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Micronta VOM

Discussion in 'Troubleshooting and Repair' started by osmo, Mar 22, 2017.

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  1. osmo

    osmo

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    Mar 22, 2017
    My Micronta 22-207 meter has a blown resistor (yes I did it...) and the value is burned off the board under it. I can't find the manual either. Anyone who has this same meter, working or not, could help me by pulling off the back cover and reading the value off the resistor at centre of very bottom of the board -- it's the one below the 9ohm and right at the edge of the board.
    Thanks, Osmo.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Can you see a 9Ω, 90Ω, 72Ω and 7.5Ω resistor in this area?

    If you can only see 3 out of 4 of them then you know which one you burnt :)

    Which range was it on when you had this little accident (and what did you do)?

    Have you confirmed that other ranges still work?
     
  3. osmo

    osmo

    4
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    Mar 22, 2017
    Thanks Steve; there's a 9 and a 90 right above it, but I find 72 and 7.5 at upper left of the board, not in the same area. I believe the cooked one is fractional, as it seems a decimal point still remains at start of the scorched label on the board, eg. .xyO (Ohm remains), so you may be referring to a different meter.
    All positions of the meter respond (not checked for accuracy) except the 500ma/10A, which makes sense as I may have been measuring something like 5A but forgot to move the red lead to the 10A jack, but that was some time ago so I don't know now. Anyway, that's the one bad range that I know of.
    I found a similar cluster of resistors in the 22-208 meter which is quite different in other ways, and it includes a 0.94 so that's how I'll start experimenting. Thanks, Osmo
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If it's a voltage range that is affected:

    I would carefully differ one end of this damaged resistor and measure it's value (with another meter). Then solder it back into place again.

    With the meter in the known faulty range, connect up a known signal that would be about half scale and note the reading on the meter and the voltage across the resistor.

    With the aid of all these measurements we may be able to calculate the correct value of the resistor.

    If it has failed either completely open or completely shorted, this won't be possible.​

    Actually, on re-reading your last post, it sounds like all of your current ranges have stopped working.

    If it's a current range, then start with this:

    It may be best if you first confirm the correct operation of ALL ranges. This is most easily done by comparing readings with another meter. For voltage readings, take them simultaneously with the meter in parallel. For resistance readings, measure separately. For current measurements, measure simultaneously while the meters are in series. In all voltage and current ranges, start with a much lower value than would be appropriate for the range to ensure that fault has not resulted in a much more sensitive meter!

    My suspicions are that you will find that only the current ranges are affected, but are all of them affected, or just some?
    Whichever it is, get back to me with your findings.

    When testing your meter I recommend you record the range, the expected reading, and the actual reading. The expected reading is the value obtained from another meter. It is vitally important that you note the range being tested! Keep this so you can check that in fixing the meter nothing else has changed.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Also, if you can post a photo of the board, that would be of great assistance.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Also, it might be worth checking out this http://www.eevblog.com/forum/repair/radio-shack-(micronta)-multitester-model-no-22-216/. It's not your model, but as noted from the photos, it appears to have a fuse in the current ranges. Maybe yours does too?

    Note that you might have faults aside from a fuse, assuming it has one, but it's not that unlikely you will be able to repair it.

    It's also worth knowing that many of us probably have old analog meters with at least one dead current range. :D
     
  7. osmo

    osmo

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    Mar 22, 2017
    _____________________________________________

    Steve, thanks for all your thoughts on this (also on the side to Tedstruk...). This damage happened in one incident some time ago, and I knew it immediately and have not used the meter since then, just trying to revive it now. Only the 500ma range is dead, all others respond, though I have not confirmed yet that they all read RIGHT.

    I experimented a bit with values to replace the blown resistor (which I remember broke in two -- nothing usable). 1ohm gets me in the approximate area, but it seems a little high, so next I'll parallel it with a trimmer and see if I can get it right and consistent throughout the 500ma range. If I can, that should be it, though I'd still verify all other ranges anyway. If the 500ma range is NOT consistent, there may be other damage, which would likely affect other ranges, so I'd take a closer look at traces, wires, and contacts, and measure all other resistors to find any other damage. For now, though, I believe this one resistor is the only victim, as try-out resistances seemed to produce current readings either low throughout the range or high throughout the range -- I just need to narrow it down to the best value. Osmo
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You're best off paralleling other resistors rather than using a trimmer. In the short term it might work, but the full 500mA is going through this, so a lot will be demanded from the trimmer.
     
  9. osmo

    osmo

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    0
    Mar 22, 2017
    ___________________________
    Hello Steve, about the tough job for a trimmer in a current range, I thought of that too, but I believe the trimmer would be about 16ohms, so carrying only about 30ma at full-scale and so dissipating under 15mW, so it should be safe. And I'll be more careful next time .... Osmo
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Remember that Trimmer is rated for the power dispassion across the entire track. Use I = sqrt(R/P) to determine the max current it should be allowed to pass.
     
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