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. Seewhich 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.