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Airco Heliwelder High current problem and solution

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  • Airco Heliwelder High current problem and solution

    Heliwelder IV High current problem and solution

    Heliwelder IV 1341-0176 code 665

    Weld current at range maximum regardless of range selection or weld current adjustment

    I obtained a copy of the welder schematic from the forum and found that the weld current control was implemented by altering the excitation of a dc control winding of a saturable reactor. By increasing this pilot current, the ac impedance of the saturable reactor was reduced, allowing a higher weld current to flow. This DC pilot current was controlled by a Printed circuit board by altering the firing angle of a Triac. The problem had to be somewhere in this circuit.

    Narrowing the search:
    I first removed the connection to the gate lead of the Triac. If the problem remained, it had to be in the Triac. If the problem went away it had to be in the control board. The problem did in fact disappear so the problem had to be in the control printed circuit board.

    Diagnosing the board:
    It was impossible to find a schematic for the control board, so I did an inventory of the major components. All the board components had manufacturer markings and most are still available. There were no Proprietary components on the circuit board.

    The major components were:
    MC7812 12 volt regulator 3 legs
    LM324 quad operational amplifier 14 legs
    LM555 timer 8 legs
    4N38 Opto coupler 6 legs
    several electrolytic capacitors

    Bench check:
    I removed the board from the welder and set up a test for the voltage regulator. I applied a variable DC supply voltage to the input leg of the regulator and observed the voltage on the output leg. From the data sheet on the regulator, I knew the output should go no higher that `12.4 volts as the input voltage was increased. In fact the output voltage followed the input voltage past 12 volts all the way to 16 volts. I didn't need to go higher. The regulator was bad and had applied full voltage (~24volts DC) to all the components on the board.

    Repair approach:
    I knew components had been damaged by the high voltage. I knew all of the integrated circuit components had operating maximum voltages of no more than 16vdc from their data sheets. I ordered the LM324, LM555 and 4N38 IC's and new capacitors (since 35 year old electrolytic capacitors are on their last legs anyway). I replaced them all and installed sockets for the IC's just in case. I also replace the faulty DC regulator with a more reliable voltage regulator board based on a LM2596 IC which has fail safe features.

    The welder now works as described in the operators manual.

    I hope someone can use this information,
    Joe the tinkerer
    Last edited by ceceljoeh; 12-30-2014, 04:49 AM. Reason: more accurate description

  • #2
    Now when I test a board I power the components with a huntron tracker. So while you are replacing parts they may not be exact,

    another problem is while you have a schematic we don't, making it impossible to follow along


    • #3
      no board schematic

      I only had the welder schematic provided to me by burnt Hands. I had no schematic for the board and no huntron unit (I am not familiar with this terminology). The only way I could find the problem is to narrow the search (current control board and inductive reactor), find the likely source of the problem (voltage regulator), and shotgun the parts most likely damaged by the known malfunction of that component.

      And it worked!!
      You can't fault success!!
      At a total parts cost of about $7!!

      Joe the tinkerer


      • #4
        Beyond my capabilities, but I like your organized approach and enjoyed reading your procedure. Happy for you that you fixed it.
        Miller Challenger 172
        Hobart AC/DC Stickmate
        Older Sears AC Stick machine


        • #5
          Thats good!!!

          I've noticed that many NTE cross overs have inconsistancys, and obsolete parts have to come from China