I have two amplifiers, one for a regular guitar and the other for a bass guitar, and they both have the same problem. When I turn them on, they make a loud buzzing sound if the guitar is plugged in, but don't do anything when I play it. Neither amp can be taken apart without destroying the chassis. The cord and the guitars are fine, as I've tried both on a friend's amplifiers and they worked fine then. Can anybody tell me what the problem is and how to fix it, or if I just have to buy new amplifiers?