Lowrance electronic LCX-17M - Manual (Page 104)

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This causes the sonar to automatically increase its Discrimination or
noise rejection feature. This can cause the unit to eliminate weaker
signals such as fish or even structure from the display.
3. The water may be deeper than the sonar's ability to find the bottom.
If the sonar can't find the bottom signal while it's in the automatic
mode, the digital sonar display will flash continuously. It may change
the range to limits far greater than the water you are in. If this
happens, place the unit in the manual mode, then change the range to a
realistic one, (for example, 0-100 feet) and increase the sensitivity. As
you move into shallower water, a bottom signal should appear.
4. Check the battery voltage. If the voltage drops, the unit's transmitter
power also drops, reducing its ability to find the bottom or targets.
Bottom echo disappears at high speeds or erratic digital
reading or weak bottom echo while boat is moving
1. The transducer may be in turbulent water. It must be mounted in a
smooth flow of water in order for the sonar to work at all boat speeds. Air
bubbles in the water disrupt the sonar signals, interfering with its ability
to find the bottom or other targets. The technical term for this is
cavitation.
2. Electrical noise from the boat's motor can interfere with the sonar.
This causes the sonar to automatically increase its Discrimination or
noise rejection feature. This can cause the unit to eliminate weaker
signals such as fish or even structure from the display. Try using
resistor spark plugs or routing the sonar unit's power and transducer
cables away from other electrical wiring on the boat.
No fish arches when the Fish I.D. feature is off:
1. Make certain the transducer is pointing straight down. This is the
most common problem if a partial arch is displayed.
2. The sensitivity may not be high enough. In order for the unit to
display a fish arch, it has to be able to receive the fish's echo from the
time it enters the cone until it leaves. If the sensitivity is not high
enough, the unit shows the fish only when it is in the center of the cone.
3. Use the Zoom feature. It is much easier to display fish arches when
zoomed in on a small range of water than a large one. For example, you
will have much better luck seeing fish arches with a 30 to 60 foot range
than a 0 to 60 foot range. This enlarges the targets, allowing the
display to show much more detail.
4. The boat must be moving at a slow trolling speed to see fish arches.
If the boat is motionless, fish stay in the cone, showing on the screen as
straight horizontal lines.