DesignNews – Sherlock Ohms: Corroded Copper Stops Dial Ton

Corroded Copper Stops Dial Tone – Dave Boccuti 

A few decades ago, I owned a home in one of Boston's western suburbs. The house had been built during World War II, when nobody with any business building houses was doing so. But that's another story. This one is about the telephone.

One day, I picked up the phone, and there was no dial tone. I went to the neighbor's house (this was before cellphones) and called the phone company to report the problem. The company said it would look into it, so I returned home. A little while later, the phone rang -- it was the phone company asking about the problem. I hung up, checked for a dial tone, and called back to report it was A-OK.

All was fine for a few days, but then the problem came back. I walked over to the neighbor's house, and this time I called my house. My wife answered, and the phone seemed OK.

This went on for a while. I kept reporting the problem, and the phone company kept avoiding it. Eventually, I discovered that, when there was no dial tone, I could fix the problem by waiting for an incoming call or by calling my phone from another one.

This got tiring, so I kept calling and explaining the problem in more detail. Service calls were made, but there was never a diagnosis. Eventually, a young guy in a Harvard T-shirt came out in a bucket truck. I explained the problem to him in great detail, and he said he would work on it. I looked out every once in a while and saw his bucket truck moving from pole to pole. This took the better part of a morning, but I was pleased that someone was trying to fix the problem.

Eventually, he came back with a big smile. "I found it," he said. He showed me a wire splice that he had cut apart. It was a plastic sleeve with a metal crimp device in it that was used by the phone company techs to connect phone wires. It had a lot of green corroded copper salts inside. He said that when it rained or was humid, the copper corroded, and eventually the wires lost electrical contact. The telephone system works on two voltages -- a lower voltage for the dial tone and a higher voltage for the ring -- and there's the answer. When the higher voltage was applied to the corroded connection by calling (ringing) my phone, it broke through the corrosion and re-established the connection (imagine a spark welding the wires back together). This would last a while, but eventually the tiny connection would corrode again.

The Harvard man replaced the splice with a new silicone-filled one, and that solved the problem. I never had a dead line again.

Dave Boccuti is a mechanical design engineer with more than 30 years of experience in a number of different industries, including consumer products, medical devices, and custom automation equipment. He is developing disposable diagnostic medical devices at Daktari Diagnostics in Cambridge, Mass.



Posted: 5 years 7 months ago by Jon Chandler #13894
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This is an interesting article, that explains why something that appears to make no sense at all was happening.
Posted: 5 years 7 months ago by lespic #13892
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It's great when a post starts " A few decades ago "
Posted: 5 years 6 months ago by Jon Chandler #13987
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The take-away point in this article, even if it is a few decades old, is that there was a logical, real explanation for a seemingly mystical cure. Sometimes figuring out why something works requires taking a larger view with an open mind.
Posted: 5 years 6 months ago by lespic #13988
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Jon , I have to agree, my post meant no disrespect for it being decades old, the reverse in fact, fault finding especially in my decade (70’s) was a hard won skill, no swapping out boards or modules, or even having any diagnostics to help. Try finding a fault on a NRTL system armed with a avo meter. It came down to dogged perseverance and experience. Success came from not assuming anything, delay fuses could look good but fail under load, operators with nylon “garments” (yes I asked), some parts ; 100 way cables : loose panels etc were exposed, so subject to coffee baths or the swinging legs of the operator … I used to tell my students “start by assuming its not the main board “ and set them faults to prove it… but that’s several other long stories.

( Negative Resistor Transistor Logic ) we called them many legged beasties.. B***** factory turned them into gray counters , flip-flops , monostables you name it !!!
Edit,new file... even now I got confused with 0 / 1 -12v and 0v !!!

Rant over...

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