| If the interference is due to high frequency transients in the MHz
region, which it could very well be, then the 10,000 uF cap is too
large -- it would not be effective above a few hundred kHZ.
Try instead a 0.1 uF ceramic 100V capacitor across the power terminals
on the depth sounder itself. Leads should be kept as short as
possible. Try for a couple of mm maximum on each lead. 1 cm can be
too long for noise above a few MHz.
If that doesn't work, then add a ferrite torroid (they're sold at Radio
Shack now) in the form of two U-shaped ferrites that meet at a machined
serfaces to form a closed square; the whole works is held together with a
plastic clamp. Wind BOTH power leads together through this ferrite
loop. At least 6-8 turns of both leads. This would help eliminate the
common-mode noise coming down the pair of power leads. The bypass cap
across them eliminates the differential mode.
Re-test to see if the problem leaves after each of these steps.
Instead of using the ferrite, you could add a second inductor to the
ground power lead.
Next, on the pair of signal wires or cable going to any second unit,
wind the entire cable around the ferrite loop, to break any common-mode
path.
Also, put the small cap and ferrite common-mode choke combination on
the remote unit if there is separate power and ground to that.
The suggestion of putting a cap on the alternator is important and
should be done.
Last, to be elegant, once you find the cure, you can start removing
some of the other things you had on to find exactly what it was you did
that was critical. Then you can work on cleaning up the source of the
interference on that lead by doing filtering at the source.
EMI suppression is difficult and requires quite a good knowledge of
that field to do it for hard problems. It is far worse when you are
trying to set up a SSB radio on the boat and find it not only
interferes with everything, but also burns some of it out! It can also
interfere with the autohelm before these suppression steps are done.
Eventually, we cured all interference -- but it required bringing a
high frequency scope aboard to do it.
Don
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| If your depth-finder transducer is aft of the propellor, you may
be observing a problem more applicable to power boats, ie. wash
and air under the transducer. When there are any bubbles in contact
with the transducer, it becomes inoperative.
The solution is the get it positioned better. If it's on the transom,
then move it away from the prop. If it's a through-hull, it's a
bit of a pain but I would suggest putting it for'd.
Cheers,
Ray.
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