Finding The Cause Of Noise - Humminbird Matrix 67 Operation Manual

Finding the Cause of Noise

Electrical noise usually affects the display with many black dots at high speeds, and high sensitivity
readings. One or more of the following sources can cause noise or interference:
Possible Source of Noise Isolation
Other electronic devices
The boat's engine
Cavitation from the boat's
propeller
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Turn off any nearby electronic devices to see if the problem
goes away, then turn them on one at a time to see if the noise
re-appears.
To determine whether the boat's engine is the source of the
noise, increase the RPMs while the boat is in neutral and
stationary to see if the noise increases proportionately; if noise
appears when you rev the engine, the problem could be the
spark plugs, alternator, or tachometer wiring. Replace the spark
plugs with resistor plugs, install an alternator filter, or route the
control head power and transducer cables away from the
engine wiring.
Turbulence created by the propeller can cause noise; make sure
the transducer is mounted at least 15" (38 cm) from the
propeller, and that the water flows smoothly over the face of
the transducer at all times.