c. Guard terminal.
This terminal is usually strapped to the common
terminal.
The terminal
is connected directly to an inner chassis shield.
By removing the strap,
a guard potential equal to the common terminal
potential may be applied
to the guard terminal, shunting the leakage current
from the common terminal
to ground.
d. Isolated
output
terminals.
Provide
connection
to
an
external
recorder.
e. Isolated output level control.
For full-scale meter deflection,
this control allows adjustment of the isolated output voltage from 0 to 1
volt dc.
f. Range
control.
Provides
selection
of
the
desired
full-scale
sensitivity.
g. OPR/ZERO controls. Zero opens input terminals and shorts amplifier
input allowing zeroing of the instrument with the lower zero control.
In
OPR position the instrument is ready for operation as a voltmeter or null
detector.
2.
To electrically zero the Model 845AR, the controls would be placed in
the following positions:
POWER
ON
RANGE
10 MICROVOLTS
OPR/ZERO
ZERO
The zero control is then adjusted for an initial zero meter deflection. The
RANGE switch is set to the 1 MICROVOLT RANGE and then rezeroed with the zero
control.
3.
It may become necessary to adjust the mechanical zero control of the
Model 845AR at more frequent intervals than complete calibration.
To
mechanically zero the instrument, the following procedure would be followed:
a. The RANGE switch is placed to 1000 VOLTS and the POWER switch to ON.
b. The mechanical zero adjustment screw is adjusted for zero meter
deflection.
c. The RANGE switch is set to 10 MICROVOLTS and the instrument is
electrically zeroed.
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