Fluke 825A - Tube Replacement; Component Replacement

Measurements аге out of
tolerance on any range
when the Kelvin-Varley
divider is dialed to any
setting other than 499910.
One of the Kelvin-Varley
tolerance.
Resistors R306, R307,
R308 or R309 out of
tolerance.
Instrument is out of
specifications on all
ranges.
Meter rattle, drift,
or error is observed
on all null ranges.
V202, У203, or V204
faulty.
Chopper G1 faulty.
Moisture or dirt on
printed circuit boards
or switches.
SYMPTOM PROBABLE CAUSE REMEDY
divider resistors is out of
Measure the voltage drop across each Kelvin-
Varley resistor with another John Fluke Differ-
ential Voltmeter. Begin by setting RANGE switch
to 500 volts and the voltage readout dials to
499.910. Reference to the schematic diagram
will show that there should be 100, 10, 1, 0.1,
and 0.01 volts respectively across each resistor
of the A, B, C, D, and E decades, except for the
two resistors of each decade that are paralleled
by the following decade. Across these two re-
sistors, there should be 50, 0.5, and 0. 05 volts,
respectively. Measure all voltages +0. 05%.
Remember that if one resistor in a decade has
increased or decreased appreciably, the voltage
drop across all other resistors in the decade will
be slightly affected also.
Check and replace faulty resistor.
Check V202, V203, and V204 by replacement.
Clean or replace chopper.
Clean instrument.
Figure 4-2. TROUBLESHOOTING (Sheet 2 of 2)
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 V102, V104, V105, or V106 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 du Pont 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 Humi-Seal 1В12.
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 а
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 32°F 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. i
4-3