Lowrance electronic Lowrance Fish-finding Sonar & Mapping GPS LMS-520C - Manual (Page 113)

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In severe cases, it can completely cover the screen with black dots, or
cause the unit to operate erratically, or not at all.
To eliminate or minimize the effects of electrical noise, first try to de-
termine 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
sure the engine is also off. Turn your sonar on, then turn off Noise Re-
ject [also known as the ASP feature (Advanced Signal Processing)].
Sensitivity should be set at 90-95 percent. 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 equipment 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 re-
route the sonar unit's power cable and transducer cable away from the
wiring that is causing the interference. VHF radio antenna cables radi-
ate noise when transmitting, so be sure 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 sure everything except the sonar unit is turned off, then start the
engine. Increase the RPM with the gearshift in neutral. If noise ap-
pears on the display, the problem could be one of three things; spark
plugs, alternator, or tachometer wiring. Try using resistor spark plugs,
alternator filters, 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 sure to use the in-line fuse sup-
plied 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 your trans-
ducer owner's manual or the Installation instructions (Sec. 2) in this
manual for the best mounting position.
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