Finding The Cause Of Noise - Humminbird 500 series 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
Other electronic devices
The boat's engine
Cavitation from the boat's
propeller
Isolation
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 reappears.
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.
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