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Dell 3110CN printing white diagonal lines

The machine is plugged into a dedicated wall socket.

I have reseated the Black toner (where the problem seems to be)

I have reseated the transfer belt/drum

I have reset defaults/NVRAM.

I have run the contamination check.

On the contamination check yellow, magenta and cyan seem fine.

Black has diagonal white lines 3/4 across the page. The last quarter looks fine.

Any ideas what the problem is and how to fix this?

I have several of these printers and this is the second to start doing this.

TIA to all answers