I'm sorry to hear of the MK issues you're struggling with your 7890 printer. I have a few questions below to help me get a better understanding of your situation. The information you provide will help me determine the cause and solution to your problem.
After reviewing your order history, I see you recently purchased the set of refillable arts and ConeColor ink bottles for your 7890 last month.
When did you first begin installing the ConeColor refill carts into your printer?
Did you remove air from the internal ink bags and prime the exit channels using the syringe and priming tip, as per our instructions?
Do you have the full set of refill carts installed, or a mixture of refill and Epson? If a mixture- which are refill and which are Epson?
About when did the MK channel begin not fully printing?
Did the MK clogging start slowly as a few missing nozzles and got progressively worse, or was it working fine then suddenly stopped printing?
What is the severity of the MK clogging- a few individual nozzles missing or a large section of the position? About what percentage of the MK nozzle check pattern would you say is missing?
Are the same nozzles missing on the nozzle checks, or do the missing nozzles move around after cleaning?
Does the nozzle check look better or worse after doing a few regular cleaning cycles or a power cleaning cycle?
Yes, my understanding is the MK and PK channels have different ink lines, and share a dual damper, but use the same print head channel- so if the PK is working well, but MK isn't- then the print head is likely fine, and the cause of the MK flow issue is caused elsewhere. Did the technician replace the dampers at the same time as he replaced the print head, cleaning pump and wiper blade?
Please let me know so I can try to help resolve this issue for you, and get you back to happily printing.
Best regards~ Dana