KONICA C552 strange printing defect

[sendpiks]

Hi Everyone and thank you for your help and knowledge.
I have very unusual printing effect that looks very common but after carefully looking its not so easy.
Please look at attached scanned test page. On the bottom you can see white strips on 50% and 25% density. I KNOW What is the problem of these strips, this is dirty corona mesh in yellow imaging unit. I KNOW how to solve this problem, just to replace imaging unit.
Most interesting goes next. After replacing the unit to the new one same defect happens on new unit after ~10K prints. I replaced unit again, and again after 10K prints I have these strips. I tried FOUR different units. All the same. I dismantled one unit and cleaned corona mesh, put it back and after 10K prints it have same defect. I replaced developer, no changes.
I have no more ideas what happened. I have about 10 printers of same model and have this defect only on one printer only on yellow color.
Only original toners and unites was used.

what is the possible problem? Why printer contaminate mesh? Why only 1.5 cm from the bottom?

Thank you for ANY ideas.

[tsbservice]

Check machine for dirty filters. It may interfere with machine internal ventilation/suction.

[Zesti]

Seems toner scattering at back end of IU. Clean laser window and which toner are you using. Whats level history of yellow.

[Bix]

Other expired consumables may cause problems for your yellow.

[mascan42]

Check machine for dirty filters. It may interfere with machine internal ventilation/suction.

This. I more often see it with cyan, but a clogged filter or ventilation duct can make the corona become contaminated very easily.

[allan]

So the problem does not go any after cleaning the laser glass and charge corona?
Ouch to change those at 10K....

[printerguy77]

I've had this before. I'd bet my house it is laser glass. Cleaning tool does not get it clean as the time I had it there was a sticky substance on the glass. Had to remove laser assy to clean it properly. It's not that tough a strip down either. Honestly. Give it a try.

[sendpiks]

Check machine for dirty filters. It may interfere with machine internal ventilation/suction.

THANK You Very Much for everyone answered for this tread.
Solution I found was based on advice by tsbservice
I replaced toner filter (that on back of machine), vacuumed and cleaned all back suction pipes and replaced suction fan to a more powerful one, and this helps! All residues was gone.

Thanks
Leo