The MA5100 is an old beastie . . . there's all sorts of things that could be going wrong. Semiconductors from that era frequently simply fail with age, as do electrolytic capacitors.
Specific problems that I remember from the 5100 were dried-out main filter capacitors that cause oscillation in the preamp circuits, bad/flakey slide switches, and worn volume controls.
Specific problems that I remember from the 5100 were dried-out main filter capacitors that cause oscillation in the preamp circuits, bad/flakey slide switches, and worn volume controls.