Thermal Arc LT-300

I would say that the biggest bang for the buck would be to take it apart and see if there is anything obvious that could be easily fixed, if not, it can probably be parted out profitably. Your 3 HP converter is too small to drive it and you'd also need a transformer, which is becoming too much of a hassle.
If you are in IL, and decide that you do not want to waste your time parting it out or fixing it, let me know, I may take a stab at seeing what can be salvaged.
i

Open it up, clean out the big chunks, blow out the little ones using compressed air and a paintbrush as encouragement when necessary. :-)
Then get in there with a strong light and look for obvious signs of "blowed-upedness". (Yes, it's a new word - we like doing that in English. It's the past tense uncertain of blowed-up.) ;-P Use your nose, many blowed-up components are easily identified by scent.
(Somehow, stuff like that sounds better with a patently fake- sounding Southern Accent slathered on. But that's just me.) ;-)
Don't waste time trying to fix it or develop a way to power it at home, that will be a big waste of time - especially if you don't even know if the stupid thing works yet.
Go find a friend or acquaintance with an industrial shop somewhere close that has some utility supplied 480V 3Ph you can borrow, scrounge up a set of cables, and see what happens when you try powering it up and striking an arc. Who knows, it might only have a small problem that can be fixed cheap - plug the loose cooling fan cord back in and replace the one diode that overheated first.
The ideal friend would have recently moved his fab or welding shop into to a bigger building with a 480V 3Ph Utility feed, and he's been running his old smaller rig looking for a big 'un just like you found. You walk in with that monster and watch him turn green...
Swap straight across for his old 240V 1Ph rig that you can plug in and use at home, with visitation rights reserved for when you have big jobs that need a big welder.
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Follow up: I took the cover off, looks like someone had already (a couple of screws missing from the cover), and found no signs of any blowed uppedness. I did find, however a broken off zener diode, which fell out of the bottom. Maybe that's all that's wrong with it, or not. I guess I'll sit on it for awhile, if I ever run across someone with 3 phase 460V, I'll replace the diode and see if it works.
Scott

Where do you live? Might be someone here within reasonable driving distance where you can test it. They might be looking for one...
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It is a manual for a similar tig welder. LS-300. I think that the difference is only that your unit has high frequency and that one does not, but double check.
there are many diagrams, schematics, etc. Worth checking.
The boards likely run on some standard voltage like 15 VDC or some such. If so, you can try powering them or some such, at your risk.
i