Lincoln square wave TIG 300 problems

Somehow the last post on this ended up in a reply to "ping Iggy" hope I got this in its own thread.
Anyway, I opened up the welder and refreshed my memory on what the
boards look like. There are actually very FEW tantalum caps in it. Most of the caps are plastic film types, so a lot less likely to be the problem.
I did find what appears to be a real problem. I think my tig torch cable has burned up. I get about 50 V drop across the cable at 100 Amps. I get less than 1 V drop on the ground cable. This is a Weldcraft water-cooled TIG torch. I will have to go to the store and get a new cable. Do these cables burn up, or just maybe corrode away over time? I have been using the miller TIG cooler fluid I got at the welding place since I have had it, but the torch came with the used machine.
Michael Terrell was looking for Lincoln schematics. They have just put most of them online in August. See http://www.lincolnelectric.com/assets/servicenavigator/LINCOLN2/SVMALPHA.pdf
which links to about 100 service manuals with board-level schematics as well as a lot of other info.
Unfortunately, they don't seem to have the S.W. TIG 300 in this list, it may be too old. But, I hope since this is all on line, I can get the manual from them by calling on Tuesday.
Jon
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On 12/30/2011 8:47 PM, Jon Elson wrote:

I had a Weldcraft 250 Amp water cooled torch do this to me. I am not blaming the torch, I got 16 years of use out of it before this happened and I was welding very close to the rated current. Make sure that your cooling water is flowing though, the wire in the torch cable is nowhere near heavy enough to support the rated current without water cooling.
Happy New Year, BobH
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BobH wrote:

Well, I have not used it a great deal, but it has been filled with TIG coolant for 5 years. I have no idea of the history before I got it, but if the torch is as old as the welder, then it is REALLY old. When I got it, water would not flow through the torch. I let the cooler run for a while, and suddenly a bunch of grey goop spewed out the return line and it started flowing pretty well. I flushed it out with two fillings of water before putting the TIG coolant in the system. I'm afraid that goop must have been corrosion coming off the cable because the previous owner probably used tap water in the cooler.
Hopefully the cable won't cost a whole lot. I'm a little worried the torch might be clogged up. The cooler provides lots of flow when I removed the torch and held a container up to the cooler's outlet. I guess I can uncouple the supply hose at the torch end and see how much flow I get there.
At least at the currents I've been using, the return to the cooler doesn't get seriously hot. I can always hold my hand on the fitting.
Thanks,
Jon
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Gunner Asch wrote:

https://picasaweb.google.com/lh/photo/jXLUv_Zh9I40taoBZlMYDtMTjNZETYmyPJy0liipFm0?feat=directlink Yup, that is the exact machine, although mine isn't quite that grungy. 2 digital meters, with pulse and crater fill, code 9298FF
I'll call Lincoln on Tuesday and see what I can get. They did send me a machine wiring drawing when I called a few years ago. What I need is the service manual, it has the schematics of the boards.
But, the more I think about this, the more I am convinced that the torch cable is shot, and has ALWAYS been bad, from the time I got this machine! It all fits! The cable must have corrosion inside, and after sitting for a while it goes open-circuit. When I try to use the welder, the HF eventually breaks through the corrosion, but it still makes a poor contact. What I can't figure out is how the thing works at all without blowing a big hole in the return hose where the bad spot is. I have put a DVM between the electrode and the torch lug on the welder and measured 50 V at 100 A. That is dumping 5 KW into the torch cable! You'd think that much heat would generate a blast of steam and pop the hose.
Anyway, I just ordered a replacement cable/hose on eBay, I'll see how that changes things.
Jon
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You could ask how they feel about posting schematics or allowing others to post them on the internet.
jsw
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Jim Wilkins wrote:

They have at least 100+ online already, starting with models just a little newer than mine. These are the complete service manuals for a variety of machines.
Jon
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Jon Elson wrote:

Thanks, Jon. I'll download some of them and see how repairable they look. I'm looking for something to do when I turn 62, and am moved from VA disability to Social Security. Currently, I'm not allowed any other income and have been working to repair my shop and build some new benches, to keep busy on the days I can work around the house.
--
You can't have a sense of humor, if you have no sense.

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Michael A. Terrell wrote:

Well, they don't have a huge theory of operation section, and many of the newer welders are computer-controlled, limiting the number of points you can probe. But, I think they are quite well-built and should be repairable with the manual. Of course, you can get one that has extremely expensive damage like a bad transformer or SCR block.
The only repair I've done on mine was a capacitor in the post-flow timer was bad. I traced that back with the machine wiring drawing and then traced it onto the board to the specific component. The current problem I'm having looks like a bad torch cable, I can't BELIEVE it took me this long to figure out something was REALLY wrong, here! I am positive it has been this way since I got it, and just never really thought why the voltage meter would always read so high. I would have had to find it except that it "heals" itself after you try to start an arc for a few minutes. But, I think the cable is slowly rotting inside the hose, and the resistance just keeps going higher.
Jon
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Jon Elson wrote:

Something still doesn't seem right. 50V * 100A = 5KW. If the fault is contained in a small area (and I can't imagine it would be the whole length of the cable, the fault would have burned open in less than a second while dissipating that sort of energy.
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Jim Stewart wrote:

YEAH! This is clearly right! But, I verified the voltage across the cable with a known-good DVM. It reads right on the nose, +/- 1 digit on the welder's meter. And, I know the welder's current meter is at least within sight or reality due to the arcs I get. When I first got this welder, I couldn't get any flow through the torch, and left the cooler running for a while, and then this HUGE amount of CRAP came out of the hose/cable. I had no idea what it was at the time, but now I think I do! I think it was corrosion off the cable. I think the previous owner had used tap water in the cooler, and when the welder went into storage, the water left in the hose badly rotted the cable, enough so it was badly plugged up.
All this misbehavior fits the symptoms. When the welder is first used after sitting for a while, I get HF but no welding current. I didn't believe it before, but now I DO believe the meter readings that the welder is putting out 72 V. It also gets over-voltage trips and shuts down frequently in this condition. After some time, maybe 5 - 10 minutes, just instantly, the welder starts delivering weld current. I suspect this is a bad contact at one end of the cable, it gets a layer of corrosion, and then the HF finally breaks through that corrosion and makes a decent contact.
Well, so the cable has about 1/2 Ohm resistance. 50 V @ 100 A. As you say, 5 KW would have made an explosion if it was all concentrated in one spot. So, I think the cable is BADLY corroded all the length of the cable, and is really a thin strand. I think that grey STUFF that came out when I first started it up was an important bit of the conductor.
Anyway, I have a replacement cable on order. I expect to immediately see the difference in the performance of the welder. And then, I will post-mortem the cable, it ought to be interesting.
Jon
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Well, final report:
I got the new torch cable last night, it made a huge difference. With the electrode in the ground clamp, I now get 4 V at 100 A instead of 50+ V. The welder seems to work much better, generally. Not completely sure this is just because I can get more amps or what, but welding just seems to work better.
So, the post-mortem on the cable : First, after not using the welder for about 4 days, the cable is open-circuit! I read about 30 Meg Ohms between the ends. I cut into the hose, and the cable inside is gray-white and not shiny at all. I bent the hose to aid in cutting through it and many of the wire strands snapped, not sure if I nicked them with the knife, or they were just brittle from corrosion. The cable is not obviously rotted away to nothing, as I was somewhat expecting to see.
Oh, also the coolant flow through the torch is greatly improved, so I am kind of suspecting the old cable had swelled up from corrosion. One final observation, running at ~100 A for some time on this project, the return water to the cooler was COLD! In the past, the return water got quite warm, not so hot you couldn't hold your hand on the fitting, but well above body temp.
Jon
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