HP 740B Operating And Service Manual page 20

Model 740B
Table 3 -3. Input Terminal Connections
Section ill
TYPE CONNECT INPUT TERMINALS
OF
MEASUREMENT
+
-
GUARD
~
Unguarded To high (most To low (most
To - Not
positive) of negative) of
terminal Connected
source
source
Guarded To high (most
To low (most To reference Not
positive) of
negative) of potential of Connected
source source
source
k. Adjust ZERO if necessary to obtain a zero
meter indication.
- - - - N O T E - - - - -
The ZERO control has the greatest
effect on the lower ranges and no
noticeable effect on the 100 V and
1000 V ranges. Paragraph 3-47
explains how to cancel the effects
of small offset voltages with the
ZERO control.
1. Depress Xl SENSITIVITY and remove short
from input terminals.
m. Connect input box terminals for the type of
measurement desired using the information
in Table 3-3.
- - - - N O T E - - - - -
1. Make sure all terminals are
tight. Use insulated solid
copper wire for all leads.
2. When measuring the voltage
drop across a current car-
rying load, be careful to
connect the + and - INPUT
leads directly to the load and
not to the current supply
wires. Voltage drop along
the current carrying wires
can cause a significant mea-
surement error.
3. Paragraph 3-49 explains
guarded measurements.
n. Meter should indicate between +0.1 and +1. O.
If necessary, change setting of RANGE switch
until a reading of +0.1 to +1. 0 is obtained.
o. Turn Xl VOLTAGE SET clockwise until the
meter deflection is between 0 and +0.1. Each
step of the VOLTAGE SET switch changes the
meter deflection by one major scale division.
p. Depress XlO SENSITIVITY and turn the cor-
responding VOLTAGE SET control cw toob-
tain a meter reading between 0 and +0.1.
q.
Repeat step p for X 10 2 , X10 3 and X10 4 SEN-
SITIVITYbuttons and VOLTAGE SET controls.
01794-1
r. Read the first five digits from the digital
readout tubes; sixth digit from the meter
(Figure 3-7).
+
Figure 3 -7 . Sixth Digit ~ VM
- - - - N O T E - - - - -
A full scale input (1 V on the 1 V
range, for example) is indicated
by the digital readout tubes indi-
cating 9 -9-9 -9 -9 (first five digits)
and the meter indicating 10 (sixth
digit) .
s. Before removing leads from device under test,
depress Xl SENSITIVITY.
THE 740B INPUT SHOULD BE
DISCHARGED FOLLOWING ALL
HIGH VOLTAGE MEASUREMENTS
TO PREVENT CARRYING THE
STORED CHARGE TO THE NEXT
POINT OF MEASUREMENT.
t. Remove leads from device under test or de-
energize voltage source and place INPUT Z
switch to 2 Meg position to discharge Model
740B input. Return INPUT Z switch to
00
after discharging input.
3-47. ZEROING EXTERNAL OFFSET VOLTAGES.
3 -48. Quite often small offset voltages exist in a test
setup as a result of contact between dissimilar metals,
thermals, etc. Offset voltage is present if a test
setup produces a voltage with no apparent drive volt-
age applied. The effects of offset voltage up to ap-
proximately
±
10
J1
V can be eliminated by the following
procedure:
3-9