Thermal Arc 201 TS TIG output is erratic

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I have a Thermal Arc 201 TS welder that I bought in 2014.  Lately the TIG output is becoming more and more erratic.  If I set the output for say 80 amps, I get a feeble little arc that will barely darken my mask.  If I bump it up to something between 160 and 200 amps I get good output.  With the high output settings, I can modulate the arc using the finger control on the torch they supplied.When the trouble first started showing up (a couple of years ago) I thought it might be dirty inside so I pulled the cover and blew out what little dust was inside and it then operated normally for a while.  Lately, blowing it out doesn't do anything and I wouldn't expect it to since it's already clean inside.  Last week, I pulled the cover and tried some contact cleaner on the potentiometer on the panel that controls welding current and at the same time I pulled and re-inserted all the various cable connectors.  It got better for one day and then it was back to the old behavior.I tried a different torch (Weldcraft) along with the "official" Thermal Arc foot pedal.  No difference.Today I tried it in stick mode and it seemed happy and when I went back to TIG, it operated correctly for two beads and then reverted to its bad behavior.I'm running out of ideas and hoping somebody else has a better idea than I've been able to come up with.
Reply:How's your ground connections?  Bob
Reply:Ground connections are good.  I re-built the attachment at the ground clamp end, checked the tightness at the DINSE plug end and checked the tightness of the connections for ground and torch leads inside the unit--the back side of the DINSE sockets.  Ground clamp is attached to the work piece at bright metal.I keep thinking it's a connection someplace but I've been over all of them that I can think of but always with the same result.
Reply:Always good to "circle back" if things get fixed.  A couple of weeks ago I found the service manual for the 201TS someplace on line...now I can't remember where.  Anyway, an erratic arc was listed as one of the faults and the solution listed is to replace the whole circuit board that mounts the control potentiometers.  I thought maybe that was a reasonable solution but I could not find that board for sale any place....even on FleaBay.  So I figured "Kill or Cure" and I pulled the whole welder apart, looking for bad connections, burned components, mouse nests, etc.  I could find nothing.  Using the manual's voltage checking charts I found the correct voltages and resistance values at each test point.  (It's a little scary running that thing open on the work bench because some of the internal voltages are pretty high--up around 380 volts.) There were a couple of sketchy-looking solder joints so I re-soldered those.  I put it all back together and now it works properly.  This is perhaps even more frustrating because I still don't know what was wrong in the first place.  But I'm happy that it works again since it's really a pretty nice little welder.