1981 big 40, rectifiers and what cables go where.
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  1. #1
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    1981 big 40, rectifiers and what cables go where.

    My bobcat lived a short life with me but did make some money saved alot of money on our equipment and got alot of fence building done at the house. The water filled onan ended up letting going on lower end.

    So im back on big 40. No weld output no 110 output. All indications led to rectifires. Now ive got new rectifieres on but the ones i replaced with are only marked positive and AC negative is not marked.

    Cable labled #5 is positive and has a jumper between both rectifiers. My original asumption was this was a dc positive.
    AC positive comes from wire 40 atleast i think thats ac positive. I have not figured out wich wire is #40. That should be ignition hot? 6 is negative.
    Im thinking out loud here. The wireing diagrams are a help but dont seem to actual show me whats going on. I see the path on positive that appears to flow out of rectifier through #5 with the jumper.
    Each rectifier has a ac hot wire. I do not know wich wires these are. Tech at miller said #40 is ignition i do not appear to be getting ignition power to either rectifier

    Heres the diagram ive been stewing over. The big 40s witeing diagrams for 1970s-90s seems to be about the same.

    Searching my vin doesnt bring up a manual but searching big 40 does and the vin gets me close on its age
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    I thought i saw a diagram with cables labeled better but ive been unable to refind it.

    This 40 has a solid f163 that runs pretty damn good. I just have to smarten up to get it to weld.
    Yesterday the bobcat quit on the job then my pickup decided to have starting issues today is pickup day and tommorrow i may dive into the big 40 in hardcore kinda way.

    Thank you all, you are a huge help.

  2. #2
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    Re: 1981 big 40, rectifiers and what cables go where.

    AC doesn't have a designated (+) and (-) as each leg is switching back and forth 60 times per second. Hence the term "alternating current".

    Wire #40 is in fact battery (+) hot to the ignition switch.

    Wire #5 is DC+ as you suspect to the center slip ring brush and wires #6 & 8 are DC- to the outer slip ring brushes.

    Will need the unit SN to determine if yours has battery (+) to the exciter circuit for initial field excitation or not so we can be on the same page.
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  3. #3
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    Re: 1981 big 40, rectifiers and what cables go where.

    Duane you are awesome.
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    These are the only markings. Im guessing negative is across from +

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    I have cables with these markings #7 #8 104 #5 #6 #90 #50
    Last edited by evan; 04-11-2018 at 12:22 PM.

  4. #4
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    Re: 1981 big 40, rectifiers and what cables go where.

    + and - are diagonally across from each other and AC connections are diagonally across from each other. If you’re wiring diagram is correct. Below is how they are wired.

    + 5
    - 8
    AC 7
    AC 104

    + 5
    - 6
    AC 90
    AC 50
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    1969 SA200

  5. #5
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    Re: 1981 big 40, rectifiers and what cables go where.

    Man you guys are amazing.

    Hooked up wires with capt b's help got nothing out of 110 outlets so i decided to jump start #5 and 6 grinder came to life dancing on the ground.

    Smoke started coming off this so i shut it down is it normal for it to get hot
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    I did see if she welds and it does. In second gear 50% its alittle cold but i did lay a bead down

    Thank you guys so so much

    Wire #7 is on ac spot on rectifer that goes to that thing thats getting hot
    Last edited by evan; 04-11-2018 at 10:06 PM.

  6. #6
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    Re: 1981 big 40, rectifiers and what cables go where.

    That's trim resistor R2 and yes it will get hot but you should check the lead connectors and tap strap for rust and clean as necessary. A poor connection will cause it to get hotter than normal or than it should. Smoke may have just been accumulated dust burning off but check those connections just the same. The rheostat to the left of your hand will get hot too and you should check its connections and clean the winding with some fine sandpaper.
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  7. #7
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    Re: 1981 big 40, rectifiers and what cables go where.

    Duane. The coarse adjust was froze up when i first got this thing so i drenched it in penetrating oil got it unfroze. The coarse knob is broke part way off before i got.

    I cleaned connections with sand paper and watched it feeling the cable on it to see if uts hot and wasnt it stopped smoking then rehostat like you said started smoking then also stopped so i then burned some beads.

    Holy cow this thing runs so much different than the bobcat in a good way. Arc strikes off with out trying and its so smooth
    I was burning 1/8 7018 trying to figure out what gear i want to be in. Ill have to look at manual and see what amp ranges are on coarse adjust damn thing runs hot and smooth.

    Thought i was going to have to tell the guy i just started welding for that my welder took a dump.

    Is this thing going to burn up my grinder, seems to be running my grinder in turbo mode.

    Im missing all auto idle parts so im going to have to rig up a linkage to manualy set it to idle then to weld rpm

    Heres a few pictures of beads
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    Im so excited. Alittle bummed im not able to run my wire feed portable, but ill get over it. Also found i often cant reach in comercial buildings due to a short cord and end up running stick anyways.

    Last edited by evan; 04-11-2018 at 11:29 PM.

  8. #8
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    Re: 1981 big 40, rectifiers and what cables go where.

    Have you checked Auxiliary Power output voltage and/or frequency? If engine speed is too high, voltage and frequency will be as well making motors run faster than they should.
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  9. #9
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    Re: 1981 big 40, rectifiers and what cables go where.

    My guess is you're going to wind up replacing that welding table top quite often too.
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  10. #10
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    Re: 1981 big 40, rectifiers and what cables go where.

    Lol. I have not checked rpm. My volt meter appears to be able to check rpm.

    Yes that welding table is meant for burn pile. I dont normaly weld rught here but did because this trees big enough to use as hoist to load and unload welder.

    Ill see if i can figure out how to check rpm with my volt meter in the morning.

    Im stocked euth your help and otheres i was able to get a peice of history welding and will put it to good use.

  11. #11
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    Re: 1981 big 40, rectifiers and what cables go where.

    I would think your better off setting rpm using voltage and frequency at the receptacle instead of actual engine rpm . My reasoning for this is to compensate for any electrical variables within the machine itself. Since each machine I've ever been around seems to have it's own personality.

  12. #12
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    Re: 1981 big 40, rectifiers and what cables go where.

    Doss,
    Ill check voltage at receptical , maybe its snowing here then suppsed to turn to rain

  13. #13
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    Re: 1981 big 40, rectifiers and what cables go where.

    If your DMM doesn't have a frequency feature, get yourself one of these for checking/setting generator engine RPM:

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    Your Big 40 is a 4-pole, 1800RPM generator so the frequency equation is:
    RPM = (Hz x 120) / 4

    Small portable 2-pole 3600RPM generators and welders such as a Bobcat, Trailblazer, Weldanpower, etc is:
    RPM = (Hz x 120) / 2

    $30 at Home Depot and available from several online sources.
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  14. #14
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    Re: 1981 big 40, rectifiers and what cables go where.

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    This is what i have. Its cold and blowing snow. Sitting in truck look at weld lead outputs. Thinking about bolting 1/8 over generator area. Move leads out to the sheet and build cable holders on the side.
    Pull the fan rubber hoses alt oil filter carb and everything off and paint the 163 cat yellow

  15. #15
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    Re: 1981 big 40, rectifiers and what cables go where.

    Sorry guys but not quite out of the woods yet.

    I flashed feilds after hooking up rectifiers and all seemed well. Then next day i had to flash again i thought maybe it was a fluke becausd i hadnt used it much.
    It wasnt.
    Put auto idle rid on today and no power. Flashed to wake it up.
    Would this mean a rectifier is bad or hooked up incorrectly allowing bleed off over night.

    I can shut it down then fire it back up 20 minutes later and its still good. Its the over night sitting that requires me to flash again.

    Also its not ramping to weld rpm when an arcs struck. I figured thats in gov sensitivity tuning. However it does work with auto idle switch when idle is turned on it almost instantly goes from weld rpm to idle

    Hoping its something silly ive done requring the daily reflash.

    Ohhh murphy switch doesnt seem to work and no lights on front panel. It used to have lights that turned off ounce running. I havent messed with any wires other than rectifiers

    Its a huge pluss to be able to go to idle with the switch even though striking an arc isnt sending it to weld rpm.

    Thanks again you guys are amazing

  16. #16
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    Re: 1981 big 40, rectifiers and what cables go where.

    I orderd that doohickey to check and set rpm. Figure im just guessing at the till i get rpm set right at idle.

    I also dont have to push murphy by pass to start engine. I dont know whats up there. I am getting a oil light that goes away ounce running.

    I welded top rail on about 20fence posts with it yesterday and shes working pretty darn good.

  17. #17
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    Re: 1981 big 40, rectifiers and what cables go where.

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    Duane thanks for tge tip. Tool works very easy as you can see rpm is off. Both at idle and weld rpm

  18. #18
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    Re: 1981 big 40, rectifiers and what cables go where.

    Yep. A bit fast.

    RPM = (Hz x 120) / 4
    RPM = (72.3 x 120) / 4
    RPM = 8676 / 4
    RPM = 2169
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  19. #19
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    Re: 1981 big 40, rectifiers and what cables go where.

    Yeah. Its idling at 1000.

    What should hz numbers be, rpm numbers aside.

    I have to pull it outa the truck to set rpm. I have a osha approved lifting procedure this time

    At idle the doohickey barely reads. Hoping raised to 1200rpm auto idle will work when arc is struck.

  20. #20
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    Re: 1981 big 40, rectifiers and what cables go where.

    Hz = (RPM x 4) / 120
    Hz = (1000 x 4) / 120
    Hz = 4000 / 120
    Hz = 33.3


    Spec low idle speed is 1050RPM

    Hz = (1050 x 4) / 120
    Hz = 4200 / 120
    Hz = 35

    That's going to be on the ragged bottom end of the Kill-A-Watts operating range.
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  21. #21
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    Re: 1981 big 40, rectifiers and what cables go where.

    Idle floats from 37-40 weld speed is solid at 60.
    Havent tested to see if arc kicks it to weld rpm.

    Duane.
    What should i be looking into to make murphy switch function fuse is good and light does light up with ignition on but not running.
    Do not have to push bypass button to start engine.

    Thanks for all your help

  22. #22
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    Re: 1981 big 40, rectifiers and what cables go where.

    Did some welding at 60hz 1800 rpm it was at 72hz . I had to go up a gear compared to 72hz. I think im going to split the difference and try 65 66hz wich is 1950.

    Is 1950 okay. Thats only 100rpm over spec in manual im reading. I want the flexibility at the upper end for big rods doing equipment bucket repairs on big metal.
    At 72hz i was running 7018 1/8" in 3rd gear 40% is on fine and now i have to go 4th gear and over 50% even if i dont get the 3rd gear back id like to gain the flexibility back in the fine adjust

    Im just jabbering

    1hz is about 30rpm

  23. #23
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    Re: 1981 big 40, rectifiers and what cables go where.

    Im wondering why my auto idle solenoid is grounded on bolt. I didnt do that. Im wondering if it should ground through board whever thats at. Currently idle switch works but it doesn't come to weld rpm when an arcs struck.

  24. #24
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    Re: 1981 big 40, rectifiers and what cables go where.

    General acceptable frequency range is from 58Hz full-load to 63Hz no-load.

    63Hz allows for governor droop (that's droop not drop) and personally that's as high as I'd go even though excessive frequency - to a point - is better than too low. Plus you have to factor in the increase in auxiliary power voltage that shouldn't be more than 10% over 120V or 132V.
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  25. #25
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    Re: 1981 big 40, rectifiers and what cables go where.

    Ok sounds good. 63 might slip to 65.
    Carb sudes agaisnt my cab. Gona try and adjust it blind.

    Decals are done from Crystal shop. Not sure if she printed a lincoln liminator one. I may change the lay out. Ive got a swiveling welding lead real on the brain lately. So it can swivel to aim at tailgate or off the side of the truck.

    Thanks for all the help again. Running beads yesterday the 163 was running very smooth at weld rpm. Ive cycled lots of fluid through lower end. Took 3 drains for fluid to not turn black right away.

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