Tektronix Phaser 550 Service Manual page 228

B
Test Prints
One primary color faded or pale OR a single color has a dark
vertical streak
This indicates a problem with a single toner cartridge. Replacing
the toner cartridge should fix the problem.
If it does not, there may be a problem with the toner cartridge
selector/eject unit. It may not be pushing the cartridge quite far
enough to make proper contact with the photoconductive belt,
resulting in poor toner transfer. Replace the toner cartridge
selector/eject unit.
Lastly, poor electro-mechanical contact my be the problem,
although this usually shows itself as light, horizontal streaks and
lines. Replace the toner cartridge biasing assembly.
Single white spot appear in the same place on the print
This indicates a bad spot has developed on the imaging unit's
photoconductive belt which will not transfer toner. The imaging
unit must be replaced. If the spot appears to migrate vertically
from print to print then the defect is on the imaging unit's
accumulator belt. In either case the imaging unit must be
replaced.
Service Manual B-6