Lowrance electronic LCX-15MT - Manual (Page 65)

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NOISE
A major cause of sonar problems is electrical noise. This usually appears
on the sonar’s display as random patterns of dots or lines. In severe cases,
it can completely cover the screen with black dots, or cause the unit oper-
ate erratically, or not at all.
To eliminate or minimize the effects of electrical noise, first try to deter-
mine the cause. With the boat at rest in the water, the first thing you should
do is turn all electrical equipment on the boat off. Make certain the engine
is off, also. Turn the sonar unit on, then turn off ASP (Advanced Signal
Processing). There should be a steady bottom signal on the display. Now
turn on each piece of electrical equipment on the boat and view the effect
on the sonar’s display. For example, turn on the bilge pump and view the
sonar display for noise. If no noise is present, turn the pump off, then turn
on the VHF radio and transmit. Keep doing this until all electrical equip-
ment has been turned on, their effect on the sonar display noted, then
turned off.
If you find noise interference from an electrical instrument, trolling motor,
pump, or radio, try to isolate the problem. You can usually reroute the
sonar unit’s power cable and transducer cable away from the wiring that
is causing the interference. VHF radio antenna cables radiate noise when
transmitting, so be certain to keep the sonar’s wires away from it. You may
need to route the sonar unit’s power cable directly to the battery to isolate
it from other wiring on the boat.
If no noise displays on the sonar unit from electrical equipment, then make
certain everything except the sonar unit is turned off, then start the en-
gine. Increase the RPM with the gearshift in neutral. If noise appears on
the display, the problem could be one of three things; spark plugs, alter-
nator, or tachometer wiring. Try using resistor spark plugs, alternator fil-
ters, or routing the sonar unit’s power cable away from engine wiring.
Again, routing the power cable directly to the battery helps eliminate noise
problems. Make certain to use the in-line fuse supplied with the unit when
wiring the power cable to the battery.
When no noise appears on the sonar unit after all of the above tests, then
the noise source is probably cavitation. Many novices or persons with
limited experience make hasty sonar installations which function perfectly
in shallow water, or when the boat is at rest. In nearly all cases, the cause
of the malfunction will be the location and/or angle of the transducer. The
face of the transducer must be placed in a location that has a smooth flow
of water at all boat speeds. Read the transducer instructions for the best
mounting position.