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Page Title:
Vacuum-Tube Voltmeter Operation |
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  TM 11-6625-599-12
e. Set the RANGE switch to the lowest
outlet is available, be sure to use the two-
range that will give an onscale indication.
prong adapter and attach the grounding lead
to the case of the conventional two-prong
f. Determine the value of the voltage being
power outlet.
measured from the VOLTS meter indication
(upper scale). Polarity of the unknown volt-
a. Set the power switch to off (down) posi-
age is indicated by the polarity switch.
tion.
b. Connect the power cord to a 115-volt, 60-
2-7. Differential Voltmeter Operation
cycle source. If the vtvm is wired for 230-volt
operation connect the power cord to a 230-volt,
a. General The vtvm is used as a differen-
60-cycle source.
tial voltmeter by comparison of the unknown
c. Set the RANGE switch to 500.
voltage being measured with a known internal
d. Set the NULL switch to VTVM.
reference voltage. When the two voltages are
equal, the VOLTS meter indicates 0. The
e. Set the polarity switch to + (positive).
magnitude of the known internal reference
f. Set all five voltage-divider switches to 0
voltage, controlled by the adjustment of the
on voltage-divider indicators A through E.
voltage-divider switches A through E, is indi-
g. Set the power switch to ON.
cated on the voltage-divider indicators A
h. Allow the vtvm to warm up for 10 min-
through E. Hence the value of the unknown
utes.
voltage being measured may be read on the
i. Advance the OPERATE-CALIBRATE
voltage-divider indicators when the VOLTS
switch, against spring tension, to CALIBRATE
meter indicates 0. When the voltage-divider
and adjust the CALIBRATE control until the
switches are being adjusted,' a VOLTS meter
VOLTS meter indicates 0.
indicator deflection to the right indicates that
the unknown voltage is greater than the inter-
2-6. Vacuum-Tube Voltmeter Operation
nal reference voltage, and the voltage-divider
switch setting should be increased. The re-
Caution: Be sure to bond the vtvm to the
verse is true if the VOLTS meter indicator de-
unit to be measured either through the alter-
flects to the left. Perform the procedures in b
nating current (at) power source or by con-
below to measure (within 0.05-percent ac-
necting a jumper (not supplied) between the
curacy) positive and negative unknown vol-
chassis ground terminal on the vtvm and the
ages between 0 and 500 volts.
chassis of the unit to be measured.
(Differential
Measurement
Positive and negative voltages between O
b. Voltage
Mode).
and 500 volts may be measured (within 3-per-
cent accuracy ) with the vtvm used as a conven-
(1) Determine the polarity and approxi-
tional vacuum-tube voltmeter. Perform the
mate value of the unknown voltage
starting procedure (para 25) and proceed as
by using the vtvm as a conventional
follows:
vacuum-tube voltmeter (para 26).
a. Set the RANGE switch to 500 and the
Leave the RANGE switch in the low-
NULL switch to VTVM.
est range that will give an on-scale
b. Connect the test leads to the common in-
indication.
put terminal and the input terminal and to the
voltage source to be measured. If one side of
(2) Set voltage-divider switches A and B
the voltage source is grounded, always con-
to correspond with the approximate
nect it to the common input terminal.
value determined in (1) above and
c. Turn on the voltage source to be meas-
set switches C, D, and E to zero;
u red.
t h e n set the RANGE and NULL
d. Set the polarity switch so that the VOLTS
switches to the positions indicated in
meter indicator deflects to the right.
the chart below.
13
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