Raymarine L750 Operating Handbook page 81

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2. The Fishfinder will not display arches unless the boat is moving. If
the boat is stopped, fish will appear on the display as straight lines.
3. The transducer may not be aimed straight down. In order to display
complete arches, the transducer must be vertical.
4. The sensitivity setting may not be high enough. Press the Gain
button and increase the gain or sensitivity setting manually.
The display disappears or is unreliable when the boat
is moving at high speeds –
1. Increased turbulence around the transducer may be "confusing" the
unit. Move the transducer to a location where there is less
turbulence when the boat is moving.
2. If the Fishfinder is using the Auto Sensitivity setting, it may be
reacting to an increase in electrical "noise." As the level of noise
increases, the sensitivity automatically adjusts, and the unit can lose
its ability to see the bottom or fish. Check for problems with the
wiring as described in the section on "The display unit freezes."
The display unit shows lots of background "noise" –
1. The unit may be set to use the manual sensitivity adjustment, and
the sensitivity may be set too high. See the information on "Setup
Instructions."
2. The transducer may be mounted incorrectly, or may be fouled.
Check the mounting as described in Section 2. Clean the transducer
as described earlier in this section.
The depth setting flashes often –
Note: Normally, this indicates that the bottom has moved out of range.
1. The transducer may not be installed correctly, or the signal may not
be reaching the display unit. Recheck the instructions in "Mounting
the Transducer" in Section 2. Check the transducer cable for signs
of damage. If the cable is damaged, the transducer and cable
should be replaced as a unit.
2. The voltage reaching the unit may be low, and this may be affecting
the transmitting power. Check for problems with the wiring as
described in the section on "The display unit freezes."
L750 Fishfinder: Installation and Operation Handbook