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Miller Bobcat weak arc, plugs work.

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  • Miller Bobcat weak arc, plugs work.

    I am working on a Bobcat 225 that will only sparkle when trying to strike an arc. The same result on AC and DC with all different amp and volt settings. So far I have cleaned and polished the slip rings and from brushes to rings is about 2 ohms. The C1 capacitor measures 950 micro farads, bridge rectifier has 43 vac in and 53 VDC out (not sure if this is what it should be). Machine doesn't idle down but 120 volt receptacle reads 57 Hz and goes to 74 Hz when engine is revved up and it will run an angle grinder. Fuse F1 ohms good. I cleaned the wiper contacts on the rheostat and it has 0-10 ohms. So far I haven't found anything that looks wrong but it just won't weld. Can anyone suggest what else to check? Please.

  • #2
    Please post your serial number.


    • #3
      Serial number is: KJ236337


      • #4
        I am not a Bobcat expert but I would not expect much at an engine speed that results in a 57 hz. output. Sorry but I don't understand this statement. "Machine doesn't idle down but 120 volt receptacle reads 57 Hz and goes to 74 Hz when engine is revved up". If it's down to 57 Hz, it must be idled down, especially if you're getting a different freq when it's revved up. Are you trying to strike an arc with the machine turning at 57 Hz and waiting for it to spin up when it senses the load? Please clarify.


        • #5
          What I mean is: the engine starts and runs well above idle, guessing 2500 rpm or more. That is when I can measure 57 Hz and 120 volts on the 120 volt outlet. Arc is just a sparkle that won't sustain itself.

          If I manually rev it up at the carburetor then the frequency goes up to 74Hz at the 120 volt outlet and the arc stays the same weak sparkle so I determined that it is running fast enough since it is over 60Hz. The engine doesn't rev automatically when I try to strike an arc and it doesn't lower to an idle (800-1000 rpm) when it has no electrical load being placed on it.

          To me it seems like it isn't getting proper excitation to the windings and the AC setting looks like it bypasses the output rectifier to the welding leads but the weak arc is the same on AC and DC. That should rule out the output rectifier and arc stabilizer. I cleaned up the welding lead terminals and the leads show 5 ohms end to end. The 120 volt and 220 volt outlets are working.

          I haven't been able to find what the input and output voltage of the bridge rectifier SR2 should be. I'm also not sure what the farad value of capacitor C1 is supposed to be but it isn't shorted or open like most capacitors show when they fail.

          Could the PC1 Idle Board have anything to do with the weld output?


          • #6
            Engine off... Green plug... wires 24 & 33 (1 &2).... Disconnect plug and ohm through brushes and rings 3.6 ohms.. Check ohms from either wire to frame, should be high ohms...wires 28 and 29 (3 & 4) less than an ohm (Check to frame)

            Looks like S2 ac volts correct...

            Plug back up and start engine... measure same wires for DC at the green plug for 6.5 − 10.5 volts dc (idle rpm) 13 − 29.5 volts dc (weld/power rpm)... wires 28 and 29 (3 & 4) 43 − 50 volts ac

            Check for AC on large rectifier wires 70 and 77 and DC on 78 and 79... Please be careful..
            Last edited by Popscott; 04-11-2019, 08:57 PM.


            • #7
              Thanks to Popscott for the troubleshooting help.

              I checked wires 24 & 33 to the slip rings: 24 = .7 ohms, 33 = 1.6 ohms. slip rings are bright and shiny. Both wires read infinity to the frame ground.
              Between wires 28 and 29 was .2 ohms and infinity to frame ground so exciter winding is good.
              Voltage between wires 24 an 33 was 20VDC with it running at estimated 2500rpm and 57 HZ.
              Voltage between wires 28 and 29 was 45VAC.
              Voltage between wires 70 and 77 was 70VAC.
              Voltage between wires 78 and 79 was 33VDC.
              Bridge rectifier SR1 had between 44 and 58 VAC in and 55-56 VDC out when engine is running. Re-checked fuse F1 and it's good.
              Verified capacitor C1 is 1000uF and it reads 935uF with my meter. Power resistor R2 = 5.7 ohms.
              Voltage between weld and work terminals was 72VAC and 62VDC depending on selector switch setting.
              I took the coarse volt switch S3 apart and cleaned the dirty contact pads but that made no difference, all terminal pads on the switch now read .2 ohms to wire 77 now.
              Off the back of the coarse volt switch S3: wire 71 to frame ground was 171 Kohms. Wire 70 to 71 was .2 ohms. Wire 70 to 72 was .2 ohms so the weld winding seems to be good unless 171 Kohms to ground is not high enough.

              I have pretty much run out of ideas on what else to check. It has voltage but no amps behind it. Everything seems to be good except it won't make a usable arc. I don't know the history of the machine but it sat for at least three years. It doesn't look abused or too beat up. Any further assistance is appreciated.


              • #8
                Just a shot in the dark--it has been sitting a while and could have lost residual magnetism. Have you checked the flashing (as in flashing the field to magnetize it, not an LED lighting up) D4? If it or it's connections are bad, you may not have enough magnetism in the rotating field to make it weld. I could be in left field with this, but it's worth a look. That would account for insufficient excitation.


                • #9
                  Is it kicking up into high rpm when you strike an arc?
                  it should be idling (low rpm) around 22-2400 rpm. It should kick up into high (36-3700 rpm) when you strike an arc.
                  Lincoln Idealarc 250
                  Miller Bobcat 250
                  Thermal Arc Hefty 2 feeder
                  Thermal Dynamics Cutmaster 52
                  Torchmate CNC table


                  • #10
                    ""Voltage between weld and work terminals was 72VAC and 62VDC""... that is exactly what is should be... check your work clamp and stinger connection or get another set and try them


                    • #11
                      Thanks to all that replied. The problem turned out to be the weld lead. It checked less than 5 ohms which was higher resistance than I would expect but I thought that it should make an arc. A different set of leads worked fine. On the plus side, I learned a lot about the internal workings of a welder and the reason I couldn’t find anything wrong was because there wasn’t.
                      Thanks again for taking the time to help me out.


                      • #12
                        Excellent! Glad you got it figured out.