Fluke 825A - Troubleshooting

825А
(5) After chopper is thoroughly dry, replace chopper
cover and install in instrument.
NOTE
If the chopper drive unit is unplugged while
the instrument is on, the chopper drive unit
will not operate when reassembled. How-
ever, the chopper drive unit will operate if
the instrument is turned off and then turned
back on.
(6) Adjust УТУМ ZERO ADJ control R232 for zero
meter deflection after a 20 minute warmup period.
4-3. TROUBLESHOOTING
a. GENERAL. To assist in localizing most troubles
which might occur, a troubleshooting chart (figure 4-2)
lists the causes and remedies for a number of symp-
toms. Since failure of the 825A is usually due to tube
failure, it is recommended that tube substitution be
tried before any other tests are made. However, the
825A is conservatively designed and should require very
infrequent tube replacement. At certain times, a check
of the DC pin voltages in the tubes (figure 4-6) is also
useful. As an aid to troubleshooting, all components
may be located by referring to the figures shown in Sec-
tion V. An understanding of the theory of operation
(Section III) and frequent reference to the functional
schematic will be of great value in troubleshooting.
SYMPTOM PROBABLE CAUSE REMEDY
Drift of the 500 volt DC
reference supply evi-
denced by the continual
need for resetting with
the CALIBRATE control.
Cannot calibrate 500 volt
DC reference supply.
Meter cannot be brought
to zero with CALIBRATE
control.
Measurements are out of
tolerance on one range
other than the 500 volt
range.
Measurements are out of
tolerance on all ranges
other than the 500 volt
range.
4-2
V104, У105, or V106
defective.
One of the sampling string
resistors (R119, R120,
R121) is changing value
rapidly as the instrument
warms up.
Drifting standard cell EMF
possibly caused by previous
shorting or inverting of the
cell (825A and 825AR only).
Excessive aging of V105.
Out of calibration.
One or more resistors in
500 volt DC sampling
string has shifted in value.
Standard cell EMF has
shifted (825A and 825AR
only).
A reference supply output
range resistor on the range
resistor printed circuit
board is out of tolerance.
R320 or R321 has shifted
in value.
Figure 4-2.
Check V104, V105, and V106 by replacement.
Locate faulty resistor by heating slightly with a
soldering iron held near the resistor while ob-
serving the 500 volt DC calibration.
Allow time for the standard cell to stabilize.
Several hours should be sufficient.
Check V104 by replacement.
Recalibrate per paragraph 4-4.
Recalibrate per paragraph 4-4 and observe sta-
bility for 48 hours. If 500 volt DC reference
supply remains stable replacement of resistor
is unnecessary.
Recalibrate instrument and observe Stability of
reference supply. Replacement of the standard
cell may be necessary.
If the trouble occurs in the 50 volt range, R324
has shifted; in the 5 volt range, R325 or R327
has shifted; in the 0.5 volt range, R328 or R330
has shifted. It may be possible to correct by
recalibration. If not, replace faulty resistor.
It may be possible to correct all range voltages
by recalibration. If not, replace faulty resistor.
TROUBLESHOOTING (Sheet 1 of 2)