digital multimeter failure.

Discussion in 'General Electronics Chat' started by xchcui, Oct 13, 2015.

  1. xchcui

    Thread Starter Member

    May 12, 2014
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    Hi.

    I have the M-832 digital multimeter.
    All the function are working good(ammeter,voltmeter..),except of the ohm meter function.
    When i attached the probes together,the display shows "000",and when i separate them,instead of showing infinity,the display is shown increasing value of resistance (again,the probes do not touch anything).It doesn't response when i try to measure a resistor and if it does the value are far wrong.
    It happens after i did a capacitor test,while connecting the dmm probes(ohm meter function) to the capacitor to charge it a bit.Maybe the capacitor damage something in the dmm,but when i opened the DMM,i didn't notice any burned or damage component.The battery is good,btw.
    What can be the component that caused this failure?
    and can i fix it?
    Thanks in advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Check the internal fuse.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    You can get proper meters for testing capacitors - but even with those a charged capacitor can damage the meter.

    A DMM doesn't seem to me like it gives any sensible reading on a capacitor, with an analogue meter the needle gives a kick as the test voltage charges the capacitor. An analogue meter is a little more tolerant of unintentionally being used to discharge a charged capacitor, but you can still bend the needle if it hits the end stop hard enough!
     
  4. xchcui

    Thread Starter Member

    May 12, 2014
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    The internal fuse in that DMM is not related to the resistance function,but to the ammeter function(low DC currents).
    This DMM can not read capacitor value,but it can give you some idea on the condition of the capacitor.
    But,my question are:What can be the component that caused this failure?
    and can i fix it?
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    My first ever DMM came out of a bin at a place I was doing work experience, the resistance and AC current ranges were dead. When I opened it up there was a LSI chip and a socketed 8-DIL dual op-amp, with nothing to lose I replaced the op-amp chip and the missing ranges were back, I borrowed a certificated instrument to check the calibration against and had a good DMM that lasted a good many years (its still about somewhere).

    That was quite a few decades ago and things have changed nowadays, we live in a throw away world where things aren't made to be repairable. A new DMM can be got for a reasonable price - you should consider watching the flea markets for an old analogue meter to check caps with.

    The Peak Atlas ESR meter also measures basic capacitance and has a built in safety feature in case you stick it on a charged capacitor - but you might have to save up for one.
     
  6. bwilliams60

    Active Member

    Nov 18, 2012
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    Can you take really good clean crisp pictures of the internal components of this meter both front and back? The more the better and post them back here.
     
  7. xchcui

    Thread Starter Member

    May 12, 2014
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    I attached some photo of the internal componenets.
    I tried several times to take a good clean crisp picture,but the best that i got are the ones that i attached.And they,unfortunately,still bad refer to the fact that it makes it difficult to follow the circuit and the components condition.
    But this is the maximum that i could achieved.Maybe the photos will be useful somehow,even though,they are not clear enough.
    Nevertheless,i assumed that my description about the failure will be enough,in order to get an answer.Only the ohm meter function is failed,so only small part of the DMM should be suspected(the component that related to the ohm meter).
    And i assumed that the scenario that i described before,will be enough to an experienced professional electrician to know what caused this scenario.
    DMM.jpg DMM1.jpg DMM2.jpg DMM3.jpg DMM4.jpg DMM5.jpg DMM6.jpg DMM7.jpg
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  9. xchcui

    Thread Starter Member

    May 12, 2014
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    Thanks for the file,but it doesn't help much.
    The DMM and solder iron are basic instrument for every electrician.
    And i believe that most of the members here experienced DMM failure.
    I would like to find out with your help,what component could failed and made that scenario.
    If it is related to the IC,i assume that it doesn't worth to deal with it,but if it related to resistor,corrosion or something like that,it might worth to repair it.
    But first,i need to know what is the component that makes that failure behavior.
    Anybody know?
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I personally don't think that it will be costwise to repair it, you can buy one for less than £5 on ebay...
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    In the UK, many hardware stores offer a basic DMM for around £5.

    Not as cheap (and probably not as good) as you can get on Ebay - but you can just stroll into the shop and buy one.
     
  12. ISB123

    Well-Known Member

    May 21, 2014
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    Same DMM can be bought for 3.50$ on Ebay. It's really not worth repairing.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    One of the photos shows a "black-blob" chip, so the chances of repair could be pretty remote whether its worth it or not.
     
  14. xchcui

    Thread Starter Member

    May 12, 2014
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    Guys,you are not helping here:)
    I know what is the price of those cheap DMM and it seems more reasonable to buy a new DMM rather spending time on that failed one,BUT all the idea in that thread is to understand what component made that failure behavior.
    And when i said"...if it worth to repair it...",i didn't refer to cost issue,i meant to say: if the repair will be possible,it will be worthwhile to spend a little time to fix it,but if it will be impossible to repair it,it will not be worthwhile.
    Anyway,whether it is possible/whorthwhile or not,i will be very happy to understand what component made that failure behavior.
    Anyone?
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I gave you a circuit diagram in post #8, clearly you cant understand it, so whats the point of continuing....
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    Part of that is understanding what you can and can't do anything about.

    A black blob chip can't be unsoldered and replaced, so unless you can find the fault among any discrete components - its scrap.
     
  17. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    A black blob chip can't be debugged either, even for the academic satisfaction of figuring out what's wrong with it as xchcui was going on about.

    xchcui -- It's most likely a problem in the chip. The external resistors and capacitors are largely irrelevant to operating or not-operating; they just set the scales. A symptom of slow increase in numbers on the resistance scale points to a major failure of something in the input circuitry of the chip. There's really nothing to be learned by poking around on the board if the chip is toasted in some way. It's a monolithic chip (one single, impenetrable part) so there's no where you can go with that, with either diagnosis or repair.
     
  18. xchcui

    Thread Starter Member

    May 12, 2014
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    Thanks,InspectorGadget,your explanation help me cleared up my doubts.
    So,can i understand from your answer,that if the chip fails,it doesn't necessary mean that all the function of the DMM will stop working and if,such in my case,all the other functions of the DMM,except of the ohmeter,is working properly,it is mean that only part of the chip is failed.Am i right?
    Dodgydave,I am not disparage at the circuit diagram that you gave me.
    The opposite is true.I had been trying before to find that circuit diagram in the web by myself but with no success,thinking it might help me to solve the problem.
    But since i didn't know what might cause the DMM behavior,i didn't know from where to begin with.
    It is not the first time that i read circuit diagrams and solder components by following the diagrams,but this is the first time that i see those small rounds in the end of the conductors with a numbers on them,like 9-13/2,so i didn't understand how to fit one round end to another round end(what mean the upper number and the bottom number with reference to the attached table(position/function).
    I assume that this is related to the function of the selector switch,that change the connections among those contacts every step.
     
  19. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If your meter works on volts and amps ok, then the chip blob is ok, i would suggest its a siicon device like a diode or the transistor near the com terminal on the diagram.

    Edit. You're going to need a working dvm to fix this, if you dont have one then its a futile attempt to proceed any further...
     
    Sinus23 likes this.
  20. InspectorGadget

    Active Member

    Nov 5, 2010
    211
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    Yes; as near as I can tell, there's a different input for resistance than for voltages. My sense is that something in the inside-chip input path for resistance has failed, whereas the general areas and the input path for voltage measurement is fine.
     
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