I got my first case of the "donut dots" a couple of days ago. For me, it appeared as a seemingly random distribution of black dots, and magenta dots ~3x the size of the black dots. I would include a sample, but it would require a microscope with camera. My 30x magnifier helped me to see it clearly.

The U131 values were pretty drastically different:
BK: 175/98
M: 174/140

I tried the U464 AC Calibration, Modes 0, 2, 3, & 4. The black dots were unaffected, but the magenta dots reduced in size ~30%

Next I replaced the black & magenta developing units. The black dots were gone, and the magenta dots were smaller but not fixed.

In situations like this I get out all my different test patterns, and try to reproduce/emphasize the problem. The weird thing about the donut dots is that on white backgound it's only a single dot. On a 50% fill background you can clearly see a dot surrounded by a void, outlined by a ring. On 100% fill it totally disappears.

I've got a Panasonic ledger test chart that is an aerial map of NYC in the negative, so it's mostly black. When this test pattern is copied single color magenta, Ecoprint (to get the 50% fill), you can clearly see all the lovely donuts. And when you stare at them long enough you can see the 96.75mm interval of repeating patterns.

Doh! It's the magenta drum. Although there is no visible damage to the drum, it's got to be some sort of arcing. At any rate the magenta drum fixed it, and the Troubleshooting Guide was not too helpful.

Maybe this will help somebody get the fix in two, not four visits. =^..^=

I find u089 more useful for this kind of testing, but as long as it works for you.

I was fooled this past week by the primary transfer belt causing the donut dots. Was following all the steps in Bulletin 10, and then started to replace the K developer unit. Since it was the new style and required the film shields to be installed on the drum unit, I had to draw out the transfer belt. Looked down, saw and then felt the dots with my fingers, and just went "Doh!"

Cleaned with alcohol and all fixed.

The U089 test patterns did not display the dots. If you've got a test pattern with the gradient blocks 10%, 20% 30%, 70%, 80%, 90% & 100% did not show the dots. 40%, 50%, & 60% did show the dots.

I have 2 machines doing this. A 400 and a 500. They have Magenta dots in bands about an inch wide running lead to trail. Put in a PM kit A which did nothing so the belt is out. Tried swapping M and C drums. No difference. Tried a new dev. Unit. No help. Tried all the bulletin 10 adjustments. no help. Any other ideas?