Depth/Fish Finder Problems

[Salmon Troller]

When we bought our boat, it came with a Standard digital depth finder and a Humingbird Fish Finder. Both were hooked up to a single bronze through-hull transducer (of unknown mfg). They had wired it through a selector switch so that an operator could select which finder was using the transducer. The problem was that neither was reliable. They would work for a bit and then they would generate false readings. I disconected the switch and hooked it directly to the Humingbird, but it is still unreliable. Are these units picky about the type of transducer they must use? Do transducers go bad? Is the transducer wire like a VHF antenna wire and tuned for a specific length? Maybe the Humingbird is simply junk and I should try again. Who makes a decent one that would likely work with the existing transducer?

[Pegasis]

Could be any number of reasons.

Splice wasn't done correctly (I've been told to NEVER splice the t-ducer cable unless it's already broke).

Air getting under the t-ducer because of another through-hull fitting in front of it or air from a lift strake.

Quality of the machine. Just to name a few.

[SomeSailor]

Yes Randy, they are very sensitive to even the slightest bit of deviation. Afterall... they're sensitive enough to sense the difference in density of a salmon's liver from the sea water around it at depths of hundreds of feet.

We had a pretty heated discussion on here once before on here, but since you're an old Navy AT-type... these function like a TDR (Time Domain Reflectometer). It looks for a reflected RF pulse from the trasnducer head. When it sees this head it then looks for a second return (nanoseconds later)... this return is the reflected energy from the bottom as felt by the transducer. These units are designed to be able to time-correct for a given length of cable. Anything more or less, and you've exponentially affect it's accuracy... yes... they are VERY much like a VHF cable.

By splicing, or switching you also add more noise to the circuit, which makes it more difficult (electrically) for the signal to get there and back. The more noise this induces... the higher the signal to noise ratio gets and then the less signal you'll be able to see from a return. I used to teach microminiature repair for the Navy in Hawaii... and I can tell ya... there is no such thing as a perfect electrical splice... be it soldered, crimped or lugged. Nothing is the same as the original wire.

The transducer itself... is for the most part... the same as any other, BUT... different manufacturers stimulate them differently, and are pretty good at keeping them proprietary.

So... long story short... if you want the best performance... don't splice, repair, split, switch or swap transducers. They're too cheap not to have a dedicated head anyhow.

[Patriot]

My fish finder has gone blind. The video display is fine, but it shows the bottom at 0 feet. That suggests to me that the transducer has gotten fouled. It is mounted on a bracket at the bottom of the transom, and I've tried to clean it with a brush, but without effect. Any suggestions?