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Factory modification to a Syncrowave 250, troubleshooting help?

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  • Factory modification to a Syncrowave 250, troubleshooting help?

    I have a 1989 Syncrowave and the current adjustment knob is "compressed" so that 0 amps starts at a about 70 A dial setting. It uses the older type PC1 board. The high end is OK so that I get 300 A at the max setting. Adjusting the pots that set max and min current will not trim this out.

    It looks like the op-amp that measures the shunt had some components replaced around it with a tacked on capacitor and spade lug to ground. Someone added a third wire to the shunt also. Does anyone know if this could have been a factory mod that isn't documented? --Not sure why anyone would do this. The foot pedal in remote works fine also (0 current to max current fully depressed, no matter the dial setting).

    Any help would be appreciated. Thanks.

  • #2
    Sounds like somebody may have accidentally installed an "Audio Taper" potentiometer during a repair... rather than the linear taper it requires ...

    just a guess...

    Cruizer may have a better answer...

    Comment


    • #3
      X2 on H80N's audio pot theory.

      Comment


      • #4
        Originally posted by duaneb55 View Post
        X2 on H80N's audio pot theory.
        I agree. If that is the cause, the potentiometer could be replaced with the correct linear taper potentiometer. It's likely that the original potentiometer went bad and was replaced with the wrong style. After all, a potentiometer is a potentiometer, isn't it? Not it's not.

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        • #5
          Thanks everyone, I measured the voltage on the pot wiper and it is linear from zero to the max setting. But when I plotted the current output as a function of the pot position, its like the first 1/3 turn does nothing. Also, the relay, CR1, doesn't actuate at all until I get past the 1/3 turn.

          Comment


          • #6
            Still betting that somebody erroneously installed an audio taper (logarithmic) pot during a repair... rather than the needed linear taper..

            Easy enough to find out....

            the potentiometer should be marked...

            Comment


            • #7
              Originally posted by engrx2 View Post
              Thanks everyone, I measured the voltage on the pot wiper and it is linear from zero to the max setting. But when I plotted the current output as a function of the pot position, its like the first 1/3 turn does nothing. Also, the relay, CR1, doesn't actuate at all until I get past the 1/3 turn.
              Voltage tests won't tell much if the potentiometer is still in the circuit. What is important is the resistance of the pot. when disconnected from the circuit.

              If the first third of the pot's movement does little or nothing, then it is by definition non-linear, i.e., Audio taper. This is further shown by your statement that the relay doesn't move until past the first 1/3 turn. This proves that the resistance is greater in that position than a similar linear pot. would be.

              I'm betting with those who say that the wrong pot. was installed. I would remove the pot and test it for resistance and, if non-linear, replace it with a liner pot of the correct wattage.

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              • #8
                Well I tried it with the Miller foot pedal and I get the same effect: No current for the first 1/3rd of the pedal, then the current picks up gradually to full current at the bottom of the pedal. So I think something in PC1 is flaky. And I cannot adjust it out with the on board trip pots.

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                • #9
                  Originally posted by engrx2 View Post
                  Well I tried it with the Miller foot pedal and I get the same effect: No current for the first 1/3rd of the pedal, then the current picks up gradually to full current at the bottom of the pedal. So I think something in PC1 is flaky. And I cannot adjust it out with the on board trip pots.
                  Well if the local control pot is not correct then your remote will be wonky as well, cause they work in tandem. Geez change the thing. Looking for a 1 K ohm 2 watt pot. Any electronics store will have it. Might cost 6 bucks

                  Comment


                  • #10
                    Either the wrong type of potentiometer or the wrong resistance.

                    Comment


                    • #11
                      Got it fixed!

                      Well thanks everyone for the input. Turned out to be a bad op-amp (A4). The unit was an RCA 4136 quad op amp and is used in the shunt amplifier and the summing amplifier that generates the error signal that defines the duty cycle of the SCRs. I was able to get a near mint condition SPM-353B manual off Ebay as the .pdf downloadable from Miller were not scanned at a high enough resolution so as to be readable. What is strange about this board is that it looks like they were modified in production: there is an added capacitor to a ground lug and several op-amps using much lower resistor values than in the schematics. My guess is that they probably had problems with noise transients killing the op-amps and so they reduced the input impedance of the amplifier circuits. Unit works great now, I can go from 4A all the way to full current and the dial stays calibrated within about 10%. Perfect for the kind of work I do.

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