DC242 - black lines but not drum. Ideas?

I have a DC242 and a DC250 - the 250 is really just a spare/backup machine.

The 242 prints Test Page 14 perfectly on Cyan, Magenta and Yellow but the pure black as vertical lines running from top to bottom - most visible in the 25% gray half of the test print.

So far I have replaced the IBT Cleaner, all of the 1st BTR rollers and the 2nd BTR Assembly and cleaned the ROS windows (not the mirrors).

Today I took the black developer tank from the DC250 (which prints a perfect pure black pattern 14) and swapped it across. At the same time I also swapped over the black drums (both are 90% good).

Nothing changed on the test print after all of these changes so I am at a bit of a loss as to what to test/change next.

 

Dirt in the ROS. Take apart and clean at your own peril

 

Which ROS given that the Magenta/Cyan/Yellow prints look fine? Living where I do I'm pretty much on my own regarding service and maintenance

 

Black/Cyan ROS. The left one as you look down on top from the front of the machine. Be careful with the ribbon cables and connectors as they break if you look at them wrongly