TM 9-4935-282-34
803D
SECTION IV
MAINTENANCE
4-1. GENERAL
(1) Blow the instrument out with low-pressure,
The 803D AC/DC Differential Voltmeter seldom requires any
clean, dry air to remove most accumulations of dust and
maintenance. Without extreme abuse, all that should be
foreign matter. Pay particular attention to the binding posts,
required is occasional cleaning, tube replacement and
binding post wiring, critical switches (NULL, RANGE,
calibration. Periodic maintenance consists only of preventing
OPERATE-CALIBRATE, and polarity switches), and the two
any leakage as discussed in paragraph 4-2. A discussion of
glass epoxy strips which insulate the null detector chassis from
troubleshooting along with a troubleshooting chart, component
the main chassis.
location diagrams, and a tube voltage chart are provided in
(2) Clean the glass epoxy strip, binding posts,
insulators, and front panel with a rag saturated in anhydrous
procedures necessary to calibrate the instrument.
denatured ethyl alcohol.
(3) When necessary, wash all exposed dielectric
4-2. PERIODIC MAINTENANCE
surfaces of the NULL, RANGE, OPERATE-CALIBRATE, and
polarity switches with denatured alcohol using a small, stiff-
a. The accuracy of the 803D is adversely affected by
bristled brush.
excessive electrical leakage from the input of the null detector
to either ground or the 500 volt reference supply. Special care
CAUTION
has been taken to prevent leakage across certain critical
Do not use Metriclene, acetone, lacquer
switch wafers and areas of some printed circuit boards due to
thinner, or any other methyl ethyl
moisture from condensation. The printed circuit boards have
ketones, since they will react with the
been coated with Humi-Seal 1B12. The range, null, operate-
Lexan rotor on the CTS plastic switches.
calibrate, and polarity switches have been dipped in Dow
Corning silicone oil. Accumulations of dust and foreign matter
(4) After washing, recoat the exposed switch
will cause internal leakage, and should be removed as often as
insulating material with a solution of Dow Corning 200 having a
necessary, depending on environmental operating conditions.
viscosity between 5 and 20 centistokes. This prevents leakage
b. To check for excessive electrical leakage, proceed as
due to moisture on these surfaces.
follows:
(1) Perform preliminary operation procedure as
4-3. CORRECTIVE MAINTENANCE
stated in paragraph 2-3.
(2) Set switches on 803D as follows:
a. TROUBLESHOOTING.
The
purpose
of
troubleshooting is to quickly and accurately determine the
RANGE
500
cause of any abnormal condition. To assist in localizing most
NULL
0.1
troubles which might occur, the causes and remedies for a
voltage read-
490.00
number of symptoms are listed in the troubleshooting chart
out dials
failure, it is recommended that the substitution be tried before
(3) Remove te st leads from input posts.
any other tests are made.
However, the 803D is
(4) Excessive leakage is indicated if the meter
conservatively designed and should require very infrequent
deflects more than one-tenth of full scale.
tube replacement. At certain times, a check of the pin voltages
c. To prevent excessive electrical leakage, proceed as
in the tubes (figure 4-2) is also useful. As an aid to
follows:
troubleshooting, all components may be located by referring to
the figures shown in section V. An understanding of the theory
of operation (section III) and frequent reference to the
schematic diagram will be of great value in troubleshooting.
4-1
