Help! my TIG often looses its ability to vary current

I have an old Hobart welder (TR-300-HF) which frequently drops to its lowest current setting at any given range (e.g. 50A at 50-250A). It
welds fine in this state, but having only 3 current levels is annoying. No amount of foot pedaling or turning the current knob seems to fix this problem when it happens. Assuming the potentiometers in the current knob and foot pedal are OK, what is the most likely cause? What should I check first?
-Jason
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Sounds like you blew your main reactaince coil. Sometimes they can be repaired, but more often they have to be replaced.
I had an old P&H 300 TIG that died that way.
The Hobart should be repairable. Thermal arc supports those old large Hobarts for parts.
Get it to a repair shop if possible. If you were in Seattle I could give you to excellent repair shops to take it to.
Those old machines do have a limited lifespan mainly from the insulation on the transformers finally drying out so much it just fails.
The old grey Lincoln Idealarcs have the same problem.
Luckily those old machines are so simple that they are pretty easy to troubleshoot.
--
Welding Instructor - South Seattle Comm. Coll.
- Divers Institute of Technology
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I'm in MA and don't know any repair shops. I'll open it up and check for shorts.
-Jason
Ernie Leimkuhler wrote:

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Check your foot cable...
Maybe you have a broken wire in it - and it reverts to low. Maybe a short does that. Up near the machine interface - where it might kink and yank a wire out or on the other end - or in the middle - look for a pinched/broken area. I don't doubt Ernie on anything - just some trouble shooting concepts.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
Jasonv8z wrote:

-
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I wonder if, on these older machines, that one could simply pour some "Formvar" into the transformer, as best they can, and let is sit near a heater or heated with a heat gun, in order to 'renew' the varnish on the wire BEFORE the old varnish dries to the point of cracking and therefore shorting?
buffalo
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buffalo wrote:

I know that it is common to take large electric motors in and have the stator (the fixed outer part) re-varnished and baked. If you have one you're worried about, why not call a motor shop and ask if their process might work on your welding transformer?
Both are large magnetic arrangments of conductors ..
Of course, it might be hard to get varnish in between the transformer laminations.
GWE
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If you were to submerge the transformer in epoxy and pull a vacuum on the thing, you might be able to reinsulate the windings.
Anyhow, After looking at my welder, I don't think its the transformer. There are actually 3 separate transformers (corresponding to the 3 current ranges), with the coils made out of rectangular copper bar. The epoxy looks to be in good shape, and each turn is spaced by insulating slats about 3/32" thick so air can pass between each loop. It looks like the whole assembly was then dipped in more epoxy. It all seems to be in good shape, and Its unlikely that all 3 transformers would fail at the exact same time and manner, but I get the same symptoms on each current setting.
I know now that its a saturatable reactor system, so the current output is controlled by a DC electromagnet wound into each transformer. I think these are OK too, because its only supposed to be ~24V DC (according to Dennis at Arcowelder, who was very helpful), and a short would cause my welder's output to go up. Full power to the DC coil should bring the welder to its lowest current, which is exactly what is happening. I'll check the output on the PC board next. This is all pretty new to me, so I could be wrong.
-Jason
Grant Erwin wrote:

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I think that you are doing the right thing, which is talking to knowledgeable people and poking around with a multimeter.
Did you check what the potentiometers are telling the control board? When you turn the potentiometer knobs, do these inputs change _right at where they enter the board_?
i
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I am not having a problem, at least not yet.
However, I am pretty sure that one could simply pour and brush on Formvar so as to permeate the interstices in the windings. Naturally, it wouldn't be like re-wound and baked but my guess is that it would do well.
I don't think it would hurt anything.
buffalo

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On Tue, 27 Jun 2006 00:46:01 GMT, Ernie Leimkuhler

http://www.arcowelder.com /
I think he has one in the bone yard Ask for Dennis
Gunner "If thy pride is sorely vexed when others disparage your offering, be as lamb's wool is to cold rain and the Gore-tex of Odin's raiment is to gullshit in the gale, for thy angst shall vex them not at all. Yea, they shall scorn thee all the more. Rejoice in sharing what you have to share without expectation of adoration, knowing that sharing your treasure does not diminish your treasure but enriches it."
- Onni 1:33
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