Eagle Electronics FISH MARK 500CDF - Manual (Page 101)

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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 sig-
nals 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 dis-
play 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 dis-
play 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.
Noise
A major cause of sonar problems is electrical noise. This usually ap-
pears on the sonar's display as random patterns of dots or lines. In se-
vere 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 Reject
[also known as the ASP feature (Advanced Signal Processing)]. Sensi-