74X Series
Calibration Manual
34
The other functions are also calibrated by following the step-by-step instructions on the
display.
Frequency Source:
When you calibrate the [
Frequency Source
] function, it requests
that you measure the frequency and voltage output. This signal is an approximate 50%
duty cycle, positive square wave, and the amplitude needs to be measured in pk volts, so
multiply the DMM reading by 2. The DMM must be a true-RMS type.
mA Measure:
When you calibrate the [mA Measure] function, the display requests 0.0
mA input. This does not equate to a short between mA high and low. Instead, set the
source device to source 0.0 mA, which is in effect the same as an open circuit.
TC Measure:
For [TC Measure] calibration, refer to the Thermocouple Measure part of
the Verification Procedure for the correct techniques. [TC Measure] should be calibrated
using ITS-90 standards.
Ohms Source:
Always configure the DMM in a 4-wire configuration
.
Adjustment of Potentiometers
There are three potentiometers on the Main PCA: R61, R77, and R82. These are factory
adjusted to tune out common mode error. Normally these should never need re-
adjustment. If after a repair you need to repeat this adjustment, proceed as follows.
1.
Carefully gain access to the Main PCA, and carefully power on the UUT.
2.
Set the calibrator to the [
DC Voltage Measure
] function.
3.
Short the front panel
V MEAS
high and low jacks together.
4.
Apply 10V at 60 Hz between the
V MEAS
high jack and TP12 on the Main PCB.
5.
Adjust R82 for a minimum output measured between TP24 and TP12. This value
must be less than 3 mV rms.
6.
Remove the jumper, then set the UUT to the [
DC Current Measure
] function.
7.
Apply 10V at 60 Hz between
mA/
Ω
/RTD MEAS
low jack (common) and TP12.
8.
Adjust R61 for a minimum output measured between TP17 and TP12. This value
must be less than 12 mV rms.
9.
Set the UUT to the [
TC Measure
] function, any type.
10.
Short the TC jack "+" and "-" terminals.
11.
Apply 10V at 60 Hz between the TC jacks and TP12.
12.
Adjust R77 for a minimum output measured between TP40 and TP12. This value
must be less than 3 mV rms.
13.
Remove the jumper.
Summary of Contents for 74X Series
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