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t37, try your gain adjustment behind the gauge and check the wires for resistance and/or corrosion by peeling back some insulator near the transducer.
You could be getting "bounce" when the sonar ping hits the bottom, then refelcts off the surface of the water, then back down, and so forth, without giving a good reading.
Don't peel back the insulation too close to the transducer to the point where you can't solder in good splices of new wire to replace the old crap. Also, squirt a bit of PB or WD-40 on the connections, and softly sand to reveal new metal. Coat them with electrical grease and re-assemble. if you're not too keen on sanding, an eraser can work too, but takes a while and is hard to get in to tight spots.
Also record resistance measurements before and after cleaning. Simply disconnect the terminals on each end, short the far end together, and read through the wire on the other end. You should have a very nominal measurement even over a 50 foot run of coax. You could even compare to new wire to see how far gone the old stuff is.
TRANSDUCER=====>Connection<======Wires to gauge====
..........check ^^^ here......check ^^^ here
on the other end:
Wires to Gauge==========>Connection<======Gauge()
...................check ^^^ here..........check ^^^ here
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