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Sherlock Ohms

Can You Hear Air Traffic Control Now?

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zeeglen
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Gold
Re: Can you here the ATC hum now?
zeeglen   2/21/2012 3:37:36 PM
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Good catch, but I'd be very concerned about why a cable shield that is supposedly grounded at one end has enough AC on it to give a nasty shock.  You have to wonder about the technical prowess of those who initially designed/installed/tested the system.

Years back I was fighting a problem with the waveshape from a T1 interface.  It turned out the whover wired that telephone central office used 72 ohm twisted pair instead of the proper 100 ohm cable.

LisaAiran
User Rank
Iron
Re: A new approach
LisaAiran   2/10/2012 11:08:44 PM
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OMG how could it possible in such ammount, I mean as per Government the labour cost may exceede $10. Looking forward for this desiogn, hope governement should approve this and apply everywhere.

 

skin care and aging

William K.
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Platinum
Can you here the ATC hum now?
William K.   2/9/2012 7:09:30 PM
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I am amazed that anybody would use BNC connectors for an audio input on a remoted transmitter. My guess is that the transmitters were not designed to be remoted like that. Transformer coupling, balanced or not, with an electrostatic shield between primary and secondary, was definitely the correct fix.

OF course, it would have been valuable to figure out why there was 90 volts AC between the two grounds.  I can immagine several explanations, though.

And of course there is always the burden of believing that the system functioned correctly at some time. Unfortunately, not everything that is built is designed, and not everything that is designed is designed correctly. So sometimes there are problems like this.

Fix-it-or-not
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Iron
Re: Can you hear air traffic control now?
Fix-it-or-not   2/9/2012 1:07:15 PM
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Yes, it was definately a band-aid moment.  The solution would have been to redesign the audio output and input stages of the communications gear to use isolated and balanced audio at both ends.  The equipment rack and consoles were separated by 6 floors.  I admit that the AC Safety ground for the building wasn't up to the challenge, or that there was some other piece of equipment that at causing the problem of a high difference in the ground potential.

But all of that aside, the real problem was a crap design using unbalanced audio extended over several floors which would have caused problems no matter what.  The equipment manufacturer, C----ns, should have known much better.

The isolation transformers where mounted on a piece of plexiglas and had a cover and warnings on it identifying the problem.  This didn't exist prior to the band-aid moment.

I would have thought that over the 15 year period that this problem existed that someone would have corrected the problem on such a critical service much earlier.

S.Wimmer
User Rank
Iron
Can you hear air traffic control now?
S.Wimmer   2/9/2012 10:44:09 AM
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Yes jljarvis,

The transformer is a bandaid.  However I have been in several situations where it was impractical or not cost effective (or permitted) to fix the 'real' problem.

Stephen

 

 

jljarvis
User Rank
Gold
Can you hear ATC now?
jljarvis   2/9/2012 9:16:39 AM
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Good job, catching the AC differential between the audio gear and the transmitter ground.   But those isolation transformers were bandaids.  

 

One should hope that no future tech has his hands on the TX chassis whilst disconnecting a BNC!   90VAC can kill ya.  

 

One wonders if there might be a common + gnd to chassis connection at one end or another of that audio run.

Fix-it-or-not
User Rank
Iron
Re: A new approach
Fix-it-or-not   2/8/2012 8:42:49 PM
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I have often thought about how much the government would have spent over the 15 years that they tried to resolve this problem.  Of course, I was never told, but I did catch heck for being $5.00 over what I had estimated the repair cost would be!  It's been 35 years since I have worked there.  I wished I did know...

I'd been playing with electronics since I was a child, and at this point, troubleshooting had become almost instinctive.  What had amased me at the time was that no one realised where the problem lay!  It was pretty obvious where the problem was coming from.

One thing that I have noticed is that when trouble shooting equipment, people tend to think that the installation was working at some point.  If everything that was checked is as it should be, it's usually the thing that no one has checked that's causing the problem!

Charles Murray
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Blogger
Good engineering is priceless
Charles Murray   2/8/2012 7:54:03 PM
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Whatever earlier solutions were tried, they had to cost more than $105 in labor alone. What it comes down to is that it's really hard to replace a smart enginer.

Rob Spiegel
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Blogger
Re: A new approach
Rob Spiegel   2/8/2012 2:41:12 PM
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Yes, Naperlou, the trick for Clint's success may have been that he didn't know what was attempted earlier. Like you, I'd like to know what that was. Clint will be checking in on these comments, so we may get the rest of the story.

naperlou
User Rank
Blogger
A new approach
naperlou   2/8/2012 10:00:19 AM
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Clint, what a great story.  You took a disciplined approach without preconceptions.  You questioned the selection of the cable and tracked down the issue.  I just wonder what was done before to address the issue.  Sometimes it pays to take a new approach without knowing what was done before.

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