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b. TUBE REPLACEMENT. Replacement of tubes will
not normally necessitate complete recalibration of the
825A.
If
V202 or V203 is replaced, calibrate the null
detector as set forth in paragraphs 4-4b and 4-4c. Re-
placement of Vl02, Vl04, Vl05, or Vl06 will necess-
itate calibration of the reference supply per paragraphs
4-4b and 4-4d.
c.
COMPONENT REPLACEMENT. Should it ever
become necessary to replace a component on a printed
circuit board or on the null detector chassis, proceed
as follows:
(1) Clean soldered area around component with Methyl
Ethyl Keytone.
(2) Remove part and install new component.
{3) Remove flux with DuPont Freon PC.
(4) Allow to dry for 10 minutes.
When it is cool or
humid, it may be necessary to place area under a lamp
to ensure thorough drying.
( 5) Coat any uncoated areas around R305 through R310
on the Range Resistor Printed Circuit Board and around
the input to the null detector chassis {tube V202, 5751, ·
and chopper) with Hurni-Seal 1Bl2.
d. STANDARD CELL PROBLEMS. The standard cell
in the Model 825A and 825AR deserves special consid-
eration if it is suspected of being faulty:
(1) The standard cell used in the 825A and 825AR is a
miniature low hysteresis unit which has excellent long
term stability and negligible temperature hysteresis.
(Hysteresis is a temporary increase in EMF immedi-
ately following a decrease in temperature; this effect
should not be confused with temperature coefficient.)
Under normal conditions this cell should last from 8 to
15 years.
In rare instances, failure has occurred in
less than 2 years. End of life is usually marked by an
increase in temperature hysteresis effect.
That is,
reading errors in excess of 0. 025% will result when the
same voltage is read with the 825A hot and cold. Should
replacement of the cell become necessary for any rea-
son, the instrument must be recalibrated (basic 500
volt range only), since the EMF of different cells may
be different by as much as 0. 05% and each instrument
is calibrated to its own particular standard cell. Refer
to the recalibration instructions in this case.
(2) Failure of the standard cell may occur if subjected
to below freezing temperature.
The electrolyte will
freeze at 1°F and operation below 32oF is definitely not
recommended. The life of the cell also will be greater if
the 825A is not operated at elevated temperatures. The
8 to 15 year figure holds for operation of the instrument
in normal room temperature.
(3)
The EMF 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 front 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 ac_!::ess 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.
The 825A can be calibrated as often as
deemed necessary. However, it is recommended that the
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) OF for maximum
accuracy under typical laboratory operating conditions.
Figure 4-2 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 figures 4-3 and 4-4. The recommended
calibration set-up is shown in figure 4-6.
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
RECOMMENDED
SPEC!-
EQUIPMENT
FICATIONS
REQUIRED
Any Fluke Model 800 series
voltmeter.
Fluke 301C or 301E Power Supply
Eppley Standard Cell lOOA
The equipment
used must pro-
Leeds and Northrup 2430A
vide 500, 50, 5,
Galvanometer
and 0. 5 volts
Electro Scientific Industries
with an accuracy
Special Precision Voltage
of 0. 01%.
Divider X194
Any single pole push button switch
with fine silver contacts
Any double pole push button switch
with fine silver contacts
/
Any 5. 6 {±10%)
n
resistor
Any 56 (±10%)
n
resistor
Figure 4-2.
CALIBRATION EQUIPMENT
4-3
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