Xerox DocuColor 242 (DC250 series) light banding black

[Detonation]

I have a DocuColor 242 that's showing significant banding OB side in black only. I suspected bad drum-developer contact or a bad developer housing, but this doesn't seem to be the case. I have done the following:

• replaced developer
• replaced black drum
• replaced the entire drum drawer with one from a good machine
• cleaned ROS
• replaced ROS
• replaced 1st BTR
• replaced entire ITB unit
• replaced MCU
• replaced charge (K) preclean HVPS
• replaced developer bias (K) HVPS
• replaced ROS flat cables

Some more things I could think of are NVM PWB, Power Unit, IOT driver PWB and flat cables between MCU PWB and IOT driver PWB. But honestly I'm getting a bit desperate and I'm also short on time. So I'd like to avoid a (further) wild goose chase and see if there's any suggestions here!

Attached is a full halftone 80% K. C, M and Y look good.

[Detonation]

SOLVED: 2nd BTR turned out to be te problem. Can't get my head around why it only failed with black, but there it is.

[i495979]

2btr change did not help for me. But K-drumunit change helped for awhile. So I suspect corotron or discharge string oxidized contacts on machine side.