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Troubleshooting First Checklist/ - 4
A maladjusted screen can mimic the symptoms of
deflection board or CRT failures.
By performing the adjustment procedures, you might determine if
one or more of the adjustments is the cause of the problem.
CRTs rarely fail.
Needless CRT replacement can be prevented by checking display
adjustments, checking the possibility of other defective modules,
and accepting small imperfections in screen display.
If you have any doubts about whether a CRT is defective, contact
Apple Technical Support.