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Table
6-l. Troubleshooting
Guide
Difficulty
No current output
on all
Probable
Cause
1 Defective
voltage supply.
Solution
1 Check 0202 and D208 for
Ranges.
"open."
Defective
Kelvin-Varley
divider.
Check S103 through
S106;
check R120 through
R152.
No current output
on one
Defective
range resistor.
Check corresponding
range
range.
resistor for open circuit.
Output too high on all ranges.
Defective
voltage supply.
Check Cl202 for short.
Output too high on one range.
Defective
range resistor.
Check corresponding
range
resistor.
I
6.5.3 Troubleshooting
Guidelines
WARNING
If the instrument
does not operate
at all, check the fuse, line
cord, and power source.
If these are all found to be in good
working
order, use the following
procedure
to troubleshoot
the voltage supply.
(Refer to the schematic
at the end of Sec-
tion 7 for component
designations):
1. Set the front panel POLARITY
switch
to + and connect
the meter between
J202 and the emitter
of D202.
The
voltage
should be -11.6V.
Disconnect
the instrument
from
the power
line
and
other
instrumentation
before
replacing
the fuse.
1. Locate the fuse holder on the rear panel; rotate the fuse
carrier
counter-clockwise
until the carrier
is free of the
holder.
2. If the voltage is about -17V. 0202 might be shorted.
If the
voltage
at J202 is much less than -11.6V.
check the volt-
age at the collector
of 0202;
it should
be 17V *ZO%.
Absence
of this voltage
indicates
the rectifier circuit is not
working
properly.
2. Remove the fuse carrier from the holder, then remove and
discard the defective
fuse. Replace the fuse with the type
recommended
in Table 6-2.
CAUTION
3. Measure
the voltage
at the anode of 0204;
it should
be
17.1V
i 10%. If this bootstrap
voltage
is not sufficient,
0203 will not operate and Q201 and 0202 will be cut off.
4. Check the reference
amplifier
and error amplifier
stages
using the schematic
diagram
voltage
levels as a guide.
Use
only
the
recommended
fuse
type;
replacing
the fuse with
a unit with
a higher
rating
may cause
instrument
damage.
3. Replace the fuse carrier in the holder and rotate the carrier
clockwise
until it seats in the holder.
6.6 FUSE REPLACEMENT
A line fuse protects
the Model 261 from possible damage
in
case of excessive
line current.
If the unit is totally inoperative,
the fuse may be open.
To replace
the fuse,
proceed
as
follows:
Table
62.
Fuse Types
Keithley
Operating
Limits
Fuse Types
Part Number
105-125V. 50.60Hz
3AG. 1/8A,
FU-20
25OV. Slow Blow
210.25OV. 50.60Hz
3AG. 1/16A,
FU-21
25OV, Slow Blow
6-2
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