Wacky Fishfinder Display

[Kiggsiaxjodlis]

I bought a Raymarine Element 7 fishfinder with HV-100 transducer four years ago. After a short while, it began to malfunction, first it wouldn’t show the depth readout, then it would begin going wacky on me, the screen getting all garbled, and in sidevision, the mapping would collapse, looking as if the boat were sitting on dry land. Then it would clear up and be ok for a while. The problem even occurred while the boat was going very slow, or not moving at all. As can be seen in the attached screenshots, the voltage is ok. The plug connections are all tight. No damage to the transducer, and it has been mounted according to the instruction manual. The puzzle is that the malfunctions are intermittent so I don’t suspect that rivets under the hull of my aluminum boat are causing all of the symptoms shown in these screenshots, especially since they sometimes occur while the boat isn't traveling. Might anyone have any input on what these screenshots are indicating? Each trip, the problems get worse. The corruption occurs whether it is in manual or autorange setting. The decent image here shows when it functions normally.