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Miller Bobcat 225NT assistance

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  • Aeronca41
    replied
    Great! A couple of extra ohms in the exciter circuit is not good!

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  • J-Mo
    replied
    Ok gang.... good news, While waiting for the new resistor to arrive I used another that I had available to me to verify the issue at hand. I adjusted the revolving voltage to read the max spec, loaded up 1/8 6011 set to the medium course setting (80-150) and saw a dramatic increase in welder output. Frankly it now behaves as I would expect it to behave. Once I install the new resistor I plan to double check the revolving voltage but from what I can see I am going to call this a success. I feel confidant enough at this point that I am willing to run it to the local welding shop for a load test for verification. Thank you to everyone who chimed in and helped me to get this project the rest of the way to completion.

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  • Aeronca41
    replied
    Anxious to see if that fixes it. Let's hope.

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  • J-Mo
    replied
    Originally posted by Aeronca41 View Post
    Seems that could cause it. Can you try another meter to be sure? Some meters aren't that accurate at very low resistance values.
    I believe what the meter is saying, it is old but a good meter... I ordered a replacement resistor tonight, more to come when it arrives.

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  • Aeronca41
    replied
    Seems that could cause it. Can you try another meter to be sure? Some meters aren't that accurate at very low resistance values.

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  • J-Mo
    replied
    I am not sure if this matters or not in the grand scheme of things but I just remeasured R2 to show 7 ohms instead of the 5 ohms listed in the parts book... when I reduce the resistance to under 1 ohm at idle my voltage readings globally go just above where we want them... should I suspect a failed ballast resistor?

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  • J-Mo
    replied
    Originally posted by LoganpClayton View Post
    I notice you did not have a range for V 6, as in, there should be a min to max for both Idle, and Wide open RPM.

    Idle should have been 6.5-10.5
    Wide open: 13 - 29

    I had a feeling it was excitation being low, which in turn would result in a low output across the board. Also, are you 100% sure R1 (the fine amperage control on front of machine) is in good working order, you also could jump the leads to R1 to rule it out as a point of voltage drop.

    Ok... V6 voltage....
    Min 7.2vdc
    Max 18.3vdc measured at the R2 output going to the brush assembly

    measuring input put voltage into R2
    min 15.8vdc
    max 39.9vdc

    i did jumper the R1 with the same high reading.
    Last edited by J-Mo; 02-06-2017, 02:09 PM.

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  • J-Mo
    replied
    Originally posted by duaneb55 View Post
    Have you checked field voltage filter capacitor C1 yet?

    I measure 40.6vdc across the cap, this is also the same measurement I get at the rheostat and the diode block. I did verify that I am still reading 10.5 ohm resistance on the rheostat when set to min, almost a dead short 0.6 ohm at max. It is worth mention that I see 0.5 ohm noise on my meter alone.

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  • J-Mo
    replied
    Originally posted by LoganpClayton View Post
    I notice you did not have a range for V 6, as in, there should be a min to max for both Idle, and Wide open RPM.

    Idle should have been 6.5-10.5
    Wide open: 13 - 29

    I had a feeling it was excitation being low, which in turn would result in a low output across the board. Also, are you 100% sure R1 (the fine amperage control on front of machine) is in good working order, you also could jump the leads to R1 to rule it out as a point of voltage drop.
    The listed measurements for V6 are from max setting only. Yes I have measured the rheostat several times now to verify resistance. The parts listing shows 11 ohms needed and the info you provided claims 10 ohms... I am right in the middle at 10.5 ohms. It is nice and smooth response from one pin to the other pin. I am waiting on the cap to arrive.

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  • LoganpClayton
    replied
    I notice you did not have a range for V 6, as in, there should be a min to max for both Idle, and Wide open RPM.

    Idle should have been 6.5-10.5
    Wide open: 13 - 29

    I had a feeling it was excitation being low, which in turn would result in a low output across the board. Also, are you 100% sure R1 (the fine amperage control on front of machine) is in good working order, you also could jump the leads to R1 to rule it out as a point of voltage drop.

    Leave a comment:


  • J-Mo
    replied
    Originally posted by duaneb55 View Post
    Have you checked field voltage filter capacitor C1 yet?
    Yes I did.... however like a dope I did not scribble it down. I did not see wild fluctuations and I did recall the voltage was smooth but I also was alone and unable to run welding current while taking measurements. I have another cap on order and will replace it once it arrives.

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  • duaneb55
    replied
    Have you checked field voltage filter capacitor C1 yet?

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  • J-Mo
    replied
    Ok, I have some voltage info for you guys....

    V6
    @ idle 7vdc
    @high idle 17.5vdc

    V7
    @ high idle 47.5vac

    V1
    @ min 57vac
    @ max 73vac

    V2
    @ min 50vdc
    @ max 64vdc

    V3
    @ min 21.3vdc
    @ max 27.7vdc

    globally the ocv readings are down and I could assume that is also the reason for my low welding output? I re leaned the brushes and repaired a ton of connections. There is a new bridge and diode block installed. As of this message I have not attempted to weld with it... I want to give the brush assembly time to "run in" again.

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  • LoganpClayton
    replied
    Okay Good, I'll wait till she is back up and running to see the voltage readings for V7, and V6, as well as exact O.C.V.'s in the separate polarity's and modes. I have seen the excitation voltage off more than not, so that the DC voltage that feeds the revolving field is low and thus in turn the magnetic field's actual strength, is low; resulting in a low output condition. That, to me, is #1 priority in terms of what to check-off the list of possible causes.

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  • J-Mo
    replied
    I have polished the rings and the brush contact surfaces.... I suspect an issue in the Connection to the brush *** or poor wire conditions.

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