Spots on paper on printing

For awhile now my printer has been printing spots about every 7 centimetres. I have cleaned the drum roller a few times and every time it seems to add a new spot.

Eventually I bought a replacement drum unit and this has fixed the problem.

BUT I want to know what caused the problem as the drum has always looked clean.

My conclusion is that the problem is on the drum because the diameter of the drum matches the distance of the spots. 7cm/pi = cylinder diameter.

In trying to disassemble the old unit I eventually discovered that the way to disassemble it is to remove the C-clip at one end on the axle, then remove the axle. On one end the c-clip has a small wall around it. Remove the other one.

It seems like there is a bit of dust on the foam roller left in the case. This might be the problem, but I am not sure as this is a smaller roller.

No answers required, unless someone can confirm that this works. I broke the apparatus elsewhere before discovering how simple it was, so I hope this helps someone else.