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Calibration and Repair
This section details calibration of the instrument.
Calibration should be done only by a qualified
electronics technician.
********* WARNING *********
The calibration procedure requires adjusting the
instrument with power applied and so there is a
risk of personal injury or death by electric shock.
Please be careful.
Most of the calibration parameters are
determined by a computer aided calibration
procedure after burn-in at the factory. These
calibration parameters are quite stable and so
will not need to be adjusted. Calibration
parameters which may need field adjustment are
detailed below.
Multiplier Adjustments
On the HIGH dynamic reserve setting, there can
be some reference frequency feedthrough. This
section describes how to null this unwanted
output.
This adjustment requires an oscilloscope and a
signal generator which can provide a 500Hz
reference signal.
Allow the unit to warm up for about 1 hour.
Reset the unit by turning it off and back on while
holding the LOCAL key down.
Select voltage input A and connect a 50 1/2
terminator or shorting plug to the A input BNC
connector. Connect the 500 Hz reference signal
to the reference input. Set the SENSITIVITY to
1mV and the DYN RES to HIGH. The PRE
TIME CONSTANT should be set to 1mS and
the POST TIME CONSTANT to NONE.
Connect the scope to the CHANNEL 1 OUTPUT
on the front panel. Set the scope to 2V/div and
5mS/div. Externally trigger the scope using the
reference input signal.
After about 90 seconds, the scope display
should show a 500 Hz sine wave on a 30 Hz
(500/16 Hz) square wave. Remove the four
screws holding the top panel on. Slide the top
panel back about half way. Using a small
screwdriver, adjust P402 at location D2 to
minimize the 500 Hz output. Adjust P403 at location
C2 to minimize the 30 Hz output.
Now set the both time constants to 1S. Adjust P404
at location F4 to zero the output. This adjustment
has a range of 20% of full scale on the HIGH
dynamic reserve setting. (2% on NORM and 0.2%
on LOW). This zeroes the DC output of Channel 1
on all dynamic reserve ranges.
Now connect the scope to the CHANNEL 2
OUTPUT. Set the PRE TIME CONSTANT to 1mS
and the POST TIME CONSTANT to NONE. Adjust
P1102 to minimize the 500 Hz output. Adjust P1103
to minimize the 30 Hz output. Set both time
constants to 1S. Adjust P1104 to zero the output.
All three potentiometers are located on the plug-in
board in the center of the main circuit board.
Replace the top panel.
Amplifier and Filter Adjustments
This section describes how to adjust the Common
Mode Rejection and Line notch filter frequencies.
An oscilloscope and a signal generator which can
provide an accurate line frequency and twice line
frequency sine wave are required.
Allow the unit to warm up for about 1 hour.
Reset the unit by turning it off and back on while
holding the LOCAL key down.
Remove the four screws holding down the top panel.
Slide the panel back about halfway.
CMRR
Set the reference frequency to 100 Hz. It is
convenient to use the SYNC output of the signal
generator as the reference input if it is available.
Connect the sine output of the signal generator to
the A input and set the input selector to A. With the
SENSITIVITY at 100mV, adjust the amplitude of the
input signal to 100 mV (full scale).
Now set the input selector to A-B and connect the
signal to both the A and B inputs. Set the
SENSITIVITY to 20µV, the DYN RES to NORM and
the BANDPASS filter IN. Connect the scope to the
SIGNAL MONITOR output on the rear panel. Set
the scope to AC coupled, 0.2V/div, and 10mS/div.
Externally trigger the scope using the reference input
signal.