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The depth sounding errors are probably due to either
A. the power to the sonar-and-c80 system being loaded down by the addition of the EV-1... in case this might be, both measure the voltage near or at the C80 under load, and also check for looseness or high-resistance spots anywhere in the power distribution to the whole / all components of the sonar system... or
B. loose or corroded or broken/cut/severely bent cable or jack/connector/plug, anywhere from the sonar transponder to the C80... take each connector apart, along that line, scrub both sides of each connection with a clean dry brush, wipe both down with a clean dry rag cloth, and re-seat the connection firmly, then on to the next connector along that line and do the same.
Let me know what you observe, and what results you get... if the error persists, please describe it in some detail, or with details at least.