HP LaserJet 4200 Print Quality Issue - Vertical banding

The attached file shows vertical banding, or more specifically, loss of toner intensity on the same central area of the page on every page that is printed. The transfer roller is new and I would just like a confirmed diagnosis from someone with more experience than myself as to the most likely cause of this print quality issue. Many thanks.
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Solution: HP LaserJet 4200 Print Quality Issue - Vertical banding

Not sure where it is oin your printer, but if you look at the toner cartridge you'll see the green drum (hidden by a flap). The laser window is in the printer next to the drum