Humminbird 717, 727 manual Finding the Cause of Noise, Isolation

531524-1_A - 717 & 727 Man_Eng.qxp 9/30/2006 7:24 PM Page 80

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
propeller	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.

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