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Miller Big 20 sparks but dosnt weld

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  • Miller Big 20 sparks but dosnt weld

    I recently acquired 2 Miller big 20s , I got one running and finally got the ac power to work , but the machine will not weld I have ohmed the rehostat, bridge resistor , brushs, and everything checks out, I had to flash it and replace a stuck brush and cleaned rings, I have bypassed rheostat, and then bypassed bridge resistor, I dont have the motor completely dialed in but at the ac receptacle iam geting around 60hrz, which it does fluctuate some but it should do more than just barely spark, also iam geting only 25 ocv at the weld terminals, the serial number is Jg029385. Jg029363 , 363 is the one I am working on, iam at my wits end with this I know it could only be a few things with such a simple machine but I dont know enough to figure it out any help would be very much appreciated.

  • #2
    Sounds like it's time go get out the voltmeter. You seem to know your way around electrical stuff. I would start by measuring each phase of the AC voltage off of the main generator stator. Open circuit DC output voltage is supposed to be 60-95 volts, and assuming your meter reads AC RMS volts, it should show about .707 times the output DC, or about 42 1/2 to 67 volts. I'm using the diagram in the front of the manual for the ...363 welder. It is updated from the one in the back of the book, and it says it applies to all serial numbers after JC607457, which would include yours. The wire numbers are hard to read, but it looks like the wires coming off of the generator are 17, 21, and 31; can't say for sure-the first one might be 11-the scan isn't clear, especially on the first one. You should be able to find these wires accessible at reactor Z1. Measure 17-21, 21-31, and 31-17. Use clip leads, not two hands on two probes--never give electrons a path through your heart if something goes wrong. My rule is "one hand in your pocket" around live circuits. They should all be the same, and should be near the voltages mentioned above. That should at least prove the weld stator is OK. Since you have power at the AC outlets, seems the rotor and utility power stator are both OK, but there is still the possibility of problem there. Let's check stator output first. The next most logical thing is the range selector switch. Cycle it throughout its range 20 or 30 times and see if it makes any difference; you may have to dig in and clean it.

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    • #3
      Dont let me fool you I really dont know much, I have only got this far by reading this forums, when I get home I'll give that a try and let you know what I find out, thanks.

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      • #4
        Also that makes sense for the range selector I didn't thank of it until now but I have seen it spark a few times while cycling it, it was froze up when I started working on it, I'll go ahead and clean the contacts on that to.

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        • #5
          Originally posted by Aeronca41 View Post
          Sounds like it's time go get out the voltmeter. You seem to know your way around electrical stuff. I would start by measuring each phase of the AC voltage off of the main generator stator. Open circuit DC output voltage is supposed to be 60-95 volts, and assuming your meter reads AC RMS volts, it should show about .707 times the output DC, or about 42 1/2 to 67 volts. I'm using the diagram in the front of the manual for the ...363 welder. It is updated from the one in the back of the book, and it says it applies to all serial numbers after JC607457, which would include yours. The wire numbers are hard to read, but it looks like the wires coming off of the generator are 17, 21, and 31; can't say for sure-the first one might be 11-the scan isn't clear, especially on the first one. You should be able to find these wires accessible at reactor Z1. Measure 17-21, 21-31, and 31-17. Use clip leads, not two hands on two probes--never give electrons a path through your heart if something goes wrong. My rule is "one hand in your pocket" around live circuits. They should all be the same, and should be near the voltages mentioned above. That should at least prove the weld stator is OK. Since you have power at the AC outlets, seems the rotor and utility power stator are both OK, but there is still the possibility of problem there. Let's check stator output first. The next most logical thing is the range selector switch. Cycle it throughout its range 20 or 30 times and see if it makes any difference; you may have to dig in and clean it.
          Oops--bad math there. I used the .707 factor based on single phase circuits; not too swift late at night, I guess.....this is a three-phase delta generator winding so the factor is .652 [1 minus (3 x the square root of 3) all divided by pi, if anyone cares..... :-) ] not .707. So, you should be looking for around 40-62 volts. Probably not that big of a deal, but if someone picks up the factor sometime in the future, I at least owe it to them to be correct.

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          • #6
            That's how I figure the three phase delta generator winding factor too, I used to use this formula...1 - (the square of somethingorother x the weight of a European sparrow) divided by the life span of a tsi tsi fly. Could never really get it to work out right though. But in my defense, you try and find the "tsi tsi fly" button on your calculator and see how that works out for ya. So, technically, not my fault.

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            • #7
              Originally posted by ryanjones2150 View Post
              That's how I figure the three phase delta generator winding factor too, I used to use this formula...1 - (the square of somethingorother x the weight of a European sparrow) divided by the life span of a tsi tsi fly. Could never really get it to work out right though. But in my defense, you try and find the "tsi tsi fly" button on your calculator and see how that works out for ya. So, technically, not my fault.
              I'm pretty impressed you understand deriving those equations :-) .....I just use 'em--people way smarter than me make them up! I always remember an engineering textbook I had where the author shows three or four equations, then finally a fourth, which was "the answer", often preceded by the phrase, "...and it is therefore obvious that....." I have to say it was very very seldom obvious to me....seemed like he must have left out about 10 more steps to get to that final answer! So, I just use the final answers created by people like that.

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              • #8
                Iam just a welder guys that's all way over my head.

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                • #9
                  I got around 25 volts between 11 and 31 , between 11 to 21 , and 21 to 31, I got 34 volts

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                  • #10
                    First, we have to be sure my reading of the not-very-clear diagram is really showing wire 11. I downloaded it again, and I'm pretty sure it says "11" but can't be positive. Is your copy of the diagram any clearer? Can you tell if the wire coming off of the top right corner of the delta-shaped winding is 11, for sure? The fact that one winding is less than the other two is a big clue, and helps lead me to the question of whether you may have a leaky or shorted diode. It is also possible, but very unlikely, that the winding itself is bad. Let's not go there for now. Do you have access to a clamp-on ammeter? Even if you do, not seeing the machine, I don't know if you have good enough access to the wires to hook it over them. But It would really be great to see what currents are present in wires 11, 21, and 31 with no load on the machine.

                    Looking at the manual, the diodes appear to be inside the generator housing. If it isn't too big a pain, it would be good to check them with an ohmmeter. They are on page 5 of the parts list section of the manual, assembly find numbers 84 and 108, each of which is a group of two diodes. Do you know how to check diodes with an ohmmeter?

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                    • #11
                      Yeah I have clear diagram, it's for sure 11 its inside the door panel, no I dont have a clamp on ammeter, to be honest I dont really now what that is, and no iam not sure how to check a diode, I have looked at the service manual but cant really tell what parts are the diodes

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                      • #12
                        Ok I found the diodes, had to go look at the machine and the manual

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                        • #13
                          I can go pick up a ammeter if I need to

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                          • #14
                            You can probably solve this without buying an ammeter, but it is a good excuse for a new tool......Just FYI--a clamp-on ammeter is a meter with what kind of looks like a spring-loaded C-clamp attached. You squeeze the handle, hook the "clamp" around the wire, and you can read the current flowing in the wire by sensing the magnetic field that always surrounds a current-carrying wire. When they first invented them it just felt like magic! And were they ever expensive! BTW, should you ever want to get one, I don't recommend the cheapie at Harbor Freight. I bought one assuming it would be "disposable", and it sure was...wasn't worth even the 12 bucks I paid for it. And, a lot of them only read AC amps, not AC and DC. If you're messing around with welders, an AC-only one is not a lot of help.

                            Click image for larger version  Name:	AMP_CLAMP.jpg Views:	1 Size:	15.6 KB ID:	597590

                            Well, you found the diodes, and we have confirmed wire 11. To check the diodes (all 4, one at a time), you have to unhook one end of the diode you're testing from the circuit. Diodes work by conducting current in one direction, but not in the other. The mounting stud is one end, and the cable coming off the other end will have to be disconnected from the machine. I'd rather avoid unbolting the diode from the plate if possible--there is probably thermally conductive paste in there, and you won't have any more to put back. Not a big deal, since there will still be some of the old stuff there, but it's better if undisturbed if you don't have any more to add.

                            So, disconnect the diode wire so it's hanging in the air. Your meter may have a diode test function, but it's not necessary. If you have it, use it; if not, no big deal--just use the ohms function. Use clip leads if you can to be sure you get a good connection. I'll describe the ohmmeter tests (assuming no diode test function on your meter) first. Connect one lead of your ohmmeter to the wire from the diode, and the other to it's body (which I guess you really can't do with a clip lead, unless you can clip it on the metal diode mounting plate). You should read either an infinite resistance (9999 or OL [for overload]), or very low resistance of only an ohm or two, or less, depending if you happen to have chosen the conducting or blocking direction on the diode. Now, reverse the meter leads to the diode--just swap 'em around. You should now read "the other" reading....if you had infinite/9999/OL before, you should have the very low resistance when you reverse the leads, If you had low resistance before, it should be infinite now. If this isn't clear, let me know.

                            If you have and choose to use the diode test function, the principle is same except instead of very low resistance in the "forward" direction, you will read what is called the forward voltage drop across the diode--generally 0.3-0.5 volts or so. BTW, that voltage comes from the battery in your meter; the diode doesn't "hold" any voltage of it's own. When you reverse the leads, you will read OL if the diode is good.


                            If you find anything else, like the same reading in both directions (whatever it is), the diode is bad.
                            Last edited by Aeronca41; 06-04-2019, 08:15 PM.

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                            • #15
                              Ok thanks I'll try it and let you know what I find out

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