Migatronic automig 180 mono - problem

  1. Jsk welder New Member

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    Hi.
    I have a problem with my migatronic.
    It worked just fine one month ago , since that it has been standing in warm garage and now there is a problem.
    So when i push the lever from welding pistol wire feed motor activates , gas valve activates and also contactor activates but when i release lever both gas valve and wire feed motor stops but contactor will stay on.
    If i try to press and release multiple times wire feed and gas valve starts and stops ok , but contactor just stays on as long as i keep power on from the main switch.
    If i turn power off then i have to wait maybe 15 seconds and after that contactor will release.
    Also if i turn power off and plug off the pcb connector then contactor will release immediately.
    So i am guessing the problem is in the pcb , but does anyone have any thoughts what component on the pcb board could be faulty?
    and is it possible to measure with multimeter?

    Thanks
    Juha
     
  2. hotponyshoes Member

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    Location:
    Somerset. Uk
    Sounds more like a problem with the contactor sticking?
    Measure the voltage going into the contactor and make sure its dropping to zero when you release the trigger.
    If it does the problem is with the contactor, if it does not then the problem might be the pcb or maybe relay upstream from the contactor but try measuring at the contactor first
     
  3. eddie49 Member

    It could be that part of the Control PCB has failed, so that the Stitch ( or Burnback, if present ) logic is erroneously holding the main Contactor on.
    This Forum thread may be helpful:
    https://www.mig-welding.co.uk/forum/threads/migatronic-automig-180-mono.31581/
    However, this information from that thread implies that the Contactor is controlled by the same component as the gas valve:

    "The Drive to the Gas Valve and main contactor is from the 24V supply, switched by Triac (BT137), which is controlled via the opto-isolater. (K3021)."

    That thread has a pdf copy of the circuit diagram for the PCB number 76116187, and that diagram does show a common control wire for both the gas valve and the Contactor. That implies the error is, as hotponyshoes has suggested, more likely to be a sticking Contactor.
     
  4. Jsk welder New Member

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    Thanks for your quick reply.
    I have been wrong with that gas valve thing.
    At least something is making clicking sound when pushing and releasing switch and wire motor is working ok.
    but it is true that gas valve and contactor are under same control.
    But it has to be pcb rather than sticking contactor becouse when you cut power from machine and unplug pcb connector the contactor will release immediately.
    so if you wont take the connector off , it will stay on maybe 15-20 seconds.
    My friend is investigating that pcb right now , and he suspects that triac has gone bad.
    is there any way to measure that opto isolater k3021?
     
  5. Jsk welder New Member

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    Ok triac has now been changed , but it is still the same.
    My friend told me that it seemed to be faulty , he will continue tomorrow so we will see if the new one has already gone too.
    We could not find that opto isolator from my pcb , this is 180 mono , is it possible that it does not exist on this model?
    I also measured voltage from contactor and gas valve.
    it is almost 0 before i pull trigger , then it will be 25 v ac until i release trigger , after that it will be 14 v ac until i turn mahcine off and wait long enough.
    any thoughts what components could be faulty in my pcb?
     
  6. eddie49 Member

    The previous thread that I linked to has several different numbers for the PCB:

    17616973J MPS-STD MPS STD Pt No 76116973

    76116982 MPS-4TF Continuous welding, spot, stitch, latching, current control
    76116983 MPS Continuous welding, spot, stitch
    76116984 UPS Continuous welding

    The circuit diagram attached to that thread is said to be for the 76116983 board, but in fact it is labelled "76116187".
    Does your PCB have any of the above part numbers?

    As you said, the diagram doesn't show any opto-isolator, the trigger signal for the Triac is generated by the BC237/BC337 ( T9 and T10 ) Darlington pair.
    As an AC switch, the Triac should be open-circuit both ways until it is triggered on by the gate lead. At every zero-crossing point of the 24 v AC wave, it should turn off, unless triggered again. You probably need to monitor the gate lead of the Triac with an oscilloscope to check if this trigger pulse is always there, and if it goes away when the torch switch is released.

    One quick suggestion - check capacitor C4 0.1uF and resistor R4 10 Ohms. These components are across the Triac. Try opening one leg of the capacitor and check if the fault is gone.
     
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