Nasa Clipper Echo Sounder

[alancollins]

I have replaced an old Navman fish finder with a Nasa echo sounder because it was a simple fit on my dashboard. I fitted an in -hull transducer close to where the old one was. It powered up as it should and gave a correct depth reading until I increased engine revs when it went haywire and gave spurious readings. I contacted Nasa and they said it was alternator interference and I needed to install a suppressor. I did this but it made no real difference. They now say I need radio frequency chokes in the power cables . I am baffled by this as the old sounder didn't suffer from interference and nor do the chart plotter, the VHF or any other electrics on board. Has anyone else had problems installing Nasa electronics?

 

[andsarkit]

No direct experience of Nasa electronics but I have spent time getting electronic products through EMC testing. It is a bit of a black art and trial and error is the only way as the coupling of interference can be quite difficult to predict. Putting RF chokes on the supply lines is a good first try. If you have any old VDU or USB cables with the 'lump' at one end cut off the plastic cover and put the ferrite around the power cables. Alternatively get a torroidal ferrite which you can pass the power cable through several times to increase the effect.
Also worth checking that the power cables do not run parallel to your alternator cables - crossing at right angles is better.
You could also try using a screened power cable with the screen grounded or twist the power cables so that any pick up is equal on both cables and should have less effect.
Obviously check that the transducer screen is well connected but this should not be a problem with a new instrument.

 

[alancollins]

Many thanks for your advise which is really helpful. I am slightly surprised that yours was the only response . I thought that with an instrument as common as Nasa's and a boat as popular as a Beneteau other people must have had the same problem.

 

[MikeCC]

The first reply was comprehensive and prompt! Ferrite 'beads' as on computer leads are available on eBay etc or places like CPC.

 

[Malish]

Perhaps it is not a common problem ?

 

[Refueler]

Cables running alongside each other where one is a signal cable - its more common than many people realise.

Ferrite rings etc are technical solutions ... if just re-routing cable is not enough.

 

[lw395]

Try running it from a completely separate 12V battery while the engine is running. That would confirm the problem is electrical, not vibration.
The problem probably comes down to 'signal to noise'.
You've reduced the signal by mounting the sensor in-hull rather than thru-hull, so you need to work on the noise.
I too used to work on getting things through EMC testing. We like to keep the art dark.
You can try chokes. I'd put one on the supply, and maybe put the whole coax to the sender through one.
A capacitor across the supply close to the display head might be worth a try.
Sounders work at typically 150kHz or so, so the typical suppressor cap used for RF interference may be a bit small. You could try adding something like a 10uF 25V cap near the alternator, from the main output to the engine or 0V. This will need to be the right way around, and will have a hard life, hence the 25V rating in a '12V' system.

As refueler says, not routing the cables near other noisy cables can help.
You can also try supplying the unit with a twisted pair cable direct from the battery.
Is it networked to anything? repeater? try disconnecting anything like that.

 

[alancollins]

Thanks gentlemen. It's helpful to have a variety of things to try.