HP 740B Operating And Service Manual page 25

Section III
deflection. Polarity of this voltage depends upon the
polarity of the meter deflection. Magnitude of the
voltage depends upon the degree of meter deflection
and the setting of the RECORDER AMPLITUDE con-
trol. With the RECORDER AMPLITUDE control set
to maximum (fully cw) the output voltage is approxi-
mately 1 Vdc into a 1
kn
load. The RECORDER
AMPLITUDE control may be useful in matching the
output voltage scale to an external recorder or moni-
toring device. Normally the RECORDER AMPLI-
TUDE control is adjusted for a 1 Vdc output (with
recorder or monitoring device connected) for an end
scale meter deflection.
3-70. Voltage gain from the Model 740B input to the
RECORDER OUTPUT terminals depends on the RANGE
and SENSITIVITY selected. Table 3-7 shows the gain
produced by different combinations of RANGE and
SENSITIVITY.
3-71. The - RECORDER OUTPUT terminal is con-
nected in common with the - INPUT and - OUTPUT
terminals on the Model 740B. All of these terminals
are isolated from earth ground
(~
) although the main
chassis and cabinet are connected to earth ground
through the center pin of the line cord plug. This
allows the Model 740B to make floating measurements
(measurements where the - INPUT terminal is re-
ferenced above or below earth ground) without danger
to the operator since the chassis and cabinet remain
at earth ground potential. The chassis and cabinet
of most recorders, however, are not isolated from
the recorder's - input terminal. This means that
floating measurements usually cannot be safely made
with a recorder connected to the RECORDER OUTPUT
terminals since the recorder's chassis and cabinet
would be connected to the floating potential. Using an
isolation transformer or 3 prong to 2 prong adapter to
isolate the power input to the recorder would not help.
The recorder's cabinet and chassis would be floated
at the same potential as the Model 740B - INPUT
terminal and a dangerous condition would exist. The
only safe way to make floating measurements using a
3 -14
Model 740B
recorder is to select a recorder with a floating input
(- input terminal isolated from the recorder cabinet
and chassis) and a grounded chassis and cabinet.
3-72. OPERATING PROCEDURE.
3-73. To use the Model 740B as a voltage amplifier.
proceed as follows:
a. Turn on the Model 740B and allow one hour
for instrument warmup.
b. Connect recorder or monitoring device to
RECORDER OUTPUT termiI1.als.
DO NOT ATTEMPT TO FLOAT
MODEL 740B -INPUT TERMINAL
UNLESSRECORDER-TERMINAL
IS ALSO FLOATED (PARAGRAPH
3-71).
c. Set FUNCTION to STD; turn STANDARD
VERNIER fully cw (m eter deflects to + end
scale).
d. Adjust RECORDER AMPLITUDE conlrol for
desired end scale voltage value as indica led
on recorder or monitoring device.
e. Return STANDARD VERNIER conlrol to zero'
(fully ccw).
f.
Set FUNCTION to toVM or VM as desired.
g. Perform steps e through
l'
of Paragraph 3-46
if toVM mode is selected; steps e lhrough
j
of
Paragraph 3-59
if
VM mode is selected.
h. Observe RECORDER OUTPUT voltage on re-
corder or monitoring device.
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