Debugging a Hobart Airforce 250a plasma cutter
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  1. #1
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    I have the use of a little Hobart Airforce 250a plasma cutter if I can get it running. A friend had used this a little guy for some ornamental steel cutting work, then lended it out to somebody. She got it back, and the next time she tried to use it, it didn't work. She told me that if I can get it running, we can share the use of it.

    I have never had my hands on a plasma cutter before, and I look forward to getting this thing on the air. It has pretty small capacity (1/4" steel) but should be handy anyway.

    Symptoms: power on, power light comes on as well as the "trouble" light, the fan spins up. The built-in compressor does not come on. I have verified that the compressor itself works by supplying 12v to it. It seems to be in shutdown mode, which can be from overheating, improper supply voltage, or shield cup is not in place. When I squeeze the trigger, I get nothing- no relay click or anything.

    I am guessing that possibly the circuit that verifies the cup may be confused. I am assuming that it has some way of verifying that the shield cup is in place by checking continuity between 2 points in the torch. I need to check this carefully with an ohmmeter.

    Obviously it is not overheated, but maybe the sensor is confused??

    This unit has seen very little use. It looks essentially new and the electrode has only a tiny little dimple, indicating a small amount of wear.

    Any suggestions out there of what to look for?

    Thanks
    Leon

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    Here is the link to the Owners manual. It looks like a common problem is the shield cup bieng loose or the other components bieng loose worn or none there.
    Toad
    http://www.hobartwelders.com/om/0900/o928j_hob.pdf

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    Thanks for the link- but I got the owners manual with the unit. I took apart the torch tonight and verified that the contacts that sense the cup indeed are closed. If I remove the cup, they open up.

    I hope Hobart can supply a service manual...

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    Replace all the consumables in the torch. Tip, electrode, and swirl ring.

    This is the same machine as the Miller and Hypertherm just different box.

    Most of the problems on these things are consumable issues. And make sure the swirl ring is in correctly. The o-ring should be away from the nozzle.

    There is no service manual. What you see is the only manual even for us.

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    Progress! After puzzling over the book, and the books of similar looking Hypertherm and Miller units, I checked a few more things.

    I observed that when the cup is installed, there is continuity between the blue wires, and not when it is removed. OK.

    I decided to chase down the thermal cutout switch. This was attached to a heat sink. I carefully pulled one of the spade lugs off, and when I went to pull the other, the male spade lug easily broke off from the thermal switch! Aha! I attach a jumper between the two wires that go to the thermal switch, and shazam- the yellow trouble light is not on when I power up! I squeeze the trigger, and the little compressor comes on and some sparks come out of the torch.

    Cool- lets cut some metal. I attach the work lead to some 1/16" steel scrap. I squeeze the trigger and the compressor comes on, but no pilot arc. I try it several times and I try dragging the torch across the work. Eventually, I get some fire and I'm cutting- sortof. It only gouged out about halfway thru.

    Thereafter, I have been unable to get a pilot arc at all.

    Reading all the admonitions about consumables, I carefully take the torch apart. I very carefuly clean all the parts with a fine wire brush. It was and is very clean. I put it all back together, and no pilot arc.

    I pull the white and red wires from the main circuit board and measure the voltage. When I squeeze the trigger, the compressor comes on, the relay clicks, and I see about 50v ac. Seems low...

    I plan to take this to a shop that sells these and see what they think. Why are the consumables so critical? I can see the quality of the cut deteriorating, but how can it keep the pilot arc from happening at all?

    Of course, if the thermal switch died, I wonder what other eletrco-mechanical components may have failed? Maybe it was dropped??

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    These thing barely cut with much more than a pilot arc. When the torch has no air passing through it check for continuity on the red and white wires It should read as a dead short. You need the torch attached to measure the output because it needs to sense that the electrode and tip are in contact.

    There should be no AC on the output. DC is what your meter should be set for. You might be getting a bit of ac signal from the plasma cutters chopper circuit.

    If it turns out the board is bad (Which is rather rare) i think I have a good used spare. 99% of the trouble with these plasmas it the swirl ring, It is recommended by the manufacturer that if you are experiencing ANY trouble change the swirl ring.

    Its the way these things work. When you turn the machine on there is a circuit that checks to make sure consumables are in place. Thats those two blue wires. There is another that senses that the electrode and tip are in contact as well. When all these conditions are good and you hit the trigger power will be applied across the tip and electrode. I think under load about 100v at 12A. Then the air compressor comes on. This pushes the electrode away from the tip drawing an arch like scratch starting tig. The pressure holds the electrode back and plasma escapes out the tip. Thats why these things are so sensitive. The air compressor is little more than a glorified tire pump. If there is any wear between the electrode and the swirl ring or the nozzle hole is just a bit to big there will not be enough flow to build up the pressure necessary to push back the electrode.

    Larger plasmas that work on the same principal (All Hypertherms to PMX1650, Most portable thermal dynamics machines, most millers, most lincolns) Have enough flow and pressure in reserve where the electrode will still pull back when the swirl ring and tip are worn.

    I dont think I have ever had a thermal switch fail on me. Measure across the remaining contacts on the temp sensor. Is it closed?

    From everything you say it sounds like consumables. Have you changed them?

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    Macona-
    Thanks for the succinct explanation. It makes sense now.

    At the moment, (with the therm switch bypassed), I have to shake the torch in order to cause the pilot arc to form. Then, when I try to cut with it, it just gouges the surface rather than cutting through.

    Both of these symptoms sound like the hole in the nozzle is too large. If the hole is too big, it will have less pressure to push it away from the electrode, and the needle of air will not be as narrow or reach as deep.

    So, it looks like new consumables and therm switch and I'll be in business.

    Re the therm switch- I cannot measure continuity on it because the terminal came off, including the metal piece that goes down inside of the unit- nothing to put a probe on. I agree that failure of this part is really rare, but the thing was broken to be sure, and that is what kept it in fault mode. Makes no sense unless somebody was in there before me and yanked on the wire.

    I wonder how much life I can expect from a set of consumables? This thing seems to be very persnickety about everything being perfect.

    Thanks for the help.

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    Couldnt tell you how long they will last. Havnt used one long enough to tell.

    How thick are you trying to cut through? Check the ground cable for a break.

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    Make sure to check the connections in the torch, Obviouslly if it is working after you shake it something could be bad with the switch or connections and or consumables.

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    Make sure to check the connections in the torch, Obviouslly if it is working after you shake it something could be bad with the switch or connections and or consumables.
    I have had the torch apart- everything is tight. My explanation for the arc not starting until I shake it is that if the hole in the tip has enlarged too much, or possibly an o-ring is not longer sealing well, then the air pressure is not enough to move the tip away from the electrode and start the arc.

    I'm getting consumables tomorrow- we'll see if that does it. I know what's going to happen- once I get this little bugger working well, I'm gonna get addicted and be "needing" a bigger one...

    Although I must say my Henrob-"cuts like plasma"-oxy-acet torch does a damn fine job cutting steel. Of course, it's not much use on aluminum or other non-ferrous metals.

  11. #11
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    Well, rats. Today I picked up a new electrode and tip (actually 5 of each). They didn't have the swirl ring.

    I see little improvement. I still need to give the torch a bit of a shake or a whack in order to get the pilot arc to kick in. When it does, it looks very nice- a very fine blue needle protrudes past the shield cup about 1/8" or 3/16" maybe. The switch always turns on the compressor and kicks in the relay for the arc, so I don't think it is bad.

    But then it cuts off after about 3-5 seconds or so. The air compressor keeps running for a while after it cuts off.

    Could this behavior somehow be caused by the swirl ring?

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    The pilot will not keep running for more than a few seconds only until you start cutting. This is to save the tip. The tips wear faster during a pilot arc than when cutting.

    Air compressor running after is normal as well. This is to cool off the torch consumables.

    Is it cutting. If it kicks out while you are cutting then there is no ground. check continuity through the ground cable. sometimes the wire breaks inside the insulation and is not visible.

    Remember, this thing will cut about 1/8 to 3/16 max. It is mostly designed for electricians and plumbers who are cutting through metal studs, electrical panels, etc. Not much more than that.

    If you still have to tap the torch to get it going after you replace he swirl ring then the torch head is probably bad. Open the handle and make sure there is noting obstructing the movement of the center pin that goes through the head. The torch heads are not repairable.

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    Macona- I'm narrowing this thing down!

    I'm sure you are sick of this, but it is kind of interesting to me, albeit interesting.

    If I put in the original electrode and tip, it works- current flows through the plasma stream to the work, and it cuts.

    Certain combinations of new and old electrode and tip will sometimes work. When it doesn't work, I do not get the cutting arc, and the pilot goes out, for the reason you describe- saving the tip. When I put new tip and new electrode it, I can simply never get the cutting arc to strike. And according to my eye and my micrometer, the new parts are exactly the same as the original parts- that is, they are the right part number (they have miller on them, but the welding store guy crossed them).

    I don't see how this could be because of the spring pressure between the electrode and the shiny stainless-looking contact in the torch, inside of the swirl ring. Both mating surfaces are pristine. It seems that it is a connection failure to the elctrode because of the following experiment.

    I took the cover off the torch, and I had the new tip and electrode in place, and the cutting arc would not strike. However- I took a heavy duty clip lead and attached it to little phillips screw inside the torch head where the white wires connect to the electrode terminal- the connection that runs down the center of the torch and moves when you screw the cup on.

    Anyway, I was able to draw an arc to the work piece off of the clip lead, yet the plasma flame would not draw the arc. Therefore, the box is putting out the cutting current, but it sometimes doesn't make it to the plasma flame.

    The one thing I have not been able to try is replacing the swirl ring- can't find one locally. 2 guys at welding shops inspected the swirl ring and deemed it ok. I took a loupe and looked carefully for clogs or damage. It looks perfect. Still, I may be missing some microscopic problem.

    I plan to call Hobart next Monday and hopefully there will be a tech person that can assist.

    A few times tonight, I had it cutting really well.

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    Nope, the swirl ring cannot be deemed good by looking at it. I have had numerous machines come in and not work until I replaced that swirl ring.

    Did you get them to order one for you?

    Hobart/Miller will end up telling you the same thing. Change the swirl ring.

    If that does not work the head will have to be replaced. I have had to change a couple in the past so it does happen.

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    Default Hobart 250ci help to fix it

    Looking to fix my Hobart 250 ci proble is the same, turn power on and only fan will blow air and temp light blinking and power light blink, this is a picture of the main board

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    Default

    Quote Originally Posted by agguilar View Post
    Looking to fix my Hobart 250 ci proble is the same, turn power on and only fan will blow air and temp light blinking and power light blink, this is a picture of the main board
    9dce8e73-800e-404b-9fe8-57ed820e2da8.jpg

    This is the board I have same problems hope this helps anyone here I have more pictures of the board if anyone needs them


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