Vintage DOD Eq610 equalizer question

I just powered up this thing, after it sat for 30 years. It is making a slight humming sound. Even acoustically without the amp, when I turn on the giant power switch, I can hear the power supply(?) humming inside it, kind of like a florescent light (ballast) sound? The power cord is 3 pronged/110 volt. Is this humming possibly a ground issue, or is this thing just old and crappy, or.... any ideas? Thanks