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Miller 320A/BP No Remote Control in High Range

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  • Miller 320A/BP No Remote Control in High Range

    I have a 1960 Miller 320A/BP. I've had it for about 3 years, and it generally works pretty well, except a couple of issues that I've noticed recently, when running GTAW.

    1. The foot pedal seems to have no effect on output current in the high current range. It seems to only be on/off. The foot pedal works normally in the other two ranges. I've studied the schematic, and I can see that the LV side of the mag amps has extra windings for a separate 15V supply, but that supply is only used in the low and middle current ranges. I don't fully understand the theory of operation of that part of the circuit, so I'm not sure what effect this has.

    2. The middle range doesn't seem to provide as much current as it says it should. I crank it to 100%, floor the pedal, and I can barely weld a T-joint of 1/8" hot rolled steel. If, as it says, it's supposed to do up to 150A in the middle range, it should be so hot that I can barely keep up with enough filler metal to avoid undercut. If I jump up to the high current range, I can weld 1/8" just fine around 25-30% on the dial, but as I mentioned in point #1, I have no control, other than on/off, with the pedal. This has also caused problems welding 1/8" aluminum on AC.

    Neither of these issues is related to HF start, as far as I can tell, because I get consistent, reliable starts every time I hit the pedal, as long as my tungsten is reasonably sharp and clean.

    What troubleshooting steps can I try to narrow down the issue?

  • #2
    see if this link has information that helps you.https://forum.millerwelds.com/forum/...0a-bp-problems

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    • #3
      Originally posted by tackit View Post
      see if this link has information that helps you.https://forum.millerwelds.com/forum/...0a-bp-problems
      Thanks for the reply, but I don't think any of that applies to my situation. The fine current control works in all ranges. The foot pedal works in both low and mid range, and the high frequency start works reliably at all settings.

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      • #4
        That's a tough issue to troubleshoot because there isn't much of a difference in the way the pedal works in low, medium, or high.

        That mag amp is a saturable core with a high inductance until it saturates. Once that core saturates, the inductance drops very low and basically acts as a short (no impedance on the rising or dropping current through it). This happens every half-cycle of the 50-60 Hz AC wave. When it saturates in a half cycle depends on the current and the number of turns through each winding on that curve. And the sooner it saturates in each half cycle, the more output current you have.

        So the more current you have running through the 15V circuit and the 24V panel/remote amperage circuit, the more current you have at the output of you welder.

        Also, the fewer windings you have for a given current, the more flux (earlier saturation). That's why the number of windings reduces as you switch from low to medium to high.

        I am not sure why the 15V circuit is only switched in at low and medium settings - to me it would switch in at medium after the resistor (limits the current through it) and high before the resistor (max current available through that circuit) to set the minimum current for those ranges. If you have an amp clamp, it may be worth checking the current through that 15V circuit in each of the ranges. If you have an AC only clamp, just clamp it between the winding and rectifier if you can). You can do the same through the 24V circuit that feeds the amperage control. That should give you an indication of which circuit isn't acting right.

        Hope this helps,
        Jon

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