Need Help with old analog diagnostic automotive ammeter

Discussion in 'General Electronics Chat' started by RICHMONDRON, Dec 31, 2014.

  1. RICHMONDRON

    Thread Starter New Member

    Dec 31, 2014
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    Hi!

    I fried my old Snap-On ammeter. It has 0 to 10 amp and 0 to 100 amp circuits and I connected the 0 to 10 amp leads in series with a 6 volt circuit on an old tractor that, it turns out, had a dead short in it. The result was smoke from a tubular container in the leads. When I opened the tube, which contains the leads for both ranges, I found the 100 amp leads connected to a rectangular metal plate in what appears to be a shunt type setup, in that the leads from the plate to the clips are a heavy gauge and the leads from the plate to the meter are a much lighter gauge. Both the lead to the clip and the lead to the meter for each side of the circuit are soldered together at each end of the plate. That part seems to be intact. there is a piece of insulation material, almost like a rectangle of circuit board, slightly larger than the metal plate, that separates the 0 to 100 amp and the 0 to 10 amp wiring.

    The smoke seems to have come from the 0 to 10 amp range wiring. The leads are the same configuration with a heavier lead going to the clip than to the meter. the lead to a clip and the lead to the meter for both sides are soldered together and to a piece heavy solid wire. the heavy solid wire connected to both pairs of leads have the remnants of "something" at the end of the heavy solid wire opposite the end connected to the leads that obviously was completely destroyed. That's what I need help with. I expect that the part that was destroyed is a shunt resistor, but I have no idea how to determine its value or what wattage I should use.

    The meter deflects appropriately when used to test voltage, so I don't think it was damaged. I have also traced both the 0 to 10 amp and the 0 to 100 amp circuit paths all the way from the wire at the tube through the rotary switch, the components on the board, the meter, and back the the tube on the other side of the circuit and there are no failed components.

    If someone could help me out I sure would appreciate it. I know just enough to be dangerous!

    Thanks!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,058
    It sounds like you cooked your shunt resistor. Any schematic, or make and model number available?
     
  3. bwilliams60

    Active Member

    Nov 18, 2012
    725
    90
    Can you post a picture of the unit and the parts in question. I am working on one as we speak. It is an Ultra-Pro carbon pile load tester and I believe it is smoked as well. Hard to find parts for these units though but show us what you have.
     
  4. RICHMONDRON

    Thread Starter New Member

    Dec 31, 2014
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    Model number is a MT 402 and there is no schematic that I can find.
     
  5. RICHMONDRON

    Thread Starter New Member

    Dec 31, 2014
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    Here's a bunch of photos.
     
  6. bwilliams60

    Active Member

    Nov 18, 2012
    725
    90
    Can you post these pictures in a jpg format and perhaps make them smaller? These are very large and very grainy to look at
     
  7. RICHMONDRON

    Thread Starter New Member

    Dec 31, 2014
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    How's this?
     
  8. RICHMONDRON

    Thread Starter New Member

    Dec 31, 2014
    5
    0
    While I was going through the circuit to check continuity, I found that the + (Red) lead into the board went through a trim pot on the way to the rotary switch. The rotary switch seems to simply connect the Red lead terminal to one side of the meter and the Black lead terminal to the other side of the meter. At the time my focus was continuity of the path so I didn't disconnect any of the connections to take readings of component values. I'll try to map out the circuit, do a rough hand schematic a little later, and post it.
     
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