Humminbird 757c, 787c2i manual Finding the Cause of Noise

531528-1_A - 757c_787c2_&_787c2i_Man_Eng.qxp 10/16/2006 10:44 AM

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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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