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Sherlock Ohms

Can You Solve this Engineering Mystery?

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patb2009
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i'm not much of a electronics tech
patb2009   6/16/2015 9:41:17 AM
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but a sharp 50 mSec pulse indicates energy acumulating and discharging.

 

now it's possible that energy is coming from outside, a RF spike coming from a RADAR

or something but wouldn't that show up on other gear in the lab?

 

so, all i can guess is a RC circuit forming in the coax driven by charge coming in

and a bit of damaged shielding.  if you took the coax and ran some tests,

checking the breakdown voltage and running RF into the cable, you might have

seen something

Critic
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Gold
Model Number, Please
Critic   6/16/2015 10:39:41 AM
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What was the model number of the sound-level meter?  Some shcematics are available online.

S.Wimmer
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Silver
Coax
S.Wimmer   6/17/2015 9:03:24 AM
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I once traced an RF problem in a test fixture to a two foot lenght of RG-58.   There was no visible problem but it was definitly in the coax.  Having the time and inclination I did an 'autopsy' on the coax.  After very carefully slicing the jacket off I found a wrinkle in the braid.  It was folded back over itself about one half an inch and caused a 'lump' in the impedance that the fixture was able to dectect.

DB_Wilson
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Re: i'm not much of a electronics tech
DB_Wilson   6/17/2015 9:23:12 AM
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I think patb2009 is on the right track with the pulsing RF source.  It may have been something other than radar.

I suspect that the conductor to connector joint in the original coax may have degraded over time to act as a diode to rectify a pulsing RF signal.  This only require a single soldered joint to degrade over time.  I see this as being more likely than the coax being just the right length to be a resonant element with the diodes acting as a rectifier of the RF signal or the coax having some other failure mode that was not readily detectable.

Some experienced hams that have worked on a lot of radios may have some insight into this problem.

jlawton
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Gold
When audio circuits misbehave strangely
jlawton   6/17/2015 9:26:25 AM
Actually I used to see a similar kind of behavior a little later all the time but with op amps. I'll bet the last stage before the diode bridge was an emitter-follower, right? So you've got a high gain (fairly) wideband block driving an emitter-follower with a very low high-frequency driving impedance (in one direction at least) driving a short hunk of coax into the bridge. What was happening was the gain block was going into parasitic oscillation at a frequency MUCH higher than the oscilloscope could show you, the one microsecond was simply the period before the oscillations were "blocked" by draining energy from some convenient energy storage element (which usually turned out to be something like the power supply decoupling capacitor). The coax could have acted as simply a source of capacitance or as a complete resonant element. The meter inductance MIGHT have played some role in generating the high negative voltage, it's kind of hard to tell without having a complete schematic and SPICE models of all the circuit elements. The usual cost-effective solution was to introduce 22 to 33 ohms between the emitter output and the coax, so long as that didn't serve to degrade the circuit performance. There's an article online by John Dunn on EDN that explains the premise pretty well called "Emitter Followers as Colpitts Oscillators", I'd include a link but the comment SW here doesn't let me do that so you'll have to Google it yourself.

 

Critic
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Parasitic Oscillation
Critic   6/17/2015 9:45:23 AM
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I agree that it could have been a parasitic oscillation, although I still want the model number so I can look at the schematic.  Some of the GenRad sound level meters used (oscillating) flyback dc-dc converters for power, which could misbehave under the wrong circumstances.  There is the possibility that the device was picking up RF pulses, too.  It would have had to have been a pretty intense RF field (during the pulse).  The PRF is of interest, too. If it was a radar-like PRF, then I would definitely lean towards an RF pulse.  

bob from maine
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Platinum
Re: Parasitic Oscillation
bob from maine   6/18/2015 11:41:14 AM
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Nothing in an electronic device operates independently. I had a similar problem and ended up going back to basics: What RC will give a 50 mS pulse? What components could combine to create that RC? The capacitance of a given piece of coax at a particular frequency could change drastically if there was an in-line repair done during the manufacturing of that roll. "Any sufficiantly advanced technology is indisinguishable from magic." Sometimes manufacturing variations will cause your technology to move into the magic realm with no warning. Good story.

Don Yates
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Iron
Re: i'm not much of a electronics tech
Don Yates   6/20/2015 10:12:14 AM
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So did anyone check the battery pickup contacts or the wires to the circuitry?
The capacitors in the onboard power feed may have compensated for some of the supply glitches but those pesky power problems often do cause mysterious effects!

JonC
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Iron
RG 178
JonC   6/20/2015 10:18:31 AM
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RG 178 has a copper plated steel core. I've used a lot of RG 174 which is similar. A nasty trait of this cable is the core punching through the dielectric and shorting to the shield. This frequently happens when the cable is flexed a lot or sharply bent. Combined with the oscillator theories, the core of the cable may have punched through the dielectric, not quite shorting, but resulting in a spark gap – when the built-up voltage reached 50 volts, it would discharge across the spark gap formed in the cable.

Larry M
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Platinum
Re: i'm not much of a electronics tech
Larry M   6/20/2015 12:20:31 PM
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Bad solder joint on the left-side coax? Good theory, but the author wrote that after replacing the coax with a good segment, he reintalled the bad segment and reproduced the failure. That would have used new solder joints. Maybe a broken inner conductor making only a "point contact" would also have caused this behavior, in effect a "cat-whisker" diode.

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