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Larry

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Depthsounder cable check

wrote in news:61d6b5e4-c19d-4b64-928a-
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Ladies & Gents

I have a Navman 3100. Depth worked fine last season bu tshows nothing
now. Is there any way to check the depth sounder cable to determine
whether is was damaged over the winter? A visual inspection of the
accessible parts of the cable has shown nothing unusual.

Many thanks in advance

Matt

The transducer of a depth sounder is a piezoelectric crystal. Left open
circuit, it generates a DC voltage which can get quite high if left open
for a day. The large, high powered transducers in a Naval submarine are
always left shorted to prevent this voltage buildup because it can kill
just sitting there on a pallet. Their crystals are huge compared to
yours.

So, if we UNPLUG the transducer from the sonar and leave it open
overnight, we can take a high impedance digital voltmeter and measure the
crystal's DC voltage, briefly as the meter's 10M ohms discharges it like
a little capacitor, IF the cable is:
A - connected to the transducer in the bilge....and....
B - doesn't have any leakage across the conductors which will drain away
this high impedance source's voltage.

Do NOT connect an ohmmeter across the transducer, which always measures
an open anyway, because the transducer's high voltage may damage the
ohmmeter.

You should see some level of DC voltage if the cable and transducer are
in working order. If not, it's 50/50 which is the problem.

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