Humminbird 755C Marine GPS System User Manual

Display Problems
There are several main conditions or sources of possible interference that may
cause problems with the quality of the information displayed on the control
head. Look in the following table for some symptoms of display problems and
possible solutions:
Problem Possible Cause
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
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.
The control head loses
power at high speeds.
If the power output of your boat’s engine is
unregulated, the control head may be protecting
itself using its over-voltage protection feature.
Make sure the input voltage does not exceed
20 Volts.