Humminbird 67 manual Finding the Cause of Noise, Possible Source of Noise Isolation

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	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.
The boat’s engine	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.
Cavitation from the boat’s	Turbulence created by the propeller can cause noise; make sure
propeller	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.

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