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Miller Bobcat 225NT assistance

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  • Miller Bobcat 225NT assistance

    Hello all,
    First time poster here so be gentile.... This is going to be a long post; I have been working on this unit for several months now as time allows.
    Last spring, I acquired a Bobcat 225NT from a friend that was not in running condition. The terms of the acquisition were literally get it out of my way. This machine has had a very hard life; it had spent its life in a concrete business. I am only able to read the first three digits on the serial (KKK) and by my homework puts this unit around '99 or '00. This unit has a Kohler engine.

    When I first started work on this unit and after a significant amount of carb cleaning I was able to make the motor run well. I verified that the AUX side of the generator was producing "Some" power. I plugged my camper (50a service) into the generator and loaded it for about an hour to see if anything died hard; I measured around 20 amps on each leg. After it survived that test I felt that it was worthy enough to put some money into and started to restore it from the ground up. I tried to weld with the unit and learned that I could not even get a spark. After some forum hunting I checked and verified the F1 fuse and continuity through the brushes and slip rings. The rings were dirty but not scoured, the brushes did not look heat loaded and there was plenty of meat left on them. I learned that the Reactor feed wire (72) was completely corroded off the reactor. I also learned that the reactor was very "fuzzy" with corrosion. I was still a bit skeptical about spending a large amount of money on what I felt was still an unknown condition unit so I located a Stabilizer/ reactor unit on Ebay that was listed to be from a Bobcat 225D. When I looked the parts manuals up they listed the same part number so I purchased it. After it arrived I took the machine apart to install the reactor/ stabilizer only to learn that there was 100# of formed concrete in the bottom of the welder. Because of the work involved and the amount of corrosion noted on the lower pan I elected to purchase a new lower pan unit. Now I know most normal people would have walked away long before this point... but I never was known to back down from a challenge and besides, I have very little cash into this machine at this point.

    So... new used Reactor/ Stabilizer install on new pan. Cleaned and painted every part removed and reinstalled, cleaned the fuel tank while it was out. It honestly looks pretty good now... LOL. Once we reassembled everything we fired it up and to our joy I now could produce welding current on all three course settings and while metering the AUX outlets I could see a change on the voltage as I adjust the Rheostat. I attempted a weld, something simple 3/16 flat steel and a 3/32 6011 rod. I could barely produce any heat at all, in any of the three settings in any position on the rheostat. After some looking and a ton of cleaning we learned that the rings/ brushes were soiled and were cleaned to fresh surfaces. The course adjust switch was removed/ disassembled/ cleaned/ reassembled. The polarity switch was also cleaned in the same manner. The F1 fuse showed good continuity but also showed a voltage drop so it was replaced to correct that issue. the Rheostat was cleaned and now provides good smooth change. We did find a significant voltage drop on the oil pressure switch of all things (13v coming in and 9v leaving). It has been corrected as well.

    So... My question at this point. We are making it just a little bit better each time we play with it, almost every connection has been cleaned or replaced inside of this thing at this point. I feel that the unit is running "COLD" meaning I do not think it is anywhere near the rated amp output for welding. I am hesitant to spend $110 to have it load tested until I feel better about the output it is creating. Some data for your enjoyment... OCV is 74v, when metering the generator output to the reactor (72) with an amp clamp I do see 70-80 amps while welding. I do have an RPM of 3800 high and 2000 idle. I show 64hz measured at the aux plugs (I know it needs to come down just a touch). My leads are older but only 15 feet in length and clean connections. I am now able to run 3/32 6011 as designed but the machine needs to long arc to run 1/8 6011. Is it possible to have good measurements and still end up with poor generator output? Is it possible that the Reactor could "Choke" the current down as I am seeing? Any and all advice is welcome, I am determined to make this old girl sing again. The hour meter shows a staggering 7700hrs.... the condition of the machine when I picked it up agrees with that number.

    Thank you ahead of time,

  • #2
    Welcome! Sounds like you have checked most everything. You didn't mention R2, an adjustable resistor in the rotating field circuit. Check especially the contact of the tap.


    • #3
      R2 has two options... the machine likes the lesser resistance of the two but the resistor does become warm to the touch. It is a large ceramic resistor and I assume it is designed for some heat load, might be normal?


      • #4
        I would expect that resistor to get a little warm. On your reactor question, it would only limit steady current while welding if it's dc resistance is too high due to a bad connection or possibly internal corrosion. Reactors only respond to changes in current, tending to smooth out instantaneous ups and downs around the average.

        Assume the brushes slide completely freely in their holders.

        You sound like you're comfortable working around electrical eqpt. I think the next thing I would do is connect one meter lead to the stinger and start working backwards with the other (extended ) meter lead looking for a voltage drop while someone is welding, all the way back to the diodes. Do the same with the work lead. I suspect you might find an unacceptably large voltage drop somewhere. Same principle you used on the oil switch.


        • #5
          Ok I will look into that... I work at a DOE high energy physics lab... electricity doesn't worry me much.

          thank you for the response


          • #6
            For what it is worth to mention... the AC side acts the same way... as I understand the drawing should eliminate almost all of the D.C. From the equation.


            • #7
              Nothing in here like you have to worry about with high energy physics! You are probably overqualified for welder repairs. :-)

              Agree on the AC comment. If you try the measurements I suggested, do it in AC. I should have included that. Simplifies the circuit a bit.


              • #8
                If he's a PH.D. at that high energy physics lab, you may need to explain it again, Wayne.


                • #9
                  HA! No PHD here.... I repaired the accelerators for years and I am now a firefighter at the same place.


                  • #10
                    WELL, J, you as the harrassee, and Ryan as the harrasser have that firefighting thing in common.

                    Actually, I've known a couple of pretty handy PhD s, but like everyone else, I guess, many are dangerous with tools.

                    Hijack alert....Ryan, speaking of firefighting, I got called Saturday to add a breaker for a friend to hook up a new electric water heater. Found an FPE panel. When I told him he had to replace the panel and I would not add a breaker, he got pretty mad. Still stewing. But sometimes you might have to make someone mad to potentially save their life. The guy is past 80 and not all that spry to escape a fire. I hope he doesn't find someone else to do it.


                    • #11
                      J-Mo, good job on all the troubleshooting and repairs thus far.

                      Another thing to check/replace would be the field voltage circuit filter capacitor. It may be failing to maintain proper field voltage under load.

                      As I'm sure you're aware, they have a finite life and with that may hours on the machine it may have out lived its.
                      MM200 w/spot controller and Spoolmatic 1
                      Syncrowave 180 SD
                      Bobcat 225G Plus LPG/NG w/14-pin*
                      *Homemade Suitcase Wire Feeder
                      *WC-1S & Spoolmatic 1
                      PakMaster 100XL
                      Marquette "Star Jet" 21-110


                      • #12
                        Originally posted by duaneb55 View Post
                        J-Mo, good job on all the troubleshooting and repairs thus far.

                        Another thing to check/replace would be the field voltage circuit filter capacitor. It may be failing to maintain proper field voltage under load.

                        As I'm sure you're aware, they have a finite life and with that may hours on the machine it may have out lived its.
                        Thank you, it is always a challenge to repair what you know very little about... these forums have been incredibly helpful. I have not tested/ replaced the cap as of yet however I do not see what I would consider wild fluctuations in the revolving or the excite voltages. I do see a 1v drop at the diode block, another is on order.


                        • #13
                          An update.. we did locate a weak spot on the diode block. I might have helped it along some but it was very brittle. The current plan while taking into consideration the multitude of poor connections is to just rewire the low voltage side completely. This should remove the unknowns and connection issues completely. So, I will back after I find time to complete this task. Oh.... and perform the weld side test as requested.


                          • #14
                            Typical firemen, breakin stuff....<br />
                            <br />
                            Eek!! FPE is no Bueno! Actually had a fire yesterday that is a likely suspect...electrical in nature, started roughly in the area of the panel. It was too far gone to tell what it was, but the house was of the right age for one. <br />
                            <br />
                            J-mo....that's pretty cool you know all those big electronical words to talk on level with Wayne and Duane, they have to dumb it down for me, like really dumb it down.


                            • #15

                              Logan here, I'm a certified Miller service tech for what is going my 4th year now, I am also Lincoln Certified and do warranty work on well, you name it. I see you have taken a lot of the suspect components, isolated, and tested as best as you possibly could to continue on with diag of machine, I do notice a few however that have been missed. So let me help you more thoroughly on that matter.

                              I would be VERY interested to check the winding's for the exciter (R2), revolving field(R1), Aux power field(R 3 & 4), and main welding field (R5 & 6). I would also be curious to see what the excitation voltages are sitting at (V 7) and what Revolving field voltages you have while @ idle and while @ wide open (V6) . If the SR2 Rectifier is going out, is poorly grounded, or if the excitation winding's them-self have gone bad/starting to fail you can see a wide variety of output issues. Be sure to remove the wires going to the slip rings when testing the revolving field resistance. Here are some pictures to aid you with testing.

                              Overview of testing points: Click image for larger version

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                              Resistance Values: Click image for larger version

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                              Voltage Values: Click image for larger version

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                              Expected Output OCV in AC, DC CC and DC CV:
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                              Let me know what ends up happening!

                              - LPC
                              Last edited by LoganpClayton; 02-01-2017, 12:40 PM.