Fluke 825A - Maintenance

825А
SECTION IV
MAINTENANCE
4-1. GENERAL
The 825A Differential DC Voltmeter seldom requires
any maintenance. Without extreme abuse, all that should
be required is occasional cleaning, tube replacement,
and calibration. Preventive maintenance consists only
of preventing contamination of the chopper contacts and
any leakage as discussed in paragraph 4-2. A discus-
sion of troubleshooting along with a troubleshooting
chart, component location diagrams, and a tube voltage
chart are provided in paragraph 4-3. Paragraph 4-4
delineates the equipment and procedures necessary to
calibrate the instrument.
4-2. PREVENTIVE MAINTENANCE
a. LEAKAGE PREVENTION. The accuracy of the
825A is adversely affected by excessive electrical leak-
age from the input of the null detector to either ground
or the 500 volt reference supply. This leakage becomes
evident when the RANGE switch is set to 500, NULL
switch to 0.1, voltage readout dials to 490.00, and no
input at the binding posts. The leakage indicated on the
meter should be not more than one-tenth of full scale.
Special care has been taken to prevent leakage across
certain critical switch wafers and areas of some printed
circuit boards due to moisture from condensation. The
printed circuit boards and sensitive areas of the null
detector chassis have been coated with Humi-Seal 1B12
(manufactured by Columbia Technical Corp. of Wood-
side, New York). The RANGE, NULL, OPERATE-
CALIBRATE, and polarity switches have been dipped
in Dow Corning silicone oil. Accumulations of dust and
foreign matter will cause internal leakage, and should
be removed as often aS necessary, depending on en-
vironmental operating conditions. Blowing the instru-
ment out with low pressure, clean, dry air will remove
most accumulations. Particular attention should be
paid to the binding posts and wiring, the NULL, RANGE,
OPERATE-CALIBRATE, and polarity switches, and the
two glass epoxy strips which insulate the null detector
chassis from the main chassis.
b. After blowing the instrument out, wipe the glass
epoxy strips with a clean dry rag. If necessary use a
rag saturated in anhydrous denatured ethyl alcohol. The
binding posts, insulators, and front panel may also be
cleaned with denatured alcohol. When necessary, wash
all exposed dielectric surfaces of the NULL, RANGE,
OPERATE-CALIBRATE, and polarity switch with this
alcohol using a small, stiff-bristled brush.
CAUTION
Do not use Metriclene, acetone, lacquer
thinner, or any other methyl ethyl ketones
since they will react with the Lexan rotor
on the CTS plastic switches.
After washing, recoat the switch insulating material
with a solution of Dow Corning 200 having a viscosity
between 5 and 20 centistokes. This prevents leakage
due to moisture on these surfaces.
с. CHOPPER CLEANING. All electromechanical
choppers
accumulate
some
foreign
matter
on
the
con-
tacts
over
a
period
of
time.
This
contact
contamination
only
presents
a
problem
in
low
level
amplifiers
of
the
type
used
in
the
825A.
It
can
cause
the
zero
to
shift
to
such an extent that the null detector can no longer be
zeroed
with
VTVM
ZERO
ADJ
control
R232.
To
clean
the chopper contacts, proceed as follows:
(1) On cabinet models, remove screws at back of
cabinet and slide instrument out of case. On rack
models, remove screws holding cover to instrument
and set cover aside.
(2)
Remove
chopper
from
null
detector
chassis
as-
sembly
and
unplug
chopper
drive
unit.
(3)
Remove
two
screws
at
top
of
chopper
and
slide
cover
off.
CAUTION
Never touch the chopper contacts. It is
also good practice not to handle any interior
parts.
(4) Immerse chopper in clean denatured alcohol until
the contacts are just under the surface (figure 4-1) and
then agitate the chopper for about a minute.