Extech DC clamp meter, large DC offset developed.

M Philbrook

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Jul 13, 2016, 10:24:41 PM7/13/16

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Just got a Extech 845 , 1000 amp AC/DC clamp with a basic DMM on it and
IR..

I used the DC amp once, it worked.. put the meter away, next day used
it again. As soon as I select DC amps now, it shows a -32 Amp offset.

I can zero it however, It looks like the value its reporting is ~
50% of the actual value.

I have not used this meter for nothing else yet, but I did plug in the
test leads to check to make sure the rest of the meter worked after
seeing this, and it seems to work ok on the other funtions..

Is anyone familiar with this failure ? The initial offset was only
about +/- 0.5 amps if I remember..

Jamie

Phil Allison

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Jul 14, 2016, 12:44:25 AM7/14/16

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M Philbrook wrote:
>
> Just got a Extech 845 , 1000 amp AC/DC clamp with a basic DMM on it and
> IR..
>
> I used the DC amp once, it worked.. put the meter away, next day used
> it again. As soon as I select DC amps now, it shows a -32 Amp offset.
>
> I can zero it however, It looks like the value its reporting is ~
> 50% of the actual value.
>
> I have not used this meter for nothing else yet, but I did plug in the
> test leads to check to make sure the rest of the meter worked after
> seeing this, and it seems to work ok on the other funtions..
>
>

** As the problem only affects the DC amps ranges, it might be the jaws or other parts of the magnetic circuit have become magnetised so offsetting the Hall Effect sensor.

Holding the meter near an analogue CRO screen will reveal if this is the case.

Otherwise the unit has developed a fault.

.... Phil

mike

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Jul 14, 2016, 3:18:01 AM7/14/16

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My DC clamp has the same problem.
Got it used and it was that way from the start.
I considered trying to demagnetize it, but fear the
voltages induced by the
attempt might bust something, so I just use the zero function.
It takes a very small magnetic field to disrupt the zero.
Unless the initial offset is 0.0, you have to zero it anyway.

I contacted the vendor and they said, "push the zero button."

I once had a DC clamp probe that zeroed itself every time you
opened the jaws. Nice feature.

N_Cook

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Jul 14, 2016, 4:28:47 AM7/14/16

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I would get inside and clean any range/function switch contacts.
Does the manual advice against use near loudspeakers, laying meter jaws
next to screwdrivers etc

M Philbrook

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Jul 14, 2016, 6:33:34 PM7/14/16

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In article <cb5fec08-5836-4b2d...@googlegroups.com>,
palli...@gmail.com says...

You gave me an idea.. We have a demagnetizer in the shop.. I can stick
the jaws in it and give it a shot.

Thanks..

M Philbrook

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Jul 14, 2016, 6:51:44 PM7/14/16

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In article <nm7ifs$7mj$1...@dont-email.me>, div...@tcp.co.uk says...

this meter is only a few days old in my hands, there should be no crud
in there.

The manual only says to remove test leads when using the clamp and
there is an obvious reason for that, to prevent the leads getting into
the equipment while reaching in. The leads were still in the package.

I will try a demagnetizer/degaussing process

Jamie

JW

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Jul 14, 2016, 6:58:34 PM7/14/16

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On Thu, 14 Jul 2016 19:01:41 -0400 M Philbrook <jamie_...@charter.net>
wrote in Message id:
<MPG.31f1e3e57...@news.eternal-september.org>:

Or get a Fluke. Odd things like this is why I won't touch B brands...
I need to trust my gear and not fuck around with crap.

M Philbrook

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Jul 14, 2016, 7:10:40 PM7/14/16

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In article <366gob9ormtvoev7c...@4ax.com>, no...@dev.null
says...

Ok, we have flukes there too, they fail just as bad ;)

Jamie

jurb...@gmail.com

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Jul 15, 2016, 3:11:46 AM7/15/16

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>"Holding the meter near an analogue CRO screen will reveal if this >is the case. "

Ha. Try finding one these days. I got a couple laying around not hooked up. Gave Ma one when her XBR took a shit and it is CRT. But I bet if I walked up and down the street and banged on people's door I would have to walk miles before finding anyone with a CRT based TV.

Years ago people were already laughing saying "TVs are flat now, stupid".

So how do we indicate if something is magnetically charged with the lack of color CRTs or whatever ? I was thinking a guitar string but how do we know it itself is not magnetised ? I fit is used it has spent alot of time right in front of a pretty strong magnet. See if it attracts to other strings ? I guess that might work.

I think this guy needs to wrap wire around those jaws, and out of phase so they don't blow the thing out (or disconnect it) and just demagnetize it with a stepdown transformer to something reasonable so as not to blow the variac either and give it a good douche of 50 or 60 Hz, depending. If the bogus reading goes away we know what happened.

Now if it doesn't, the plot thickens.

jurb...@gmail.com

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Jul 15, 2016, 3:15:04 AM7/15/16

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I fucked up the quoting on that.

MJC

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Jul 15, 2016, 4:26:45 AM7/15/16

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In article <b4a7ee92-e039-42f3...@googlegroups.com>,
jurb...@gmail.com says...

>
> So how do we indicate if something is magnetically charged with the
> lack of color CRTs or whatever ?

Because now we have GPS no-one has a Boy Scout compass any more?

Mike.

Phil Allison

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Jul 15, 2016, 5:45:54 AM7/15/16

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jurb...@gmail.com wrote:
>
>
> >"Holding the meter near an analogue CRO screen will reveal if this
> >is the case. "
>
>
> Ha. Try finding one these days.

** Most techs have one - I own four.

Analogue CROs are what we all used until DSOs with LCD screens came along.

The trace is very sensitive to a magnetic field.

.... Phil