Eagle Electronics SeaFinder 250 DF - Manual (Page 65)

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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 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 Re-
ject [also known as the ASP feature (Advanced Signal Processing)].
Sensitivity should be set at 90-95 percent. There should be a steady
bottom signal on the display. Now turn on each piece of electrical
equipment on the boat and view the effect on the sonar's display. For
example, turn on the bilge pump and view the sonar display for noise.