Fixing an old DC voltmeter!

Discussion in 'The Projects Forum' started by Wargon, Jun 25, 2012.

  1. Wargon

    Thread Starter New Member

    Jun 25, 2012
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    Hey Forum!

    I recently bought an old Western Electric DC Volt meter: Model 931 at an antique store. I wanted to show it working but when I hooked it up to a DC power source nothing happened. After opening it up and using a circuit meter to test I realized it probably is the old resistor in the circuit that is fried. I was wondering if someone could help me out by figuring out what kind of resistor it is so I can replace it to get the meter working again! It says that 1000ohms equals 1 volt and the range of the meter is 0 to 30v! I have included pictures in the attach files section and thank you!
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Keep that thing closed! Don't let anything get near the meter movement (where the needle pivots), especially things like a brillo pad or iron filings maybe from a grinder! Metallic dust will be attracted to the magnet and absolutely screw it.

    Looks like there's already some crap in there, not good!
     
  3. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
    2
    Thank you for the reply! I did put the lid back on after I took the picture and I am working on it in a very clean environment so no worries there!! I know that the meter motor still works!
     
  4. #12

    Expert

    Nov 30, 2010
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    Can you take pictures? Can you post them here? Can you read color codes? Is the resistor burnt beyong reading colors or printing?
     
  5. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
    2
    Hey #12,

    I already posted two pictures right under the description of my original post! I can read color codes but I did not see any colors or printing on the resistor! :( Its just black all around it!
     
  6. #12

    Expert

    Nov 30, 2010
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    I was thinking about a picture of the burnt thing, or circuit board, if it has one.
    It's just that I don't have anything to work with because I don't know how to figure the resistor size. Is the whole thing just a resistor and a meter movement?
     
  7. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
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    That's correct! The second picture I posted has a yellow arrow pointing at the resistor! I have attached to this reply three more photos that might help you get a better look at the resistor and its size! The resistor is about the size of a quarter and is about 5 quarters thick if that helps with the size of it! The resistor has no markings on it at all! It has the two wire connectors on top and that's it! Its not a normal resistor I am used to seeing then again it was built in mid 50's! The whole thing just has the resistor and a meter movement! The resistor in this circuit just resist the current to allow the voltage to be measured between 0 and 30v from the meter movement!
     
  8. #12

    Expert

    Nov 30, 2010
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    Honestly, I wouldn't recognize that as a resistor if you didn't tell me what it was!
    I would guess that the label (1000 ohms per volt and 30 volts) means the resistor is 30,000 ohms minus the resistance of the meter coil! In my experience, the coil is about 35 ohms but that meter is older than I am!

    It is dangerous to try an ohmmeter on the coil because it might be a 100 microamp coil and most ohmmeters use 1 milliamp! I suggest putting 150,000 ohms in series with the coil and applying a double A battery to the resulting circuit! Then you can measure the voltage across the resistor and the the voltage of the battery and calculate the resistance of the coil! By looking at the movement of the needle, you can see how far 10 microamps is moving it and calculate the full scale current!
     
    strantor likes this.
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Seriously looks like you have some corrosion issues in the movement! are you sure it's good to go?!
     
    #12 likes this.
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    745

    If it says 1000 ohms per volt/ so for 30volts fsd you will need a 30k ohms resistor, try a 22k ohm preset and a 22k resistor in series, and adjust it to 30k.

    Test the meter on a variable dc supply with a calibrated Digital meter to compare.
     
  11. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
    2
    Thank you #12, Strantor, and Dodgydave, for responding!

    Thank you #12 for that recommendation on how to test the meter coil because that was the one variable I did not have to use the equation to figure out the resistor's resistance! I will try that today to see what I get!

    Strantor that is one thing I am worried about as well! The test that #12 suggested should answer that question as well if the coil is even still in good enough condition to measure voltage accurately!

    Dodgydave I will give that a shot as well to see if it will work and allow the meter to accurately measure a voltage power supply!
     
  12. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
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    Before I use a resistor, do I need to worry about the watt rating on the resistor or will the standard .25w be enough for this meter?
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Yes, as the current will likely be under 5mA, probably well under. At 2mA, for instance, the power dissipated is I^2•R, or 0.000004 times your resistor.
     
  14. Wargon

    Thread Starter New Member

    Jun 25, 2012
    7
    2
    Hey Everyone! Thank you for everyone's help! Sorry for the long update, was a crazy week so finally got around to messing with my old vintage voltmeter and after everyone suggestions and help was able to get it working again!! I had a hard time figuring out the coil's current so I did some trial and error and with a 33K resistor I am able to get an acurate reading every time compared to a modern digital one! Thank you again for everyone's help! Was able to bring a piece of history back to life!!
     
    Metalmann and strantor like this.
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