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b. Set power switch to ON and allow a 20 minute warm-
up period.
c. Set RANGE switch to 5 or 0. 5 volts, voltage readout
dials to zero, and NULL switch to 0. 001 volt.
d. Adjust R232, VTVM ZERO ADJ, for zero meter de-
flection. This control may be reached through the venti-
lation holes
in
the cabinet model (see figure 4-3).
In
the
rack model, an identified access hole is provided on the
rear panel.
2-3.
PRELIMINARY OPERATION
The following procedure prepares the voltmeter for oper-
ation.
a. Connect power plug to a 115 volt AC power source.
If
instrument has been wired for 230 volt operation, con-
nect to 230 volts AC.
NOTE
The round pin on polarized three-prong plug
connects instrument case to power system
ground. Use three-to-two pin adapter sup-
plied with instrument when connecting to a
two-contact receptacle.
For personnel
saiety, connect short lead to suitable ground.
b.
Set controls on 825A voltmeter as follows:
RANGE
500 volts
NULL
VTVM
polarity
+ (positive)
all voltage
0 (zero)
readout dials
power
ON
c. After a warmup period of at least ten minutes, ad-
vance OPERATE-CALIBRATE switch against spring ten-
sion to CALIBRATE, and adjust internal 500 volt
DC
re-
ference supply with CALIBRATE control for zero meter
deflection. Release OPERATE-CALIBRATE switch.
2-4.
OPERATION AS A DC DIFFERENTIAL VOLT-
METER
a.
After completing preliminary operation, connect
unknown voltage to input binding posts.
If
one side is
grounded, always connect it to the lower input post
(middle post).
b. Turn RANGE switch to lowest range that will allow
an on-scale reading and note approximate value of un-
known voltage as indicated on upper meter scale.
c.
If
meter reads to the left, turn polarity switch to
the negative position. The meter needle will deflect to
the right. This is because polarity of unknown voltage
is negative.
d.
Noting the position of illuminated decimal light,
set five voltage readout dials to approximate voltage
detern:iJned
in
step b. For exarnple,
if
approximate vol-
tage is 35 volts, the decimal light between the B and C
voltage readout dials will be illuminated.
Therefore,
set A dial to 3, B dial to 5, and C, D, and E dials to 0.
e. Set NULL switch to successively more sensitive
null ranges, as indicated in figure 2-2, and adjust voltage
readout dials for zero meter deflection
in
each null posi-
tion. When the meter needle indicates to the right, the
voltage under measurement is greater than the voltage
set on the voltage readout dials. When the indication is
to the left, the voltage is less than that set on the readout
dials.
INPUT
*RECOMMENDED
VOLTAGE
NULL
RANGE
SETTING
50-500
10 then 1
(then 0. 1 for voltages
from 50 to 100 volts)
5-50
1 then 0.1
(then 0. 01 for voltages
from 5 to 10 volts)
0.5-5
0. 1 then 0. 01
(then 0. 001 for voltages
from 0. 5 to 1 volt)
0-0.5
0. 01 then 0. 001
*Any null range may be used with any input vol-
tage range; recommended settings are those most
useful.
Figure 2-2. RECOMMENDED NULL SETTINGS
f. Read unknown voltage directly from the five voltage
readout dials.
2-5. OPERATION
AS
A CONVENTIONAL VTVM
If
it is desired to use the instrument
as
a VTVM only,
additional ranges can be made available by converting the
NULL ranges to VTVM ranges. This is made possible by
setting the voltage readout dials to zero.
Proceed
as
follows:
a. Perform preliminary operation procedures as stat-
ed in paragraph 2-3.
b.
Consult figure 2-3, and select full scale voltage
deflection desired.
If
the approximate value of the vol-
tage to be measured is unknown, select the 500 volt
range initially.
c. Set RANGE switch, NULL switch, and voltage dials
as
indicated for the range selected.
d.
Connect voltage to be measured to input binding
posts. Deflection to the right indicates that unknown
DC
voltage is of positive polarity.
e.
Read voltage from upper or lower scale
as
listed
in figure 2-3.
2-3
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