first turned on and is
referred to as the “cold” calibration. The second calibra-
tion is performed after the instrument is thoroughly
warmed up and is referred to as the “hot” calibration. By
calibrating the Model 263 at these two temperature points,
the actual temperature coefficients (TC) of the lGQ, 1OGD
and lOOGO resistors are determined and stored for use on
these resistance ranges and the lower current ranges.
“Cold” calibration consists of calibrating the volts and
ohms ranges right after the instrument is turned on. After
allowing the instrument to warm up for a minimum of
three additional hours, “hot” calibration is performed.
“Hot” calibration consists of again calibrating the volts and
ohms ranges.
Calibrating the >lMQ ranges can be done by two methods;
direct resistor measurement or the internal calibration ‘lad-
der” technique. The direct resistor measurement method
requires equipment that can accurately measure the
resistors. The measured values are used as the calibration
values. The accuracy requirements for this method are con-
tained in paragraph 8.4.2. The “ladder” technique com-
pares each >lMfl resistor with one of the previously
measured resistors to determine its value. The “ladder”
method is the preferred method here because a single
precision DMM (Model 196) can be used to calibrate the
Model 263.
8.4.2 Recommended Calibration Equipment
Table 8-2 lists recommended calibration equipment. An
alternate DMM may be used as long as it meets the ac-
curacy specifications in the table.
If using the direct measurement method to calibrate the
>lMO ranges, use equipment with the following specifica-
tions. Note that these accuracy specifications represent the
accuracy required of the entire measurement set-up.
lOM0 Range - 0.02%
1OOMQ Range 0.045%
1GCl Range - 0.05%
lOGa Range - 0.19%
1OOGR Range - 0.19%
8.4.3 Environmental Conditions
Calibration must be performed under laboratory condi-
tions having an ambient temperature of 23’C + 1°C and
a relative humidity of less than 70%.
8-2
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