Are you talking about the little fuse in the back of the TNC with the voltage dial around it?
If this is blowing, your control is at fault. The control can be removed through the front. The wired in the back are actually plugs that come out sideways. Then the top of the control can be removed exposing plug in boards. It is probably the power supply board that is blowing the fuse. I hope your parameters are backed up.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)