Lincoln Wirematic 250 problem

Hi all... thanks for reading this. I'm wondering of someone has experienced this problem or perhaps has a solution to it.
I'm not a welder in any way, so please excuse some of my decriptions below. Since I specialize in electronics, my friend has approached me to check his welding machine out as it has been doing weird things lately. He is using a Lincoln Wirematic 250 welding machine and what happens is that it will lay a nice weld for about 5 or so seconds which will be followed by sputtering, bubbly and a rather ugly weld, this would be followed by a nice weld once again and so on so forth... The sputtering can actually be heard by an untrained ear. There are four filter capacitors that parallel the gun to the ground. All of them check fine on my cap meter at 30,000 microfarads ( I did check them separately). There are also two SCR's that are used to control the output voltage. I checked them and they seem fine, although those have been checked in a low current application - about 500 mA, since I had no access to a 150 A source to give them "the real test". All that's left is the transformers, chokes, clamping diode (checked OK), a shunt resistor and a control board. I don't have the schematics for the control board but was able to follow a few traces to see where the control signals are going to and where the current and voltage sampling enter the board for processing. So far the board seems fine but I can't be 100% sure.
Can anyone help me with this? Am I being led down a garden path by looking in the wrong place? Has anyone ever experienced such a problem?
All your replies would be mostly appreciated.
Thank you. Alex
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You haven't mentioned much about what or how he's welding except that it involves a wire feeder, but if he is using solid steel wire with gas shielding, he could be losing the shield from either an electrical or mechanical problem. That will give a sputtering, bubbly and ugly weld.
You may need to answer a few of the following to get more specific help.
Is it mild steel or aluminum, you are welding? Are you using solid wire, flux core? Are you indoors or out in the wind?
Before you go too far with the hard stuff, check out the obvious. You need a steady flow of shielding gas for welding with solid wire. Too fast of a rate can cause suction of air into the flow. Clean metal helps a lot.
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There are others that frequent this newsgroup that can probably be more helpful but I have a couple of suggestions. If you haven't already checked, make sure the gun and the ground cable have firm, clean connections. If not they can easily heat up and cause problems. Also, have you checked that the wire is feeding smoothly? If it's skipping or hanging up in the liner it can cause problems.
Also, if he's using gas make sure he has consistent flow.
Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com
"The universe is full of magical things, patiently waiting for our wits to grow sharper." -Eden Phillpotts, A Shadow Passes, 1934

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Alex i had the same problem at work the voltage was fine then it would change five volts next time i put down the weld. It turn out to be the scr but how they describe it is one regulate the voltage in and one regulate voltage out. but if the voltage going in is not right it will confuse the voltage going out. example: 32 volts in standered, but actually it is 28 then the output should be 24 but in reality it is 20 volts. I dont know if this helps. im dont work on welding machines but i do know what happen to me.
shipyard worker
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On 15 Sep 2003 18:55:57 -0700, snipped-for-privacy@hotmail.com (Alex Wiecek) wrote:

sounds simple bit check the welding wire for rust or feed problems doug
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snipped-for-privacy@earthlink.net (doug) wrote in message (Alex Wiecek)

Hi Doug...
Being an experienced welder, I'm pretty sure he would have noticed feed problems but this sure is worth exploring. We tried three different spools of wire with the same effect.
Alex
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Alex Wiecek wrote:

This sounds to me like you've got either an obstruction in the regulator or the tank valve isn't allowing adequate flow.. it sounds like the shield gas is accumulating in the system but the reg. isn't supplying enough volume, it's not keeping up with the demand. It recovers between attempts to weld and again stores that 5 second amount of gas. This is assuming that you're stopping to look and dial the machine each time it go wacky and then the thing works for a bit again, but I'd say that there's little in the machine and the supply of power that'll give what sounds like the typical case of a lack of shield gas.
Does the machine have a real flowmeter (with a little tube and a ball in it to indicate volume) or is it one that has a gauge to indicate flow? If it's the gauge type you might find that there's an orifice on the inlet, this could do exactly what you're describing.. as could foreign matter in the regulator inlet
John
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John...
if I recall correctly, the weld goes from good to wacky to good to wacky in a rather predictable cycle. During all that, I've been measuring the voltage accross the bank of capacitors and while the weld was good, the voltage measured a steady DC voltage with small amount of AC ripple on it. As soon as the weld begun to screw up, the DC voltage varied all over the map and the AC component was about 6 VAC. To me this indicates poor regulation due to an overcurrent condition or the inability of the control board to regulate the output voltage or even perhaps one of the SCRs going wanky. Both SCRs were bench tested good although I was only able to test them at much lower current. I wonder whether or not the problem with the gas supply might have something to do with voltage regulation but I can't really see why it would. I don't know what sort of a flowmeter this machine has. I took a quick peek at the user's manual at http://content.lincolnelectric.com/pdfs/products/navigator/OBSO_IMS/obsomenu.pdf but haven't seen anything there. I might have missed it.
Alex
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On 17 Sep 2003 03:34:51 -0700, snipped-for-privacy@hotmail.com (Alex Wiecek) wrote:

I'd think that if the weld goes wacky it would definitely affect the voltage. During normal welding the wire is shorting to the puddle in a high speed smooth manner. But when loosing shielding gas the weld becomes much rougher with the wire shorting in a unpredictable manner. The welder can only do so much trying to keep the voltage up. At some point it has to drop.
Wayne Cook Shamrock, TX
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>http://content.lincolnelectric.com/pdfs/products/navigator/OBSO_IMS/obsomenu.pdf
Hi Wayne...yes, this is a possibility that I will definitely explore. I've seen many people mention the gas flow problem and maybe I'll get lucky with this thing? I will try to meet up with him tomorrow to see if I can spend some more time with his machine.
Thanks Wayne.
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On 15 Sep 2003 18:55:57 -0700, snipped-for-privacy@hotmail.com (Alex Wiecek) wrote:

Hello Alex, I would like to suggest that you fault find this unit as though it was a normal electronic power supply.
To do that you would connect up a load resistor to the output terminals and monitor the current flowing in the resistor with an ammeter and monitor the DC Voltage across the resistor with an Oscilloscope.
Now you can take all the time you need without worrying about wasting wire and gas as you would have removed the wire and turned the gas off.
From your junk box you could probably find resistors of a few ohms to test the unit with a light load of a few amps. Then parallel your resistors to obtain a load current of several amps. If the intermittent or rhythmic problem does not show up with low current then test with a higher current.
Finding a cheap high current load is going to be a problem. Maybe you could scrounge a few yards of thin stainless steel mig wire from somewhere and wrap it tightly round a house brick. Use some screw tight electrical connections to hold some thick copper insulated cable to the stainless wire and submerge the brick dummy load into a bucket of water.
Maybe you will need a few buckets with brick loads, in series and then in parallel to experiment with different heavy load currents.
With your load current flowing you now have plenty of time to poke, prod, look for hot bad joints and make measurements.
That would be my approach Alex, Regards, John Crighton Sydney
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I think you just need the bucket of water as a load resistor. Measure the resistance pure and add salt or baking soda as needed to reduce resistance to the desired experimental level. To get even "fancier", put screws into the bucket sides below water level to use as electrodes! Replace them as needed.
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Hi John...well, when you think about it, it really is a power supply.

John, that's a fantastic idea. Problem is with resistors... I might be able to scrounge a few here and there but I'm not sure how high I'll be able to bring the amperage without smoking them. I do have a 50 ohm/200 W load resistor for ham radio applications and that should give me a few amps of load there. But then again, I would probably want to go the other route (ie. brick and a wire) as I really don't want to blow my dummy load. :)
But nevertheless, excellent idea there John. Thank you!
Alex
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How about high intensity lamps for active/passive loads.
Not so much the screw in type, but they work - filament type not gas type...
I was thinking SUN GUN or long strip glass tube with coil lamps. Parking lots have them ..... Lighting store. - Electriction might have some junk/second hand holders - buy a couple of lamps - 1kw lamp goes a long way.
Martin
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Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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OK guys... the Wirematic saga continues. I haven't had time to look at the machine for a few days and this might have helped things.... Today, I either had a brainstorm or a brainfart... and remembered something that now I think I should have investigated further when I took the SCRs out for testing.
When I removed the SCRs for bench testing, I've noticed that the surface of both SCRs as well as the surface of the heatsink where the SCRs make contact with, have either signs of pitting or corossion. Where there was a bump in the heatsink, the corresponding point on the surface of the SCR would have a dip. At first, I didn't even think that this is pitting or corossion. My initial thought was that this allows the SCR to sit tightly against the surface of the heatsink and/or allows precise repositioning of the SCR in case it is ever removed, etc. Now, don't laugh as I have NEVER worked on high current equipment and I really don't know what to expect when examining such problems.
Now that I'm thinking about it, it doesn't make any sense to me. You'd expect to have a clean point of contact between the SCR and the heatsink to allow the maximum transfer of current from the SCR to the heatsink (and consequently to the gun). The uneven surface that I'm seeing there is probably a result of moisture getting in there and corroding the surface of both, or a sign on sparking. It may not be as critical in low current equipment but I'd think that it would make a heck of a difference in instances where a very high current is needed at a steady voltage.
Can you guys confirm this for me? I am planning to go back to my friend's either tomorrow or the day after.
Your thoughts would be muchly appreciated.
Alex
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I don't know of any SCRs that are 'keyed'. It is not necessary to install an SCR, Triac or diode in any special mechanical orientation.
Cass

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