My old Navman 8120, after years of great service, has started to go on the blink with respect to the sonar function.

The bottom trace is now indistinct and broken. When I turn the gain up little happens. It used to flood the screen with every particle detected below the boat when the gain is turned up. The depth reading is fine. I can compare it with my back-up device.

So far, I have cleaned the transom transponder, cleaned the LT6 connector, done a factory reset and checked the spliced cable (all soldered joints).

Anyone seen this before? I am loathe to start swapping in new parts like a new transponder. It is too hard on the wallet.

I have thrown myself on merci of Navman Marine but they have no effective representatives in the UK.

Thanks in advance