Ultrex Fuse Blown

I was getting some interference on my sonar so I went to change out the fuse in the head unit of my 36V Ultrex. I read that if you change from 3 to 5 amp, the interference will go away.
The problem is, when I go to put a new fuse in, it blows every time. Even when I turn off the battery power to the trolling motor and depth finder. What would cause this?

So... changing the size from a 3amp to 5amp should NOT help interference... If the fuse is blown, it could have fome from a number of thing...Example, a spark, a cable that was connected while the power was on, etc... HOWEVER, if the fuse is blowing when you replace it, there is a direct short somewhere in the head unit would be my guess... Look for a pinched wire, perhaps the transducer cable is pinched?...

Thanks