Why does machine turn on, motors work, but no audio input/output?

An accurate description of the problem is to imagine an unplugged cable at the tape heads' output. Naturally I have checked; what I need to know is if there is an adjustment or troubleshooting procedure (bias, output voltages, etc.), or if there is any audio processing circuit board available. Please help! Lots of music to digitize and an almost perfectly good machine to do it on...