Canon iR4570 Series Service Manual page 719

<Field Remedy>
Use the following service mode item to decrease the developing bias, thus suppressing the symptom:
COPIER>OPTION>BODY>FIX-SMR
0: default
1: decrease the developing bias by a single step (-20 V)
2: decrease the developing bias by 2 steps (-40 V)
If '2' is selected, the lines can become thinner.
<Image Sample>
F-14-7