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  • Bobcat 225G

    Hello yall my name is Karin and I have a Bobdat 225G. S# KC329812. It's been a project for a while now and being a industrial Maintenance mechanic/welder I thought I could figure it out but I guess not. Its now been sitting for about a year. I finally got it running after cleaning the carb. But its still spraying to much gas. Ok my main thing I'm stuck on is I don't have the voltage on my auxiliary plugs and the welder will make an arc but it's so week and welds cold. ( not actually what I would call welding) Any info would be greatly appreciated. Oh and the rectifier under the fuse on the left side if looking in from front is bad. It shows open in every direction.

  • #2
    Obviously, you're going to need to change the diode. I don't have one of those machines to look at, but if I'm guessing right from your description and the picture in the parts list in the manual (I think it's diode D4), any silicon diode rated 6 amps or more at 600 volts or more will work--you don't need the specific (and potentially more expensive) Miller part number. If you order it on line, it will cost more for shipping than the diode. Mouser.com has 13,000 of them in stock and they're $0.61 each. Again, if my guesses are correct, that is the "flashing diode" (it doesn't flash light--it "flashes" a current through the field winding in the generator on startup to reestablish the magnetic field if the welder has been sitting a long time and lost its residual magnetism.). Not sure of your level of electronics knowledge, but the end of the diode with either a band around it or a plus sign is the cathode--make sure you install the new one in the same direction.

    https://www.mouser.com/ProductDetail...44ZIU3pFOkUJM% 3D


    After replacing the diode, the first thing to check is if you have the proper engine RPM, which your post leaves me reason to doubt. Do you have a digital multimeter that will measure frequency? If not, go to Home Depot or Harbor Freight and get a Kill-A-Watt meter for twenty-some bucks. Connect it to the 120v utility outlet and check the frequency. You should be running at 62.5 Hz at high idle (no load). Do you have the manual? If not, here's a link:

    https://www.millerwelds.com/files/owners-manuals/O431F_MIL.pdf

    Well, after I said all that, I just looked and I have five 6 amp 1,000 volt diodes in a plastic bag--if you want one, send me a PM with your snail mail address and I'll give it to you. If not, you can get it from Mouser, Digikey, Allied, TEDSS, or your local electronics parts place if you have one around--they're getting pretty scarce. (Parts places, not the diode--you can always find a diode that will work in this application.)
    Last edited by Aeronca41; 03-25-2020, 06:52 PM.

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    • #3
      Thank you so much for your help. My Hz on the 110 plug is like .54. At high idle and no load. I know the bridge rectifier is bad and the capacitor is also bad. Not sure about the flashing diode yet. Just got the part numbers from miller on these 2 parts. Where do I find the flashing diode?

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      • #4
        If you are only seeing 0.54 Hz, you are not generating enough voltage to even measure. The bridge rectifier SR2 produces the DC for the excitation voltage for the generator, so that makes sense. You need to replace that first.

        The flashing diode is D4 on the schematic diagram in figure 7-1 in the manual, just below and to the right of SR2. To physically find it, look at Fig. 10-4 in the manual. It is Find No. 14 on the picture--it appears to be mounted somewhere just to the right of the SR2 bridge rectifier. I thought that's what you were referring to when you said the diode was bad, but you must have meant the bridge rectifier.

        By "the capacitor" I assume you mean C1, just below the bridge rectifier on the schematic; shown mounted above the rectifier in fig. 10-4. Not surprising it's bad; electrolytics die regularly from old age, and this one has been around for awhile. You should be able to find a 1,000 microfarad, 75 volt electrolytic for around 15 bucks. Try TEDSS.com--they have a lot of capacitors. And then the other places I mentioned earlier. just look for one that will mount properly and that you can connect up ok. Does not have to be identical. If possible, buy one with 105C temp rating rather than the standard 85C rating. They will last a lot longer. My experience has been that the better ones don't cost that much more. If the 105C is really expensive, the 85 degree one will work fine, just not as long.

        Miller4less.com has SR2 bridge for $22, but you can get one from Mouser or Digikey for about 5 to 7 bucks. 40 amp 800 volt . Just pick the one with the right shape.


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        • #5
          Thank you for your help. Sorry it's taken me so long to get back to this.

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          • #6
            Could I get a picture of a flash diode. I believe that is what the guy told me to check.

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            • #7
              Karin, I don't have a welder like yours to send a picture of, but here are the pictures from the manual, and picture of a replacement diode; not sure what yours will look like, but you should be able to locate it from the location in the pictures. It is on the center baffle, near the top, below the big capacitor. Yours might be plastic like the picture; it could be silvery or even gold-colored metal, and is likely round, but could be rectangular. It is probably 1/4-3/8" diameter and about the same or just a bit longer, with a single wire coming out of each end. At least that's the most common shape. Click image for larger version

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              • #8
                Thank you so much. I will check on this in a few.

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                • #9
                  Well I found the flashing diode and checks good. It shows .530 so good there. Still no auxiliary power or power at the brushes.

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                  • #10
                    Good. I'm hoping replacing the SR2 bridge rectifier will get you back on line.

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                    • #11
                      Replaced bridge rectifier and still nothing...... I don't know what else to check, I'm at a loss completely.

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                      • #12
                        OK--time to dig deeper. Maybe a bad connection somewhere. First, remove both brushes and check the resistance between the slip rings. I'm not sure what the reading should be, but let's see if its in the ballpark. Maybe someone here will know the value it should be, but off the cuff I'm guessing something under 100 ohms, likely by a lot. Then, while the brushes are out, measure the resistance from each of the slip rings to the generator shaft to make sure you don't have a shorted rotor. You really need an insulation resistance meter or megger to measure this correctly, but let's see what the ohmmeter says.

                        Next, put one lead of your ohmmeter on wire 21 where it connects to the diode bridge, and the other to wire 24 where it connects to the brush. Keep the brushes out and not touching anything while you make this measurement. While you have the meter connected, turn your fine current adjust knob from one end of its range to the other; you should see the ohms changing on the meter. The highest you should read should be something less than 200 ohms (depends on where the tap adjustment on the big resistor R2 is set) , and the lowest should be less than 100 ohms (again depending on the tap setting). Take a good look at the fine current potentiometer (which is wired as a rheostat), and the big resistor R2 (mounted horizontally on the center baffle). Look for cracks, bad connections, evidence of overheating, etc. with a good bright light. Sometimes just loosening and retightening the tap the big resistor will help--just put it back in the same place.

                        Next step is pull wires 22 and 29 off of the bridge and check the resistance between the wires.

                        Then, check resistance to a good ground from each of those wires.

                        Also check resistance of wire 33 at the bridge to ground. You have zero or close to it.

                        Does your meter have a capacitance measuring capability? If so, disconnect and check the big capacitor C1 by the bridge.

                        Let's see what we learn from these measurements.

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