its not the developer unit, per se. Its the developer powder in the unit. Time wears on it an it clumps. Especially if there is too much moisture. If your not on a service contract then doing this could be a challenge but not impossible. However, when i did it when i had a contract i was warned away from doing it again, by the techs boss. But the simple solution is replacing the developer powder after cleaning the unit out of old developer. As it is magnetic you could use a magnet but the powder would be difficult to remove from the magnet so put a bag or something to collect it over the magnet. When i did it i had to remove the 'blade' and wipe under it. Then put the blade back with the same width it started with, without the right tools this may ruin things, it did for me once. try to steer clear of removing the blade. but use some paper to slide out any clumps. The best choice is having a tech do the work, as they are trained in the procedure and know what not to do.
Hope this helps
I found your useful reply when searching for the remedy for the same problem. Is this problem typically fixable? as you said a tech should do it easily. I'm struggling to find an engineer to come to us, and all the advise to date says 'replace the whole developer unit' which seems over the top without at least trying to fix it. Do you know what the tech would do to fix it?
the problem is a clumped toner in the developer for magenta. The clump refuses to pass out of the narrow channel that moderates the amount of Developer/toner on the roll. Because it can't pass neither can the deveoloper in that spot leaving a line. The tech should be able to easily fix this. If it happens often with the color you may need to have the developer changed.
Our Phaser 7800 has developed a white line across the page. If you look @ an 8.5 x 11 sheet it is width wise across the 8.5" .
We have cleaned the LED via the wand and it still appears. It is approx 1" in from front of the page as it exits the printer.
We have also replaced the drum and it still appears.
Below is a pic of the problem.