Tektronix Phaser 550 Service Manual page 225

B
Test Prints
Small, repeating light dot in one color
This can usually be traced to a dent-like defect in the developer
roller of the toner cartridge in question. The dents should be
1.5 inches (3.4 cm) apart or 1 inch (2.5 cm) for 1200 dpi printing.
No toner is transferred from the developer roller to the
photoconductive belt at the defect point.
If the sports are about 5 inches (12.5 cm) apart, then the problem
is a defect on the fuser roller. Replace the fuser.
In some instances, the spots may be dark instead of white. They
may also be shaped something other than round.
Single light, horizontal line in all the prints
This is probably caused by a defective photoconductive belt onto
which toner will not transfer. Replace the imaging unit.
Service Manual B-3