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Bobcat 225G Plus blowing capacitor

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  • Bobcat 225G Plus blowing capacitor

    Hello all, I am having some issues with my bobcat. A few days ago it stopped welding and putting out AC power. Took the cover off cleaned the switches, rheostat, terminal leads, checked for loose connections everything looked in tact. Upon further inspection I noticed the capacitor was blown (little pressure burst port on top was open, confirmed with capacitor tester) I bought a cap with the same value and hooked it up. Meter showed 120V at outlets struck an arc briefly and after a minute or so of running the new cap blew. I am not sure it the new cap blew because it was slightly smaller form factor wise than the original, or if the mounting stud at the bottom happened to short on something. That being said I would really like to make sure things are OK before I put in another $50-$60 dollar cap.

    So far I have checked the following
    SR2 rectifier and D4 diode both just under .5V drop with diode checker
    R2 resistor 5ohms good connection
    R1 Rehostat smooth resistance change 0-10 Ohms
    Fuse holder had loose connection on F1 so I replaced that, F2 is good
    welder diode is good
    rotor windings are 3 ohm good connection on brushes
    exciter windings right at 0 ohms at PLG6
    checked both rotor slip rings and exciter windings for short to ground and got 20-30 Mega ohms but it was very humid out so I am thinking it may have been moisture conduction ( consulted with a friend of mine who repairs generators who said that was normal)
    Start switch and idle board both function correctly, I am getting 12V flashing current to exciter circuit when the switch is in the start position.

    Can any of you think of anything I haven't checked that might cause a cap to blow before I try to put another one in?

    Thanks in advance.

  • #2
    Without looking up a schematic I question your test for SR2 and D4. That sounds fine for forward voltage drop but what is your reverse resistance? If you have a shorted diode in the rectifier and are putting AC on an electrolytic cap, that will blow it in short order. Need your ser no to look at a diagram.


    • #3
      I checked the reverse on the diodes and they all showed open. The only other thought I had regarding that was that one or more of the diodes shows ok on the multimeter but once the higher voltage/ amperage shows up in operation they short out. Not really sure if that can happen, most of my electronics experience is with vintage stereo equipment. I searched for the 225g plus manual online and it has the same diagram as the interior decal on the side panel so I think i got the right one.


      • #4
        I don't think I've ever seen an "intermittent short" in a diode in 50+ years of troubleshooting systems small and large. Not very likely but I learned decades ago to never say never. Please post your ser no so we can see the right diagram. Not to be condescending but you did have the polarity correct on the replacement, right? Check for AC voltage to ground from the cap--assuming there should not be any.


        • #5
          Wow. That's odd. Normally when the cap is shorted it takes out D4 diode. I've never had a new cap blow after installation. The only thing that I can think of is make sure your wires on the cap are correct. 33 on the negative and 21 on the positive. You are correct that the new style cap is smaller in size but it is still rated the same. Miller has been using it for 15 years. But even if you have a diode problem it shouldn't blow the cap. I'm still leaning toward wiring at this point.


          • #6
            Serial number KF994729 no measurable AC across the wires to the cap but upon further inspection i think i had the cap hooked up backwards. I bet that is why the replacement cap blew and my whole issue is still stemming from the first faulty cap


            • #7
              No AC is good! Amazingly, it appears the shorted cap did not take out the rectifier.

              Reversing the leads on the cap would definitely blow it. A really irritating error, but we all do something like that somewhere along the line. I always say the only people who don't make any mistakes are those that don't do anything. You may be well on the way to success here!