Need more big 40 help - no weld and no aux power
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  1. #1
    Join Date
    Mar 2018
    Posts
    41

    Need more big 40 help - no weld and no aux power

    You may have seen my other posts about this Big 40 I bought about 2 months ago. I now have issues with no welding current and no aux power.

    History since I have had this machine:
    -Once I got the engine running it did not want to weld or produce power at first. After a few minutes it just started work.
    -Later I found a broken wire in the fine adjust rheostat at about 65% position. I was able to solder bridge between adjacent wraps and restore full fine current control. Before it only welded above 65% fine control
    -Last weekend I hauled the machine about 50 miles to weld pipe fence. At first I had no weld and no power. I was probing around on the rectifiers and found no power, either on the AC side or the DC side. Suddenly it started to register on the DC side and all functions worked. I ran the machine about 15 hours over 3 days without another issue.
    -Yesterday I took the welder to go weld up some more fence. Fired the machine before I left the house and verified I had welding current by touching stinger to ground. Good strong arc.
    I drove about 10 miles and started the machine less than 1 hour later. No weld and no power.


    Today I have been poking around on the machine trying to figure out the issue. No luck so far. What I have done:

    -Measured resistance between middle and outermost slip ring. Measured 48 ohms with brushes pulled back. I even pulled wires 5 and 6 loose from the rectifier and measured about 56 ohms across them (50 after polishing slip rings) to check for broken wire with brushes back in place

    -Measured from center to innermost slip ring at 16 ohms with brushes pulled back. Pulled wires at rectifier and measured 16 ohms between #5 and #8

    -Polished slip rings

    -Checked that brushes are free to move - all looks good.

    -Flashed field with machine off. Connected 12 volts from back of ignition switch to wire 5 and connected wire 6 to frame ground. I got good spark when doing this. Started machine and still no weld or power.

    -I noticed the schematic calls out a varistor between wires 5 and 6. I can see extra terminals on the wires at the back of the rectifier where something like that could be installed. Nothing is there and it looks like the terminals have never been crimped. Not sure this impacts anything - just an observation.

    -Yesterday when I was checking things out I broke a terminal in wire 51. I fixed that today.

    This is driving me crazy. Every other component in the system looks like it would impact either aux power or weld current, but not both. I suppose both rectifiers could have died, but seems unlikely.

    Anyone have any thoughts? I'm out of ideas. S/N HG023663

  2. #2
    Join Date
    Mar 2018
    Posts
    41

    Re: Need more big 40 help - no weld and no aux power

    I found a video and checked both rectifiers. I ran through both twice and they check ok.

  3. #3
    Join Date
    May 2008
    Location
    Orange, TX
    Posts
    16,645

    Re: Need more big 40 help - no weld and no aux power

    Have you checked for corrosion between the tap strap and winding of the large "log" type resistors R2 and R3?
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  4. #4
    Join Date
    Mar 2018
    Posts
    41

    Re: Need more big 40 help - no weld and no aux power

    Nope, but I can. I did check resistance between wires 50 and 51 while disconnected from rectifier. I think this should include windings, fine adjust rheostat, and the large resistor associated that that. Measured around 20-50 ohm depending on rheostat position with some readings as high as 1 Kohm while turning rheostat. The resistor for the aux windings is only tapped on each end. The sliding band is rusted to about nothing. Output aux power somewhere between 120-125 volt at 63 hz so I didn’t mess with it.

  5. #5
    Join Date
    Mar 2018
    Posts
    41

    Re: Need more big 40 help - no weld and no aux power

    It’s fixed. Thanks for the help Duane!

    I checked resistance at each large sliding resistor. Weld resistor measured 12 ohms from end to sliding tap. Aux power resistor measured 23 ohms end to end - no sliding tap on this one. Found a lose connection at top end of that resistor and replaced. Turns out that was not the issue.

    Found one rectifier installed backwards (+ and - reversed). Once again not the issue, but corrected.

    Finally found another post about flashing the field with a grinder switched on in the aux power outlet. Tried that with engine running. After a few seconds grinder took off. Now I have power and she welds!

    Hoping this is not an ongoing occurrence, but she works for now!

  6. #6
    Join Date
    May 2008
    Location
    Orange, TX
    Posts
    16,645

    Re: Need more big 40 help - no weld and no aux power

    Glad to hear it's up and operating for you.

    Quote Originally Posted by biguglydave View Post
    Finally found another post about flashing the field with a grinder switched on in the aux power outlet. Tried that with engine running. After a few seconds grinder took off. Now I have power and she welds!
    What did you do with the grinder besides pulling its trigger?

    Quote Originally Posted by biguglydave View Post
    Found one rectifier installed backwards (+ and - reversed). Once again not the issue, but corrected.
    On the contrary. This combined with the poor connections could very likely have been the cause.
    MM200 w/Spoolmatic 1
    Syncrowave 180SD
    Bobcat 225G Plus - LP/NG
    MUTT Suitcase Wirefeeder
    WC-1S/Spoolmatic 1
    HF-251D-1
    PakMaster 100XL
    '68 Red Face Code #6633 project
    Star Jet 21-110

    Save Second Base!

  7. #7
    Join Date
    Mar 2018
    Posts
    41

    Re: Need more big 40 help - no weld and no aux power

    This is the thread I found:

    https://weldingweb.com/showthread.ph...-Miller-Big-40

    I actually used a drill and locked the trigger on. It did not turn at first. Once I touched the wires to flash the field it started to spin and rampedup to full speed over 2-3 seconds. After this all worked. I’m not sure I needed the drill plugged in, just following the thread I found. I tried flashing once before on Saturday, but that was engine off and only for a second or so.

    I’ve thought some about the backwards rectifier and how that might affect the circuit. To be honest I got a C in my Electronics for Engineers class, so I have not been able to convince myself of an answer.

  8. #8
    Join Date
    May 2008
    Location
    Orange, TX
    Posts
    16,645

    Re: Need more big 40 help - no weld and no aux power

    The "drill flashing" method that you may be thinking of is you plug a brush type drill into an auxiliary outlet and with the welder/generator running, pull the drill trigger and spin the drill chuck by hand until it starts running on its own. The spinning drill motor produces a low voltage that back feeds into the stator winding which creates a small magnetic field around the rotor that restores its residual magnetism that is required to get things started.


    Using an outside voltage source such as a 12V battery at the brushes just energizes the rotor directly to establish the revolving magnetic field needed to produce output. Once energized, the rotor should maintain its residual magnetism for quite a period of time to kick things off each time it's started.

    I'd say with the loose connections and reversed rectifier, the machines rotor didn't know if it was comin' or goin'.
    MM200 w/Spoolmatic 1
    Syncrowave 180SD
    Bobcat 225G Plus - LP/NG
    MUTT Suitcase Wirefeeder
    WC-1S/Spoolmatic 1
    HF-251D-1
    PakMaster 100XL
    '68 Red Face Code #6633 project
    Star Jet 21-110

    Save Second Base!

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