Using the machine in a room where the humidity is too low may cause dark lines in halftone areas at 75mm intervals. This is because low-humidity conditions tend to cause variations in light sensitivity across the surface of the drum.
Selecting [On] for [Low Humidity Mode] under [System Settings] (User Tools) may help to prevent these lines from appearing.
When the low humidity mode setting is enabled, the drum is rotated slightly every 15 minutes. This keeps the light sensitivity constant across the entire surface of the drum.