Lincoln Model DC400 Poor Arc With Mig

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I have a dc 400 machine made in 1985 . It is a stick mig and aluminum machine. The mig runs good until it cuts out pauses, pops ,like the arc is being shut off. The wire keeps going actualy pushing the gun from the work. You have to let off the trigger and start again, then it runs good for a bit. I changed the drive wheel,ground cable, mig gun liner,took the cover off and blew the dust out of it, had a load test preformed on it. After all this I recheck my wire speed and heat, start running fine and later it starts popping, arc is cutting out again. Sometimes it runs good for awhile. I almost always weld clean new metal in fabercation. I want the welds to look as neat as possible because they are mostly in plan sight of my customers. this can be very frustrating when you have stop and start again in the middle of a weld. Is it starting circuit with the mig? A circuit board trouble?Last edited by Thomas Clark; 12-15-2007 at 04:54 PM.
Reply:Have you tried a known good gun and cable?  Your problem sounds like a poor connection in the weld circuit.Check the voltage at the DC-400 weld terminals while welding.  If the arc goes out or goes weak, but the voltage is still normal, or goes high, at the DC-400 terminals, there is most likely a poor connection somewhere in the cable feeding the wire feeder, the MIG gun and cable, or work clamp and cable.If this happens feel the MIG gun cable for hot spots, especially near the points where the flexible cable attaches to the solid ends.  If you have a hot spot, the gun cable is most likely bad.  (It's somewhat common for cables to breakdown in that area after long use.)Good luck,Bob
Reply:I looked into it further and found out it could be a reed switch,a (CRV) circuit. This is in the lower left, inside the machine. I got a hold of a manual for it . If you herd of this before let me know. But I think the first thing to do is to go ahead again and check the leads. The leads do hang on a over head track so there might be wire break down. A backhoe bucket did roll over the mig gun lead last year but it kept working. Not enough to cut the rubber hose to the gun. This machine is old and gets run five days a week I wonder how much more its going to take. Is the DC400 Ideal-a-arc made in 1985 bullitpruff?....tomcatt.
Reply:I'll get a ohm meter and check it out .Last edited by Thomas Clark; 12-18-2007 at 10:40 PM.
Reply:I would expect the reed switch to cause other problems, but I'll admit that I don't have much experience with this particular machine.  If the book says it may be the problem, definitely check it out.The suggestion I made above was a first step.  I simply didn't address what to look for if the voltage was cutting out at the DC-400 terminals.  If that happens, there are many other things that could be causing the problem, like a faulty relay in either the DC-400 or the feeder, a bad control cable, a poor internal connection, bad PC board, etc.I also wanted to point out that checking welding cables and heavy current carrying components with an ohm meter may not tell you anything useful.  An ohm meter simply doesn't put enough current through the conductors.  In welding circuits, we need to know what is happening with high current passing through.  Voltage drop is the best and easiest way I know of to find a problem.Good luck,Bob
Reply:I'll bet on contactor trouble, probably tip wear.Does the contactor make a nice solid slamming sound when you trigger the gun?Do you hear the contactor drop out when the weld current quits?Pull the cover and check condition of the contactor's contact tips.
Reply:Yea, I'll have to get my buddy to check for voltage drop when it cuts out. If we get time. I know once you get in the shop you have to get all this stuff done right now. And I'm  trying to write this now and my wife is bitch 'n about working for her manager at the kwick mart. But one thing I do here the contactors click pretty good when I start welding. I can't tell whats going on when it cuts out because the arc is loud and popping. But thanks for the tips on this so far. I'm tying not to come to a dead end. If I figure this out I'm going to tell ya because this is what its all about, and its great!  Thanks guys..tomcatt.
Reply:I found lose wires from the mig gun running to the inside of the dc400 and tighten them up. Found the contact tips or (points same thing?) duals inside a clear box, on the mig gun. It looks like you pull it off the circuit board to replace it. But it looks clean and shinny inside. I run it again with wires tight and it runs the same. I could talk my boss into getting another lincoln repair shop to look at it. I wonder if its worth the money?...
Reply:Originally Posted by Thomas Clark... Found the contact tips or (points same thing?) duals inside a clear box, on the mig gun....
Reply:What feeder are you using? The interconnect cable between the machine and the feeder could be suspect. Does your DC 400 have a terminal strip or amphenol plug for the cables. The control relay (ice cube) on the board in the feeder could have carbon build up on the contacts. Depending on the feeder some need to feel 'feedback' voltage through the interconnecting cable, when this is lost they drop weld voltage. Get me codes on your Dc400 and feeder. I will go from there. I am assuming you are using a standard Tweco style 400 gun.
Reply:Ya I have the tweco gun on this. I found that ice cube inside the wire feeder head but I didn't take it out to look at it up close. I don't know if I can remove it without damaging somthing. I have codes on this. LN-7 wire NEMA rate feeder input 115v-2.0 amps 50/60 hz serial # 137186 code 910. The machine is a Lincoln DC-400 code 8785 serial # AC-640752. Now when I run with my 7018 rods I do seam to have trouble like the arc wants to stop. So I turn the heat up. I turn up the arc force also . I can't always run too hot. I set it for the thickness and type of welding, like vertical up or over head, etc, and go for awhile then it acts up. The other welder that works with me says if your arc goes out turn your machine up more. This machine is a combination aluminum gun, rod, and mig gun machine. I wonder if it is something inside the main part of the machine. The dc400 has a terminal strip inside a little door on the front of the machine. This is where the mig gun wires go to and they seem to be screwed on all tight. They plug in on the wire feeder head. All the metal I weld is new and clean.Last edited by Thomas Clark; 02-01-2008 at 03:56 PM.
Reply:LN7 code 9100 manual http://content.lincolnelectric.com//...r/im/IM351.pdfDC400 code 8785 manualhttp://www.mylincolnelectric.com/Cat...t.aspx?p=31762So you are saying weld is dropping out on mig and stick? So its not limited to your feeder, don't worry about the relay. When it breaks arc will it re-establish an arc immediately?
Reply:I agree with denrep it sounds like it might be the main contactor in the back of the power supply. When it starts to mess up can you let go of the trigger, retrigger & it works OK?
Reply:Does this DC-400 have a multiprocess switch on the front to change from Stick to MIG?  if so, there could be a intermittent poor connection in the trigger circuit of the multiprocess switch.  You could try jumpering #2 to #4 on the terminal strip of the DC-400 for test purposes.  This will cause the DC-400 to have output all the time while the jumper is connected, so the gun will be always hot for this test. If it welds fine while jumpered, check anything connected to the #2 to #4 circuit.I did a little bit more checking on the reed switch you had mentioned earlier.  It functions only in CV mode, so it shouldn't be causing your stick problems but  It can be causing the problems you have described regarding MIG welding.The reed switch signals the DC-400 to switch from hot-start to weld mode when it senses weld current is flowing.  If it's not closing right away, you could be starting out welding in the hot-start mode, which is several volts higher than normal weld mode.  If the reed switch finally closes after several seconds, your volts will drop suddenly to normal weld mode voltage.  If the volts were set for a good weld when in hot-start mode, normal weld mode will drop the voltage too low to weld properly.  You should be able to test this by jumpering out the reed switch.  If you get a good steady weld output with a not-so-great arc start, the reed switch may be at least part of the problem.Good luck,Bob
Reply:Yes, when I run the mig it will start and go fine, then it will pop, pop ,pop, and you leave off for a second start again and it runs good again. The thing is it may not do it again for the next several welds. The stick will start and run good but it might just stop and stick to your work like someone shut the arc off for a split second. I don't like the rods we switched to there alot cheaper and harder to get what you want them to do. So running the stick might be somthing I'm doing wrong. But I have turned up the heat on stick and it seems the same. Is the terminal strip where the leads come to from the shoes or copper fins in the multiprocess switch?Last edited by Thomas Clark; 02-05-2008 at 10:54 PM.
Reply:Try taking the back cover off (with the power turned OFF!) & take an air nozzle & blow it out all around the contactor. See if that helps any. Those contactors are real bad about chattering because of dirt & that will cause the weld to turn off & on[SIGPIC][/SIGPIC]www.migfix.com
Reply:The terminal strips (I'm almost certain there are 2) are behind the front panel.  this is where a wire feeder control cable would attach.  Terminal #2 and #4 are on the larger terminal strip, and must be connected before the DC-400 will activate the SCRs and turn on the weld output.  If anything causes an electrical break, even for an instant, between those two terminals, the output goes away.  It will come back the instant they are reconnected.  A poor connection in the #2 to #4 circuit could cause the output to stop for an instant than come right back.Placing a piece of wire between these terminals will shunt out a lot of things that could be causing trouble, like: micro-switch in multi-process switch, LN-7 control cable, LN-7 relay, gun trigger and all the wires and terminals connecting these things.I need to warn you... ONLY DO THIS AS A MEANS OF TESTING.  THE DC-400 WELD OUTPUT WILL NOT SHUT OFF WITH THIS JUMPER IN PLACE, (unless the power switch on the machine is shut off).  The ground lead protection circuit in the LN-7 will also be disabled, if the electrode were to touch the case of the feeder, the control cable could go up in smoke, and the gun will always be electrically hot!Good luck,Bob
Reply:I found the contactor in back . I did have the cover off before but not the gray box over the fan. The contactor is inside and it was a brown box with the main cables coming in. I took the screws out and there was spring loaded plates in there and it was really dirty. It had black dust bunnies in there. It's a wonder there wasn't a fire inside it when it was running. I blew out the main part of the machine before but not inside the gray box part over the fan. The contacts were pitted, so I polished them with fine sand paper about 400 grit. I put everthing back together and had to run the stick. It seams to run more responsive and hotter then I use to set it at. But I didn't get a chance to run the mig yet. We also have a big red lincoln box, with the big crank wheel on the front. It looks like a real old machine ,but I swear the thing is the most forgiving stick welder I ever used.  Bob I did find the right terminal strip, my manual refers to two. The one under the 840 multi purposs switch and behind the upper front panel beside the arc control. The one buy the arc control is the one your talking about. However I'll get back to ya on how the mig runs, I didn't get a chance to run it yet. Only had enough time to clean the contactor. I'm thinking #2 &#4 terminal post left to right to run the test? Thanks guys for the info. I'll try to help you to if I can. Maybe a beer tester at the marti graus.Last edited by Thomas Clark; 02-08-2008 at 10:22 PM.
Reply:I got a to run the mig and Its running fine now. It has a strong constant arc. That guy at the repair shop missed that one.  Thanks for helping me better understand this machine, it's well appreciated. This old lincoln isn't ready for the bone yard just yet! Thanks again guys!