Replace the power supply’s C1 5/12 volt Filter Cap

“…Unfortunately C1 at 2900 mfd (or 4700 mfd) is way too small of a filter cap, and the fact that this cap is 30 years old isn’t helping things either. Replace this cap with a new 6800 mfd to 10,000 mfd 16 volt or higher filter capacitor.
When replacing this capacitor it is NOT necessary to take the whole power supply apart! Just cut of the old capacitor off the power supply board, leaving the old cap leads as long as possible…”

-from http://www.pinrepair.com/sys1/