Intermittent echo sounder

[lydiamight]

Recently purchased a used Dehler 36 CWS fitted with a Raymarine/Autohelm ST 50 echo sounder which is driving me mad with an intermittent fault. Power supply to head is fine as the display is always on when it's powered up. Depth display will sometimes show correct depth and then "count down" slowly to zero over a few seconds, display does not go from correct reading to zero in one go. Will stay like this for a random period and then come back on. Transducer is moulded in so it can't be short of oil. Previous owner says it was working OK last year (well he would say that wouldn't he). Could the yard have trowled too much antifoul on ounder the transducer? Any other thoughts or advice would be welcome.

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[AndrewB]

I had a something similar with an ST50 some years ago, that for some reason would unexpectedly move towards reading 9.5 meters, and then away again. A bit heart-stopping on a couple of occasions when I thought I was in deep water.

Turbulence was suggested. A manufacturers fix costing £50 failed to make a difference. The "fault" was traced to interference from an unsuppressed fluorescent cabin strip-light: it took nearly a year for the penny to drop. Try checking whether this happens when ALL other circuits are off.

Another reason may be an intermittent connection caused by damage or loose terminals in the transducer line, particularly where it leaves the transducer or enters the master unit. If the line was ever cut, so as to enable it to be passed through narrow conduits, check there. The return to zero appears to happen slowly because the unit 'damps' the output, averaging over successive measurements to prevent abrupt fluctuations in readings.

 

[AndrewB]

P.S. ...

It is most unlikely to be caused by painting over the transducer.

Similar problems were discussed in threads last year, check HERE.

 

[scarlett]

Try drying the head out. My ST 60 Autohelm head failed three times due to water ingress.

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[VicMallows]

I think this is a highly probable explanation. The mini-inverters in flourecent lights typically work around 100-200KHz. If you're unlucky, and your's is 150KHz (most echo sounder transducers work at this frequency) .............

Have people forgoten the problems flourescents (not to mention alternators) used to cause DECCA?

Vic

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