call B&K - ask for Jerry in Tech Support. He is by far the most familiar and forthcoming about the older amps. Describe the problem - he may be able to give you a fix over the phone (or at least what/how to check it). You'll probably have to send it in for repair, unless it's something simple like a loose wire. It doesn't sound to me like it can be fuses but I'd check them anyway. B&K can tell you where they're located (inside the amp). If I had to guess I'd expect the repair cost to be ~$100 plus 2 way shipping. They'll fix what's wrong and go through the rest of the amp too - give you all new fuses, power switch, reset the bias, etc . . . so it's not a bad deal for the money. Especially compared to most shops who'll charge tou a min $200.