Airconditioner fuse (1 amp , glass) blows. What could cause it?

[lake of the woods]

I have the original Lunaire / MTC (controls) air conditioning system for my 1989 40 DC and recently it started blowing a 1 amp glass fuse located in the MTC controls. I have done no recent work on the electrical that would cause this. The fuse is for "system power" is is one of four glass fuses in the control (system power 1 amp, compressor 30 amp, fan supply 6 1/2 amp, and pump and valve 6amp). the other 3 fuses are all fine so it is only the "system power" fuse that blows. The wiring is all in excellent shape and all connections are tight and corrosion free (the joy of owning a well cared for fresh water boat)

Does anyone have any suggestions as to what may be causing this fuse to blow? where should I be looking?

the other air conditioning system (identical) on the boat works fine.

Mark
Lake of the Woods
1989 40 DC