Xerox phaser 7100

[Pessimist]

The printer had an error 092-321 - Patch K Fail. Cleaned the toner tube, refilled with AHK toner, replaced the chip. The printer became ready.
Now in the black drum unit, half of the developer goes to waste in 2-3 prints. I've been putting the developer back in, but the problem persists.

 

[Pessimist]

Found the cause.
A broken off curtain on the black cartridge blocked the contact to the charge shaft in the drum unit.