I bought a cheap Chinese yellow fish finder last year. It worked very well and accurately measured depth. But I tried to turn it on this year and it does not turn on. I thought it was waterproof, but it is not.
To resolve this problem I disassembled the device.
2. Remove 6 screws and remove front panel. Be careful! Short wires inside.
3. Carefully inspect the board for the presence of oxides. In the center you can see the 8-bit microcontroller Holtek HT46R64.
4. Remove the PCB (5 screws on the bottom of it). On the other side I have found some oxides, so I cleaned it with extra refined petrol lighters refill. Looks like new.
5. When assembling do not forget to return the rubber buttons and strap! First of all install the screen into the body and then you can return the PCB.
It works well now after the cleaning! :)