Repairing a Digital Multimeter?

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somitomi
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Repairing a Digital Multimeter?

Postby somitomi » Mon Nov 27, 2017 2:57 pm UTC

I'm not sure this is the right place to ask, but it's my best bet.
Recently I made the mistake of measuring AC current with a multimeter only capable of measuring DC current (Voltcraft VC130-1). Since then the resistance measuements are all wrong and the diode tester beeps continuously even if I remove the fuse from inside the meter. I'm guessing the current measurement modes are dead too, but I haven't checked that.
I know nothing about the workings of these digital things, so I'm wondering if it is possible to fix this and how hard that might be. Does anyone know what component I might have killed? Do these things have some "fusible" part, that I could replace?
Thanks in advance.
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wumpus
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Re: Repairing a Digital Multimeter?

Postby wumpus » Mon Nov 27, 2017 3:08 pm UTC

somitomi wrote:I'm not sure this is the right place to ask, but it's my best bet.
Recently I made the mistake of measuring AC current with a multimeter only capable of measuring DC current (Voltcraft VC130-1). Since then the resistance measuements are all wrong and the diode tester beeps continuously even if I remove the fuse from inside the meter. I'm guessing the current measurement modes are dead too, but I haven't checked that.
I know nothing about the workings of these digital things, so I'm wondering if it is possible to fix this and how hard that might be. Does anyone know what component I might have killed? Do these things have some "fusible" part, that I could replace?
Thanks in advance.


The first thing to do is open it up and look for a fuse. If it isn't that simple a quick check is to smell each component* (the smell of burnt electronics should be obvious). I'm afraid that modern multimeters are pretty integrated and that it would make more sense to replace the whole thing (presumably when you realize the "voltmeter" chip is bad).

* I learned this trick from my best professor in college. At the time I thought it was hilarious, but considering that thermal failure is a common means to destroy a chip *and* a common means of semiconductors that failed (failure shorted something to ground, so it let the smoke out) that it makes a great "first pass" to determine which parts are bad.

ericgrau
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Re: Repairing a Digital Multimeter?

Postby ericgrau » Sat Dec 09, 2017 12:00 am UTC

First take out your multimeter and... dammit.

Ok but seriously it's a ~$25 multimeter. Unscrew the thing, check the fuse. Hopefully you have a way to check connectivity. If not with a multimeter, then stick it in a quick circuit. If the fuse is fine: drive Uber for 2-4 hours (up to 4 due to costs of gas, repairs and depreciation) and buy another multimeter. There, problem solved in under 4 hours.

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somitomi
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Re: Repairing a Digital Multimeter?

Postby somitomi » Thu Dec 14, 2017 9:24 pm UTC

wumpus wrote:The first thing to do is open it up and look for a fuse. If it isn't that simple a quick check is to smell each component* (the smell of burnt electronics should be obvious). I'm afraid that modern multimeters are pretty integrated and that it would make more sense to replace the whole thing (presumably when you realize the "voltmeter" chip is bad).

I tried that, but didn't see or smell anything indicating where the fault might be
ericgrau wrote:First take out your multimeter and... dammit.

Yup, I had the same conversation with myself.
ericgrau wrote:buy another multimeter.

That's probably what I'm gonna end up doing and relegate this one to voltage measurement purposes (that seems to work fine). Maybe use the new one to figure out what's jacked in this one, then I'll have two. I don't think it's possible to have too many multimeters anyway...
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somitomi
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Re: Repairing a Digital Multimeter?

Postby somitomi » Wed Jan 17, 2018 7:54 pm UTC

So today I bought an identical second multimeter under the assumption, that the old one was half-toast and it occured to me I never tested the fuses in the other one (why?). Sure enough one of them was blown and the old meter works fine with an adequately sized bit of metal substitute [DON'T TRY THIS AT HOME, KIDS] . This kind of makes the new one redundant, but I don't mind that too much. Never heard of someone having too many multimeters to be honest.

I'm kind of puzzled by the fact that a blown fuse results in the diode tester beeping continuously as if there was a big fuckoff short on the terminals. I'll have to look up how these digital meters work and see if I can make sense of it.
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arbiteroftruth
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Re: Repairing a Digital Multimeter?

Postby arbiteroftruth » Mon Jan 22, 2018 10:22 pm UTC

The resistance/continuity/diode modes of multimeters work by connecting a small internal current source to the terminals and measuring the resulting voltage. The details can vary from design to design, but in this case I'm guessing the current source is routed through that fuse. So when the fuse is blown, the current can't reach the terminals, and there will be 0V. If the measurement path is connected to the external side of the fuse, then the measurement will see 0V, which looks like a short circuit if you assume the fuse is intact.


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