New i50 not working

Hi I have two st40 depth finders networked via seatalk. The transponder is a in hull P79. The system has had intermittent problems, blinking display and reading --- especially in shallow waters (10 to 5). It it works very well in deep water. It has been suggested that the p79 is not compatible with the st40, lacking sufficient power to the transducer.

Bought a i50 depth and plugged the seatalk to seatalk ng adapter cable to the 3 way junction box, plugged in the 3 wires for the transponder and powered on. Got solid ---. What am I doing wrong.

Thanks
Duane