NorthStar Multi-Function Unit M121 Installation And Operation Manual page 107

i Electrical noise from the boat's engine or
an accessory may be interfering with the
transducer(s) and/or the Instrument. This
may cause the Instrument to automatically
decrease the Gain unless using Manual Gain.
The Instrument thus eliminates weaker
signals such as fish or even the bottom
from the display. This may be checked
by switching off other instruments,
accessories (e.g. bilge pump) and the
motor until the offending device is
located. To stop problems from electrical
noise, try:
- re-routing the power and transducer
cable(s) away from the boat's other
electrical wiring.
- routing the display unit's power cable
directly to the battery with an in-line fuse.
4-2 Bottom is not displayed:
a The Instrument may have Manual Range
selected and the depth may be outside
the range value selected. Either change
the Instrument to Auto Range or select
another depth range (see section 8-5).
b The depth may be outside the Instrument'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 Instrument may have Manual Range
selected and the selected Range value is
too high for the depth. Either change the
Instrument to Auto Range or select another
depth range (see section 8-5)
106 Northstar M121/M84 Installation and Operation Manual
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 Instrument'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
Instrument to work at all boat speeds.
c Electrical noise from the boat's motor can
interfere with the Instrument. 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 9-2).
b In shallow water, the echoes may bounce.
Reduce the gain setting (see section 9-6)
and/or reduce the sonar pulse power (see
section 19-3).
c Decrease the Range.