My early 70's SVT seems to be in trouble again. At practice the other day I was adjusting the mid-range and it suddenly quit on me. Not the amp just the mid-range knob. No amount of turning makes any difference in the sound and the amp seems to be giving a lot less power. It's like there is no midrange control at all and the amp is half the power it should be. It's not the first time this has happened either. When it last occurred a few years ago I had the amp repaired but the techie didn't find an exact cause of this, just various problems that needed looking at. Any ideas?