Fluke 825A - Page 26

825А
(3) The ЕМЕ of the standard cell will change if the
cell has been inverted, or if the cell has been inadver-
tently short-circuited. If the EMF has changed, the 825A
will naturally be out of calibration on all ranges. In
either case, the cell will return to its original EMF. If
the cell were inverted or shorted for only a few seconds,
the 825A should be able to measure voltages within speci-
fications after several hours recovery time.
e. DECIMAL LAMP REPLACEMENT. The decimal
lamps are normal 6. 3 volt pilot bulbs. To prolong their
life these bulbs are operated from a 5 volt transformer
winding. However, these bulbs do occasionally fail. To
replace, proceed as follows:
(1) On cabinet models, remove screws at back of
cabinet and slide instrument out of case.
(2) Remove four screws that hold tront panel assembly
to chassis.
(3) Being careful to avoid wiring damage, gently lift
front panel assembly from instrument and set face down
in front of instrument.
(4) Remove screws holding range resistor printed
circuit board and Kelvin-Varley printed circuit board to
front panel assembly.
(5) Push these printed circuit boards aside enough to
allow access to pilot light holders.
(6) Remove pilot light holder from bracket by applying
pressure to each side of holder.
(7) Remove cardboard light shield and replace pilot
light.
(8) On rack models, remove screws holding cover to
instrument and set cover aside.
(9) Remove pilot light holder from bracket by applying
pressure to each side of holder.
(10) Remove cardboard light shield and replace pilot
light.
4-4. CALIBRATION
a. GENERAL, Тһе 825A сап be calibrated as often as
deemed necessary. However, it is recommended that the
R329 R323 R318 R326
Figure 4-4, ADJUSTMENT LOC ATIONS
Zener diode models be calibrated every 45 days and
standard cell models every 90 days. For special appli-
cations where extreme accuracy is required, it may be
desired to calibrate the instrument more frequently.
Calibration should be accomplished in a draft-free area
with ambient temperature of 72 (+3) ОЕ for maximum
accuracy under typical laboratory operating conditions.
Figure 4-3 lists the type of test equipment required for
calibration along with the applicable specifications and
recommended instruments. All controls may be located
with the aid of figure 4-4 and 4-5. The recommended
calibration set-up is shown in figure 4-7. While the
Bureau of Standards will certify the Eppley 100 cell to
only 0.01%, it may be maintained to 0. 002% if treated
with care and not subjected to sudden temperature
SPECI-
FICATIONS
REQUIRED
RECOMMENDED
EQUIPMENT
Any Fluke Model 800 series
voltmeter.
Fluke 301C or 301E Power Supply
Eppley Standard Cell 100A
Leeds and Northrup 2430A
Galvanometer
The equipment
used must pro-
vide 500, 50, 5,
and 0. 5 volts
with an accuracy
Electro
Scientific
Industries
of
0.
01%.
Special Precision Voltage
Divider Х194
Any single pole push button switch
with fine silver contacts
Any double pole push button switch
with fine silver contacts
Апу 5.6 (410%) О resistor
Any 56 (410%) Q resistor
Figure 4-3. CALIBRATION EQUIPMENT
Figure 4-5. ADJUSTMENT LOCATIONS