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anolog vom repair

Discussion in 'Troubleshooting and Repair' started by seemore, Nov 21, 2012.

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  1. seemore

    seemore

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    Nov 21, 2012
    i am new to this forum with a little more than basic understanding of electronics circuits and wanting to learn more. with that said i have recently acquired a mura model 80m analog vom.while replacing the batteries i noticed a burned resister in the rx1 ohm circuit. i can not read the value of the resister so i am in need of some assistance to determine its value.the rx10 circuit has a 1.05K ohmF value and the rx100 circuit has a 20K ohmF value. i have not been able to find a schematic for this meter on the net. any help would be very much appreciated. this is made in japan and was a high quality meter in its day.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What condition is the resistor in? If the Rx1 range still works, check its accuracy by measuring (say) a 10 ohm resistor on the Rx1 and Rx10 range and see if you get a similar reading (should be 10x different obviously).
     
  3. seemore

    seemore

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    Nov 21, 2012
    i have taken the resistor out and it burned out, open circuit
     
  4. john monks

    john monks

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    Mar 9, 2012
    I would suggest putting in new batteries and substituting in a 100
    Ohm variable resistor and adjusting it until your R1 scale reads correctly. But don't let the resistor get to near 0 ohms or you might waste the meter movement.
    The value of this resistor is to hard to figure out because the meter movement or other parts in the VOM are most likely connected in series with the resistor making the calculation difficult.
     
  5. seemore

    seemore

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    Nov 21, 2012
    thank you i will give that a try
     
  6. JMW

    JMW

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    Jan 30, 2012
    On the meter face, you will see a couple of readings. The ones we need is ua reading full scale deflection, and the "ohms per volt". Also the type of batteries used. Typically it is 1.5 V for R1 scale. If you can give us the resistance of the zeroing pot, that will help.
    IIRC many meters had 50 ua movements with 20,000 ohms per volt.
    It has been awhile( like 40 yrs) since I've done this, but I'm certain we can get you up and running
     
  7. seemore

    seemore

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    Nov 21, 2012
    thank you for your reply.i tried to reply this morning in quick reply but when i was finished i was told i was not allowed to so i am trying again. on the face of the meter there is dc 20,000 ohm/v and ac 8,000 ohm/v model 80m, no. 70 large space 1 large space u, made in japan. the 0 ohm adjustment is b-10k ohm. there is one d cell and four aa cell batteries i will loosen the meter this weekend to see if there are any other markings. thank you again for the response.
     
  8. seemore

    seemore

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    Nov 21, 2012
    mura 80m anolog vom

    i am looking for a schematic and the value for the rx1 ohm resistor.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Have you followed John Monk's advice?
     
  10. JMW

    JMW

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    Jan 30, 2012
    Guessing that is a 50 uamp movement, try a 2 k resistor. Can you read the resistors in the R10,R100 and R1000.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Answered in an earlier post

     
  12. seemore

    seemore

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    Nov 21, 2012
    i will try johns advice this weekend - the rx10 is1.05k, rx100 is20k, looks to be 94k which is powered by the 4 aa batteries and into the zero adjust pot
     
  13. seemore

    seemore

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    Nov 21, 2012
    the rx1000 is 94k powered by the 4 aa batteries
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    As has been mentioned, if you use a pot to try to determine a correct value, ensure that it doesn't fall to zero ohms. It might be a good idea to place (say) a 10 ohm resistor in series with it.

    Once you have a ballpark figure for it, get some resistors in the range 1 to 100 ohms and see how well the meter's scale reflects their value. You can probably just go off their labelled value, but if you have another multimeter I'd measure them.

    Once you have the meter reading correctly you have no other choice but to read the value if the pot (and series resistor) on another meter... OK, maybe you could read it on the x10 scale of this one (but disconnect it first!)
     
  15. seemore

    seemore

    9
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    Nov 21, 2012
    anolog meter repair

    i want to thank all who responded to my problem. i got a 1k 15 turn .75 watt cermet potentiometer and dialed in the correct resistance that ended up being 88.75 ohms. i tested four different resisters with good vom and with my one in need of repair and adjusted it until they both read the same. i am going to solder the potentiometer in the circuit permanently in case further adjustments are needed. thanks again.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You would be far better of getting a 100 ohm pot (and adjusting it to this value) or to get some fixed resistors to make it up.

    the 1k pot will only be using 9% of it;s track, which means only 9% of 0.75W = 67mW. And that will happen with a current of 9.5mA.

    Put another multimeter on a mA range across the leads of this one on the x1 resistance range to make sure that the current does not exceed 9mA.
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You might find it better to use an 82 ohm 1% resistor in series with a 20 ohm trimpot. That will make it easier to adjust exactly, and will prevent you from accidentally turning it all the way to zero ohms.
     
  18. seemore

    seemore

    9
    0
    Nov 21, 2012
    thank you for your input. i will redo.
     
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