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Continuity: So Easy to Check, Except When It's Not

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MeasurementBlues
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Re: One wire try a thousand
MeasurementBlues   4/8/2014 11:32:04 AM
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@tb100

I used a cable toner to identify cables running through my house. There are 9 jacks located through out the house. I'm sure glad I put one on every possible place where a TV might go. We ended up moving the TV a few months after we moved in. I'll I had to do was connect a different cable in the wiring closet. All cables are marked as to their destinations.

Sheepdoll
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Re: One wire try a thousand
Sheepdoll   4/8/2014 1:12:33 PM
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@zeeglen - The air valves are electromagnets.  The cable controls the signal flow from the keyboard to the coils.  A mechanincal, electric or electronic muliplexer called a relay is used to activate ranks of piles called stops.

These can be large instruments, the size of rooms.  Cables are cut when the organs are moved or the building pulled down.  An electromagnetic instrument can have a sizable relay, This looks like a telephone switch exchange.  Same inventor Robert Hope-Jones.

 

MeasurementBlues
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Continuity in Apple cables
MeasurementBlues   4/8/2014 2:41:01 PM
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Bill,

Today I discovered yet another way that fake Apple cables are made. Put an ohmmeter across the USB connector metal shell and the shell of the Apple connector (I use the 30 pin, not the lightning connector). On genuine Apple cables,  the connector shields on both connectors are shorted. Some fakes leave off the internal wire or connection between connectors.

I have a few Dynex charge/sync cables and the connector shields are shorted together. That's clearly the better design and they are only $8 at Best Buy.

I tested a Belkin Apple charge/sync cable. The connector shields are shorted together.

The lack of shielding means no protection from EMI of from emitting EMI. That would make another good test, check eimissions of cables with the connectors chields connected.

bellhop2
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Re: One wire try a thousand
bellhop2   4/8/2014 2:59:30 PM
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Robert Hope-Jones (1859 -1914) had a telephone background. He transfered this technology to the pipe organ. Nowadays, they multiplex everything on to one wire or fiber optic cable. Pipe organs, however, are built today with technologies ranging from mechanical linkages to fiber optics.

mhrackin
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Re: One wire try a thousand
mhrackin   4/8/2014 3:39:45 PM
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I immediately thought of my cable toner, which EVERY telecom tech or engineer is (or should be) familiar with.  This is a pair of devices: a signal source, with 2-wire output, and a sensor probe with a skinny nose.  Connect the 'hot" (red) wire of the source to the wire in question and the other to any handy ground. Connectivity of the ground to the far end is UNIMPORTANT for this tool!  Go to the other end of the suspect wire with the probe, and see if it picks up the signal. If so, there is continuity; if not, there isn't!  It's also useful for identifying pairs in a multi-pair bundle (its original intended primary use).

mysterylectricity
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Pocket calculator and transistor radio.
mysterylectricity   4/9/2014 10:09:16 AM
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When I wired my first house at the age of 16 (about 30 years ago) I faced a somewhat similar prolem at some point. My solution was to loosely wrap one end of the wire around an LED pocket calculator. This was guaranteed to inject a good deal of distinctive RF hash in the line. I then traced the wire with a pocket transistor radio. The idea came to me years after I'd spent a little time listening to my "Dataman" calculator/game thinking to itself on the AM band while drifitng off to sleep one night. Not sure how I discovered that phenomenon, except to say that I was so into calculators and transitor radios at the time that it's likely I chanced upon it. Or maybe Forrrest M. Mims III mentioned it somewhere in Radio Electronics or the like. Now your only challenge is to find an LED or VFD calculator!

zeeglen
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Re: Pocket calculator and transistor radio.
zeeglen   4/9/2014 11:07:56 AM
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@mysterylectricity My solution was to loosely wrap one end of the wire around an LED pocket calculator. This was guaranteed to inject a good deal of distinctive RF hash in the line.

What a great idea

A few years ago some squirrels got into the walls and chewed through an alarm cable leading to a group of windows.  I knew the cable was open from ohmeter checks, but finding it's route through the wall was another matter.  I connected an RF signal generator to one end in the attic, then used a scope probe as a small antenna (did not have a transistor radio handy) to locate the cable in the wall.  Then used a 4" hole saw to get at the cable.  The nice thing about cutting into drywall with a hole saw is it is so easy to repair afterwards.


jimfordbroadcom
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RF continuity tester
jimfordbroadcom   4/9/2014 1:01:15 PM
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Speaking of continutity testing, the other day I was frustrated because I needed to check RF continuity through a path with series AC coupling caps.  DMM is going to show near open circuit, and I wondered if a device to inject an RF tone at a certain frequency (user settable of course) and detect it at the other end of the path would be a marketable product.  What do you think?  I'd buy one!

What about step attenuators with pre- or post-amplifier or both to compensate for non-0 dB insertion loss, bypassable for those situations where you don't want the noise or distortion from the amp(s)?  And/or built-in power detector for ALC (automatic level control)?  Any takers?

I'm just throwing things out there, lest you think I'm doing my own marketing in a public forum.  If anyone from Mini-Circuits or Telemakus or wherever is listening, maybe this is an opportunity to make some useful instruments.

@MeasurementBlues, you make an excellent point about the audible (DC) continuity testers - what is the resistance threshold for the beep?  I often say, "I don't trust the beep." and set the DMM to read the ohms, thank you very much.  Now, if I could set the ohm threshold, I might be inclined to buzz my circuits out.

Bill_Jaffa
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Re: RF continuity tester
Bill_Jaffa   4/9/2014 1:09:55 PM
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I actually measuredthe buzz-out ohms value on my el-cheapo Radio Shack multimeter (easy enoug to do)--the continuity threshold was about 5 ohms. Of course, your value may differ!

jimfordbroadcom
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Re: RF continuity tester
jimfordbroadcom   4/9/2014 1:12:28 PM
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Yeah, that's the problem!  You may think things are OK with one meter, but another tells a different story.  Usually it's best to see the value and make your own conclusion.

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