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Multimeter giving incorrect readings

Discussion in 'Misc Electronics' started by Logan W, Nov 5, 2005.

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  1. Logan  W

    Logan W Guest

    I have a DMM which I got a while back for $20 or so, from Jameco.
    Lately, it has been giving me incorrect readings - when the test leads
    are not connected to anything, it will say there are 800v, and this
    does not go away when the test leads are detached. If I open up the
    case, then it works normally, and if I do not have the back fully
    snapped in, it works, but as soon as I do that, it starts saying 800v.
    <a
    href="http://jameco.com/webapp/wcs/stores...toreId=10001&catalogId=10001&productId=119212">Here
    is the multimeter I bought.</a>

    Does anyone have any ideas on how I can fix it?
     
  2. DaveM

    DaveM Guest

    DMMs frequently have a foil shield that helps to protect the instrument from
    outside interference. These foil shields can sometimes make contact with
    the circuitry and cause strange operations, such as you are experiencing.
    When you open the case, make sure there are no holes in the shield that
    aren't supposed to be there, caused by parts of the circuit board piercing
    the foil.
    Another thing to look for is a broken circuit board. Look at it under a
    magnifier and see if there are any cracks. If you find any, that's likely
    the cause of your problem. If your DMM has a multilayer board, you might be
    able to fix it, but it's not likely to be easy. A soldering iron with a
    very fine tip is mandatory here. Bridge the breaks in the copper traces
    with strands of very fine wire (30 Ga wire-wrap wire is a good choice here).
    Also,make sure that there aren't any foreign substances
    (coffee/coke/beer/etc) that has found its way inside the case. If you find
    any gunk of the sort inside, a thorough cleaning is necessary, the last step
    being a flush with distilled water. Allow to dry completely ( an afternoon
    of exposure to the warm sun should work).
    As a final suggestion, you might flex the display (assuming that it's an LCD
    display) and see if it causes your erroneous display symptom. If so, you
    might have an intermittent contact by the Zebra connector, which is a
    polymer strip of alternate layers of conductors and insulators. This strip
    is placed between the display module and the PCB, and makes electrical
    contact between the two. If that's your problem, you might try cleaning the
    contacts on the PCB, the Zebra strip and the LCD module with pure (99%)
    isopropyl alcohol. Be very careful with the LCD contact area.. no
    scrubbing on it. just a few swipes with a clean soft cloth saturated with
    alcohol.

    Good luck with your DMM.

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
     
  3. Logan  W

    Logan W Guest

    Thank you, this was the problem! I fixed it by putting a couple layers
    of scotch tape over the affected portion of the shield. Will this
    affect the multimeter in any way, or is there a better solution?

    Thank you,
    Logan Williams
     
  4. I hope you used Scotch electrical tape......
     
  5. CWatters

    CWatters Guest

    Many meters give strange readings with the probes open circuit or in the
    presence of RF fields. Got a wireless lan?

    Put it on the voltage range and connect the two probes together. It should
    read zero volts obviously. What does yours read? Hook it up to a 9V battery,
    what does it read?
     
  6. Logan  W

    Logan W Guest

    Of course!
     
  7. DaveM

    DaveM Guest

    No, that shouldn't affect the DMM unless you taped over an area that is
    supposed to make contact with a prong on the PCB. Enjoy your DMM.

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
     
  8. Guest

    For $20, throw that piece of crap into your nearest trash can.

    Now run out and purchase a quality name brand DMM from a mainstream
    reputable supplier. Wavetek is good, Fluke and Tektronix are better.

    A DMM that produces wildly inaccurate measurement is about the last
    thing that you need in your life!

    I have an inexpensive Wavetek Model 5XL (around $35) that travels with
    me and produces measurements with errors of less than 1% overall. I
    would prefer to own a Fluke or Tektronix, but simply can't justify
    their cost.

    Harry C.
     
  9. Guest

    To that I would add, hook it up to the power line and see what it
    initially reads, and while leaving it connected see what voltage it
    reads at 5 minite intervals. If you see the voltage reading gradually
    increase, immediately throw it in the trashcan because its readings are
    meaningless and you can't repair such a fundamental design defect.

    Harry C.
     
  10. Guest

    Not likely John, since I have a conventional analog meter that
    constantly monitors line voltage to compare the DMM reading against.

    Also, it is not typical of the power lines here in the US to start at
    115V and half and hour later have drifted up to 140V and still
    climbing, while the analog meter still reads 115V.

    Then too, cheap DMMs are well known to be labeled for 1 or 2% accuracy,
    when the reality is that if they can measure voltage to 10% accuracy
    they are doing quite well. On the other hand, a Fluke is usually a safe
    bet!
    :)

    Actually, my inexpensive Wavetek Model 5XL (which I purchased as an
    expendible throw-away for field work) does quite well, IF you remember
    to keep the batteries fresh. IIRC, it sells for between $30 and $40.
    Its readings are generally within 2% of those measured with the Fluke,
    but I don't know if this is normal for the model, or if I just got
    lucky.

    Kindest regards, Harry C.
     
  11. Guest

    John posted:

    "With a single meter I would expect to the see the mains rise from
    dusk (as air conditioners switch on less and less), and we do have a
    +/- 10 spec on the mains, which means _most_ of the time you can
    expect to see them somewhere between 108 and 132V."

    Actually John, the residential service requirement here in the
    Northeast (and most other parts of the US) is 108-126V, with a nominal
    of 117V. The electric utiliity becomes liable for damage and/or loss of
    service for voltages outside these limits provided that they are
    notified that either specified limit is being violated and take no
    action to correct the problem.

    Typically they can resolve the issue by changing taps on the
    distribution transformer. More extreme solutions (constant voltage
    transformers) are sometimes temporarily applied to end of line load
    variation problem until the gauge of the feeder lines can be increased
    to reduce IR drop in the line. (Usually this temporary fix is done only
    in rural areas still governed by federal REA requirement.)

    I note that the electric utilities are still lobbying for liability
    immunity during brown-out situations, but fortunately these efforts
    appear to be falling on deaf ears, since the entire responsibility for
    brown-outs falls squarely on the shoulders of the electric power
    providers themselves.

    Kindest regards, Harry C.

    p.s., John, the reason I have a dedicated line voltage meter is a
    carry-over from the days when I dabbled in color photography, where
    color enlargers needed to be powered at a constant voltage of 125V,
    otherwise the color temperature of the prints will vary. The Variac is
    now gone, but I kept the line voltage monitor as well as the Triplett
    630-NA (mirror scale) used as both a sanity check as well as a trusted
    secondary voltage and current standard.
     
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