C75 blotched gray output

Hello,
Our C75 printouts are not homogeneous flat grays, but quite visible large gray blotches. In comparison my Phaser 7800 does a much better job in this regard. Is it possible to improve this quality somehow? Replacing the developer powder? Or the black drum?

SRA3 output:

 

Only black appears this way? How long ago was the drum/developer replaced?
There's a slight possibility if you've recently replaced the charge wire/roller that it may not be mounted properly, but improper mounting and/or an aged unit usually produces linear artifacts in the print quality.
My guess would be something with the drum/developer.

If all colors appears this way, i'd lean my interest into the ITB.

*Correction*
On second look, I do see a definitive, hard line (quarter of the way down from the top), as well as the blotches moving in the print direction. Maybe a new charge unit would be a fix.

 

Thanks! Yes, only the black looks like this. The drum is at 50%, don't know about the developer, the machine is over 700k clicks.
The charge unit is the black drum's corona right? I've tried to clean the wire mesh now with a paper towel, it had some toner on it but didn't change the result.
A new black drum is on the way, hopefully it will fix it

 

Yes the corona wire, you should use a lint free cloth to clean the grid, any specs of dust or lint that lingers on the grid can cause a quality issue. You may also consider wiping the wire itself. Gently. Otherwise it may time to replace the wire altogether.

Do you run a lot of 11x17/12x18" stock? The drum life is based on 8.5x11 20# bond, so if you run all oversized media you can take that 50% life and bring it closer to 0%.

 

The new black drum has arrived with a new charge unit and nothing has changed. I've made a magenta test print too just to be sure, and it looks okay. It must be the black developer then?

Yes, I only run SRA3 stocks which is the EU equivalent of the 12x18" size

 

I'm not sure which component it would be, but our service tech had told us one day that he has seen new items out of the box have issues. It seems like a possible QC issue with some of the consumables.