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89Northstar Explorer 657 Installation and Operation Manual
4-2 Bottom is not displayed:
a The 657 may have Manual Range selected
and the depth may be outside the range
value selected. Either change the 657 to
Auto Range or select another depth range
(see section 8-5).
b The depth may be outside the 657’s range.
While in Auto Range, the display unit will
display “--.-” to indicate that there is no bottom
detected. A display of the bottom should
reappear when in shallower water.
4-3 The bottom is displayed too far up
the screen:
The 657 may have Manual Range selected
and the selected Range value is too high for
the depth. Either change the 657 to Auto
Range or select another depth range (see
section 8-5)
4-4 Bottom echo disappears or erratic digital
reading while the boat is moving:
a Ensure the back of the bottom surface of
the transducer is slightly lower than the
front and the front is as deep in the water as
possible in order to minimise the generation
of bubbles through cavitation. (See the
Transom Transducers Installation Guide, for
more information.)
b The transducer may be in turbulent water.
Air bubbles in the water disrupt the echoes
returned, interfering with the 657’s ability to
find the bottom or other targets. This often
happens when the boat is reversed. The
transducer must be mounted in a smooth
flow of water in order for the 657 to work at
all boat speeds.
c Electrical noise from the boat’s motor can
interfere with the 657. Try some suppression
spark plugs.
4-5 There is a double bottom trace displayed:
a The boat may be in an area that generates
shadows (see section 8-2).
b In shallow water, the echoes may bounce.
Reduce the gain setting (see section 8-6)
and/or reduce the sonar pulse power (see
section 17-3).
c Decrease the Range.