Machine has 55.997 mono/55.331 color pages on the clock. Suddenly tiny black spots and a spotty slightly black "overtonered" looking background came up. All TCR readings are around 6.xx% and TCR-K had 6.22%. Since I have no new K-developing units at hand, I took from one of my stock machines a very good working K-developing unit with around 80K on it and tested it, to be sure, in my office machine which also had originally a K-unit around 70K or so.
I was able to switch those K-units without problems backwards and forwards between the machines so I was confident that swapping of units, though undesired, would be possible. However, later at the customers machine I got, with the previously tested and good working K-unit, after around 10 prints a C2557. The TCR-K was showing 2.41%. After putting back the original unit everything was OK except the before mentioned print quality ofcourse. Putting the replacement K-unit back and executing manual TCR-K toner supply, machine appeared to be extremely overtonered with black toner.
A 3rd K-unit from my stock machines showed exactly the same behavior, this time starting with a TCR-K of 1,98. For the moment the machine is running with the original developing unit but the customer wants the issue to be solved.
Question: Can this swapping of units really be that critical? Would the only solution be a new unit and is setting it up with the TCR set up procedure inevitable?