Ricoh A151 Field Service Manual page 200

If dirty background occurs only in ADS mode, do the following:
If the ADS voltage is not within ± 0.1 volts of the standard voltage (2.5
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V), readjust the ADS voltage.
If the signal at CN101-5 stays HIGH, check the harness and sensor,
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and replace defective parts.
If the signal at CN101-4 stays LOW, replace the main board.
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A151/A152 6-6 FSM