HL-4570CDW Diagnostic Question

[jccorbi]

Hi .. I have a Brother HL-4570CDW that is printing a heavy cyan line (see attached scanned image) down the left side of the page.
I've cleaned the interior and replaced the Drum unit and the Belt unit and, of course, all the cartridges.
The line is visible on the Belt when I check it after printing (not before).

Any ideas on what to look at next is appreciated!

 

[PeterBJ]

Here you find several manuals for the Brother HL-4570CDW printer, including technical reference manual and service manual.