Sounds like the contrast control has a burned or corroded spot on it; set that
old most likely has manual controls for adjustments; will probably need to
replace the control, unless adjusting it either up or down (off the bad spot)
gets it to say put. Also could have bad solder connections on control. This
series of chassis also had problems with "goop" used to adhere wires and small
components in place on circuit boards; it would become corrosive/conductive
eventually, and must be removed/cleaned, and sometimes traces needed to be
repaired.