Post in Audio Karma McIntosh forum & you may get some advise. Good luck.
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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.
Any older unit that has tape monitor switches - these are the first thing to check -especially if you aren't using the switches."Exercise" the switch(s) by flipping ,punching or rotating them repeatedly - if your problem changes or goes away even in the short term - this is probably a dirty contact - trying cleaning it if you can get to the switch with a spray cleaner.