c6501 a (not straight) line, lead to trail, only in grayscale (not in full black)

[GeorgeK]

Appreciate your help in a strange CQ problem.
Only in light grey or dark grey prints I have a lead to trail, centered, not straight line (as per attached files).
- Solid black is ok.
- Problem is NOT from fuser (checked image with panic stop and problem appears before entering fuser).
- Not from drum or corona (both replaced).
- Developing assy also ok (no dirt or sth strange)
- Laser cleaned
- Ferrite cores inserted in all cables (power/ethernet) for electrical noise.

Any suggestions would be much appreciated.
Thanks in advance for your attention.

[Synthohol]

maybe check the transfer belt and roller under the belt.
the smallest fuzz on the mylar in the laser path right before the drum can do that too.
just to eliminate the DV unit, tape the holes and swap it with magenta DV unit. leave the toner door open so it wont make a mess trying to add toner while testing. see if issue follows the DV unit.

[DigiMan]

You can panic stop it to see if this occurs on 1st or 2nd transfer just below the belt.
Synth is right about the belt,disassemble it and inspect/clean the associated rollers.
I presume printing an 125K on 53 test pattern you get same results?

[Mick01]

Looks to me like a fine hair in the laser path. This will move with the fans whilst operating giving you the vavy line.
Does it only occur in black?
As mentioned, panic stop and look at the image both on the drum and transfer belt. Doing this should isolate where in the process the issue occurs.