LaserJet 4050N with blank line lengthways down middle of each print, about
5mm wide. Only appeared after being unused for 3 months.

Service manual says I need to start replacing huge parts of the printer, but
I was hoping for other ideas.
I've dismantled it as far as I can and can't see any alien objects, or signs
of wear.

Haven't started looking at main printing stage, as I wouldn't know where to
start. Are there manuals that doc dismantling this sort of thing?