I got a DOD fx25b second hand, and it has a problem. When its on, it works fine and sounds great, but when I turn it off, the signal from my bass just disappears, and slowly goes back to its orignal volume after about 30 seconds. I power it with a 9v DC power supply, so its not the battery, has anybody got any idea whats wrong with it, and if it will be able to be fixed?