Here’s how I diagnosed and fixed it…
Firstly I just tried power cycling the thing, hoping it maybe was some kind of glitch cured by the standard fix. No joy.
A quick Google brought up no immediate suggestions, but the overall consensus was that vertical lines like this are either caused by bad pixels on the CCD or a fault in the calibration.
Faulty pixels seemed unlikely to me – the line down the pages that were being scanned was a couple of millimetres wide, and detail was visible within it, but everything was just a lot lighter.
I figured it was therefore calibration that was the issue. From the little I’d read from Googling the issue, it appears that the scanner does some kind of “white balance” before each scan. Maybe there was dirt on wherever the scanner CCD was calibrating?
Getting into my scanner was easy – disconnect the mains (of course), three screws on the back, two at the front and the top pops off reasonably easily. Looking underneath the top, I could see that the glass extended a little under the case, at the top, where the CCD normally sits when it’s not scanning. There was a piece of white plastic or card under the glass (not visible from the outside) where presumably the CCD did its white balance calibration before each scan.
And lo, and behold, there was a little blob of dirt, which looked exactly lined up with the vertical line on the output. Quite how it got there was anybody’s guess, as it was sandwiched between the white plastic and the glass. And the glass is glued into place. Maybe it was always there and just recently shifted. My theory was that scanner was calibrating itself, and the CCD’s exposure was bumped up on that section because the dirt was making it think that that section was underexposing.
So as the glass was glued in place, I figured my best bet was simply to stick a white strip of paper along the top of the glass where the CCD comes to rest, obscuring that pesky piece of dirt. Worth a try, I thought.
Problem solved. Reassembled the scanner, no vertical lines at all any more.