The same mark keeps repeating

A mark that repeats at a regular spacing down the page is coming from a roller, and the gap between marks tells you which one. Measure it. Roughly 94mm points at the drum unit, and something stuck to the drum surface will usually come off with a dry cotton bud rolled gently across it. Never use anything wet or sharp on a drum. Shorter, tighter repeats generally mean the toner cartridge instead.

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