Xerox 7535 low print quality

[CarecaIII]

Hi, I bought a used Xerox 7535. I did a test to evaluate the print quality and I noticed some defects on black, magenta and cyan that I report in the attached photos.
The photosensitive tabures still have at least 50% of life remaining.
What could it depend on? How to solve it?
Thank you.

[Detonation]

The life on the drums isn't always accurate, as it ticks down only per every print, while the units also degrade over time. Have you tried cleaning the LPH's (Led Print Heads) already? That should hopefully help with the light bands. As for black, the developer might need replacing.

[Jettdog]

Bad drum units most likely. Use the cleaning wand that should be on the inside of the drop down door and clean the leds. If the defects still happen then swap the drum units around to see if the image defects follow the drum. If it doesn't then you have bad developer unit which is a little more involved to replace

[CarecaIII]

Thanks to both of you for your replies. In fact I had already tried cleaning the LEDs, but without any noticeable improvements. I will try to invert the drums to see if the situation improves. Otherwise how should I proceed to replace the developer? Thank you.

[Detonation]

I'm not sure what you paid for the machine, but personally I would not bother to change developer on these machines anymore. Developer housing + powder are around € 150 per color.