OHAUS SD Series, Service Manual

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CHAPTER 2 TROUBLESHOOTING
2.4 CHECKING THE LOAD CELL’S SIGNAL
Open the Indicator by removing the four screws in the bottom housing.
Note : Do NOT open the base on smaller models – doing so will destroy the internal
electronic components, requiring the base to be replaced.
2.4.1 Output Voltage Test
Measure the no load, 50% load and full load output. The reading should meet the Load Cell
specifications. The Load Cell output should be very close to linear over its capacity range.
NOTE : The following steps involve power applied to the scale. Load Cell solder
contacts can be used as measuring points. See Figure 2-2.
Figure 2-2. SD Series PCB connections
The EXE+ and EXE– wires should be connected to the PCB, and the SIG+ and SIG–
wires must be disconnected. Record the colors for each wire connection before
disconnecting. (See Table 2-2 for typical color code for SD Series.)
TABLE 2-2. COLOR CODE FOR LOAD CELL WIRING *
WHITE GREEN YELLOW RED
EXE+ SIG+ SIG– EXE–
* Color codes may vary.
1. Power on the Indicator with AC Adapter.
2. Using a voltmeter, with no load on the Platform, measure the voltage across points 1
and 4 of Load Cell connection on the PCB (+EXE and –EXE). This voltage must be
approximately 3.0 Volts dc. If the voltage is lower, disconnect the Load Cell cable
from the PCB and measure again. If the voltage is 3 Volts dc, the Load Cell is
defective and must be replaced. If the voltage remains low, the PCB is defective and
must be replaced.
CAUTION:
IN THE NEXT STEP, DO NOT OVERLOAD THE SCALE BEYOND FULL
CAPACITY RATING.
Ohaus Corporation www.ohaus.com 2-4 SD Series Scales Service Manual