I have an old Hartke 3500 head sitting around that I no longer use and want to get rid of. It worked the last time I used it (a couple years ago), but now it doesn't. It gets power, the fan runs, all the lights light up, the compressor light acts like it's getting a signal, but the only thing coming out of the speaker cab is a humming noise. The hum gets louder as I turn up the volume, and the tone controls affect the hum as well (bass control adds bass, each of the EQ sliders add that frequency...) but no matter what, no sound from the bass. I've tried everything - different cables, different basses, plugging into all the different jacks - nothing. Does anyone have any ideas?