I have a Flextone II XL which I have been using for several years with no problems. Suddenly, it can do nothing except generate a horrible buzzing noise. The noise is at the full volume of the amp, and happens no matter the settings on any of the dials, including the volume. Even if I set everything to zero, it still happens from about 2 seconds after I turn the power on. Even with no guitar plugged in.  I have tried different power cords and wall outlets with no effect. I suspect the cause may be that this guy kicked the extension cord out of the wall and cut off the power while I was playing. It worked after that for about ten minutes, and then the noise started. Any ideas about what I should do?