Again, can't emphasize enough ... Larry Pina's books! and I'm sure other compact Mac self-repair as well as Tom Lee's document: http://68kmla.org/files/classicmac2.pdf
The problem could be the cut-off resistor, Try adjusting R-53 (labeled "cut-off" on the analogue board). It could also be a bad R5 needs to be replaced. This is also the likely reason you have screen burn-in as the screen was operated in an over-bright condition for too long. The first thing you should do with any new acquisition is to check all the voltages and adjust them to optimal conditions to prolong the life of the Mac.
The problem could be the cut-off resistor, Try adjusting R-53 (labeled "cut-off" on the analogue board). It could also be a bad R5 needs to be replaced. This is also the likely reason you have screen burn-in as the screen was operated in an over-bright condition for too long. The first thing you should do with any new acquisition is to check all the voltages and adjust them to optimal conditions to prolong the life of the Mac.