Black strak adv 6265

[Uniquer]

A black streak at the top of print or copy.
I already replaced unit drum , developer , transfer from a known good machine but no hope.
Surprisingly, when I replaced corona it worked for some printing but come back.when i put the old corona it worked normally in other machine.
When I decrease primary charge it works for aome print and come back any suggestions.
Thanks

[Brian8506]

Slit glass above drum, transfer cleaning blade. Assuming it happens before fuser.