SARTORIUS PMA 7500...
7
Calibrating/Adjusting the Scale
Calibration/Adjustment Sequence
Check and, if necessary, adjust the following factors:
1. Zero-point offset value
2. Off-center load
3. Linearity
4. Span adjustment
Zero-Point Offset Value
The zero-point offset value is necessary for adapting the bridge voltage of the
strain-gauge load cell to the operating range of the scale electronics.
This may be necessary:
– after replacing the strain-gauge system
– after replacing the main PCB
– if the span cannot be adjusted (even with the service software) and the
error code “E 02” is displayed
– when the zero-point offset voltage has changed and is outside the
tolerance limit (e.g., due to the strain-gauge load cell being “bent”)
Adjusting the Zero-Point Offset Value
Adjust the zero-point offset value using a digital voltmeter.
– Open the scale (see page9)
– Connect the cable from the digital voltmeter (DVM) to plug ST2, pins 1
and 2 (Pin 1: measuring voltage (OP output IC6 / pin 6), pin 2: ground)
– Replace the pan support on the scale and fasten it with the screws, place
the pan on the pan support and re-connect the scale to the power supply
– Use the DVM to check the preload voltage with no load on the scale:
Target (mV)
PCB installed at the factory -120 to -1,600
Replacement PCB (with menu access switch): -120 to -350
– Adjustment when the preload voltage does not equal the target:
Factory PCB: Check the strain-gauge load cell;
replace if necessary
Replacement PCB Solder or de-solder the bridges
(with menu access switch): on the main PCB as indicated below
Zero-Point Offset Voltage J5 J6 J7 J8 J9
< -580 mV Check the strain-gauge load cell; replace if nec.
-580 mV ... -350 mV open close open open open
-350 mV ... -100 mV open open open open close
-100 mV ...+120 mV open open open close open
+120 mV ...+360 mV open open close open close
> +360 mV Check the strain-gauge load cell; replace if nec.
– Check the zero-point offset voltage again; re-adjust if necessary
After the zero-point offset voltage is reset, the span must be adjusted (using
the service software, if necessary).
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