Miller Big 40 won't weld no AC
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  1. #1
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    Miller Big 40 won't weld no AC

    Hi I have A big 40 that I can't get no arc or ac voltage I have tried cleaning slip rings, flashing fields and have checked ohms from slip rings which I thing was about 48 form center to back and about 13 center to front. Also checked stator windings which I think is #50 to 51 #3 to 4 and #1 to 2 which was all 1 to 2 ohms If I plug in a grinder and apply 12v to wire 5 and 6 the grinder will run very slow until I take away the 12v. I did find the s/n which showed it to be a 1978 but some wire #s and it showed the block to change to 240v which this don't have so I use a manual from 1976-77 which matched perfect so I believe the front panel has been changed. The manual I used was 450d from miller site. I was thinking main diodes but wanted some more ideas. Would appreciate any help. Thanks very much.

  2. #2
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    Re: Miller Big 40 won't weld no AC

    Get a package of 4 rectifiers off ebay for 5 bucks.
    Change both and reflash feilds.

  3. #3
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    Re: Miller Big 40 won't weld no AC

    OK Thanks didn't know that would cause this but will try. Just to make sure you are talking about sr1 and sr2 in diagram, correct? Also can someone explain how the voltage gets to fields to excite them as I am not quite smart enough to figure out where the voltage for that comes from. Thank you
    Last edited by RRWITTER; 06-15-2018 at 08:44 AM.

  4. #4
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    Re: Miller Big 40 won't weld no AC

    Would sure appreciate any more help on this. will replace rectifiers but any other ideas would be great. Duane if you see this would like to hear from you. Thank you

  5. #5
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    Re: Miller Big 40 won't weld no AC

    RRWITTER, evan hit on one of the more common "no power/weld output" causes but you should also check the tap straps on trim resistors R2 and R3 (see the circuit diagram and parts breakdown to identify) to make sure rusty straps aren't preventing good contact with the resistor windings.

    You can check both bridge rectifiers and both resistors if you have a multi-meter to avoid replacing any properly functioning parts. If you're not sure how to test them, Google is your friend.
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  6. #6
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    Re: Miller Big 40 won't weld no AC

    OK I will check that. Do you know what r2 & r3 should read? Also someone has added a wire from hot side of start button to #5 wire and looks to have a small diode in that wire but I don't have 12v to the center and back brushes. Thanks

  7. #7
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    Re: Miller Big 40 won't weld no AC

    Ok I have checked r2 & r3 and looks good ,ohms on both from end to strap shows about 12 ohms. The rectifiers I think are bad but one question is that the parts list shows that it should be a 6 amp 600 volt but when I look up that part it shows it as 40 amp 800v. will that be ok? The part # is 035 704. Thanks for the help.

  8. #8
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    Re: Miller Big 40 won't weld no AC

    Hi anyone know the answer to questions above? Sorry if I'm a pest just want to be sure on this, also would like to understand where voltage for brushes comes from better. Thanks

  9. #9
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    Re: Miller Big 40 won't weld no AC

    I see SR1 and SR2 being listed as 30A/600V. You can always go higher in the ratings but shouldn't go lower so a 40A/800V would be fine too.
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  10. #10
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    Re: Miller Big 40 won't weld no AC

    Ok thanks I'll get those ordered. Does that sound ok for the r2 & r3?

  11. #11
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    Re: Miller Big 40 won't weld no AC

    Quote Originally Posted by RRWITTER View Post
    OK I will check that. Do you know what r2 & r3 should read? Also someone has added a wire from hot side of start button to #5 wire and looks to have a small diode in that wire but I don't have 12v to the center and back brushes. Thanks
    From the "hot" side of the start button or the "start" side? The start side makes more sense as a field flashing by-pass fix. The diode would prevent the starter from remaining engaged once the generator started producing output from the exciter circuit

    Center brush is positive and the outer brushes are the fixed (outer) and controlled (inner) negative for auxiliary and weld output respectively.

    R2 & R3 should have a resistance of 20ohms +/- from end to end. Resistance from either end to the strap would depend on the actual strap location on the resistor.

    With the brushes lifted off/insulated from the slip rings, resistance between the center and outer slip rings should be +/- 48ohms and between the center and inner rings should be +/- 13.6ohms.
    Last edited by duaneb55; 06-17-2018 at 10:50 AM.
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  12. #12
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    Re: Miller Big 40 won't weld no AC

    Do you have the unit owner's manual for the machine? If not, go here to download it. Due to its age, you'll have to look it up by model number and select the correct manual edition based on its serial number.

    https://www.millerwelds.com/support/manuals-and-parts
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  13. #13
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    Re: Miller Big 40 won't weld no AC

    I checked r1 & r2 which was both 20 +/- ohms and checked brushes which checked 13.8 from center to one next to motor and center to one toward end of generator was 48.2. Also have cleaned slip rings. I do have manual but the one for the sn that is on it showed a 1978 but some of the wires didn't match such as #104 is #4 on this one and it showed the conversion block for 110v - 240v and this one don't have it so I am using the one for 1976-77 that matched. I thing someone has changed front panel. I checked sr1 & sr2 with meter with diode tester and it reads both ways on some of the terminals so I think they are bad if I done it right and I think I did. Is the sn stamped anywhere else on these? Also the flash wire is coming from hot side of switch which hot when ignition switch is on. I did try flashing with this unhooked didn't make a difference. Thanks for your time
    Last edited by RRWITTER; 06-17-2018 at 11:50 AM.

  14. #14
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    Re: Miller Big 40 won't weld no AC

    Sounds as if SR1 and SR2 are your problem.

    Your description of the additional wire source location makes more sense now. Hot with ignition switch in ON position.

    Did you check to see if the diode in the add-on circuit is open both ways?
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  15. #15
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    Re: Miller Big 40 won't weld no AC

    Ok I checked that diode and it seems to be open. I have ordered sr1 & sr2 so I shouldn't need that correct? This was setting for a few years but was supposed to have worked before. I can't find anything else wrong. I did notice the rheostat had a couple of dead spots but no burns I could see. I will clean that. I did have in good spots when flashing. I also checked brush wires on sr1 sr2 #5 6 & 8 to slip ring and showed low ohms, I think was 1-2 ohms on them. I'll get sr1 sr2 in a few days and see what happens. Thanks again

  16. #16
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    Re: Miller Big 40 won't weld no AC

    With new SR1/2 and if everything else is working as it should, the added "flashing" circuit shouldn't be needed.

    A previous owner may have added that if they only used the machine once or twice a year and lost residual magnetism to kick things off was the result.
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  17. #17
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    Re: Miller Big 40 won't weld no AC

    Yes That may very well be right. Someone may have flashed the wrong way is why sr1 & sr2 is bad. Am I right in thinking the voltage for initial startup comes from stator windings unless it lost residual magnetism? Just curious. Thanks

  18. #18
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    Re: Miller Big 40 won't weld no AC

    Yes, residual magnetism causes a small stator exciter AC output that is rectified to DC that then feeds into the rotor (revolving field) that results in a build up of stator AC output voltage until maximum design is reached.
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  19. #19
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    Re: Miller Big 40 won't weld no AC

    Yes about what I thought. I do quite a lot of automotive electronics but this on the ac to dc etc sometimes I don't know. I will wait for the rectifiers and see what happens. thank you

  20. #20
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    Re: Miller Big 40 won't weld no AC

    The key basic to remember regarding AC output generators and engine driven welders is DC in, AC out. That is, DC to the revolving field (rotor brushes) and AC out of the stator (stationary windings).
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  21. #21
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    Re: Miller Big 40 won't weld no AC

    Yes I do mostly understand that but get confused sometimes because I don't work on it that much. I will get the rectifiers sometime this week and let you know how it goes. I sure do appreciate the help Thanks very much

  22. #22
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    Re: Miller Big 40 won't weld no AC

    I pointed it out as one can use any appropriate DC voltage source to test a generator (technically an alternator) for output. I routinely use a 115VAC-to-12VDC power source, 18V cordless tool battery or 12V automotive batter to excite a rotor to check for stator output.

    If I know a 120V generators' normal field voltage is +/-24VDC and apply 12VDC to the rotor and get +/-60VAC output from the stator it's safe to assume all is well with those components. Applying 18VDC in the above example should yield approximately 90VAC.

    Same test can be used for engine driven welders.
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  23. #23
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    Re: Miller Big 40 won't weld no AC

    Ok that helps to understand this. One thing is I have seen a couple people saying they flashed these with the brushes, wires #5 & 6 still hooked to rectifier, Is that the wrong thing to do and I take it you do not have flash the #8 wire also just #5 & 6? Thanks, sorry again for asking so many questions.

  24. #24
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    Re: Miller Big 40 won't weld no AC

    If just flashing with good rectifier(s) one doesn't have to disconnect them. In fact you wouldn't want to disconnect them so everything functions properly if all it takes is flashing to get it operating. I was referring to testing for a good/defective rotor or stator.
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