Problems with Lincoln square wave 175

[Tcat]

I have a Lincoln square wave TIG 175 that I bought new 20 years ago. It works well 98% of the time. I use a Magnum LA 9 air cooled torch with 3/32 2% thoriated tungston; mostly mild steel, polarity DC-. Occaisionally when I start the arc it flashes way back in the cup and it seems that I am at full power times 3, burning a good sise hole in whatever I'm working on. I've ruined several items that I've spent a lot of time fitting an setting up. after restarting the arc several times on a piece of scrap it goed back to working fine. Also tried a different torch (Magnum PTA 17) with same result. Your thoughts and suggestions please. Tcat

[Tungsten dipper]

Is your gas interrupted when this happens?

[Tcat]

No, I'm quite sure it is not

[Tcat]

I've become convinced it is something with the machine rather than the torch, cup, tungsten, gas (argon) etc

[BigMike782]

Are you welding on plated material?

[Tcat]

No, clean mild steel, usually sand blasted first

[danlb]

I've seen that happen when I had a contaminated electrode. The contaminate vaporized and the resulting plasma was hard to control. Is there contamination inside your cup?

[Sparky_NY]

Could the footpedal be the problem? Maybe its allowing full or near full power at times, dirty pot? It would be easy enough to check with a ohm meter.

Does it do it if you turn the front panel current way down low? The panel pot could also be dirty, worth checking.

[Tcat]

..A contaminated electrode could be a possibility although it has happened with a clean freshly sharpened tungsten. I am guilty of using the same cup for a long time but have new now with same result. I've had the cover off the footpedal looking for something obvious; have a multimeter I guess it's time to see what I can learn. To danlb, I'm in east bay also on the cool side of the hills. (C.V.)

[Tcat]

Sparky I always leave the power at the machine turned full on and control the heat with the footpedal

[Sparky_NY]

Sparky I always leave the power at the machine turned full on and control the heat with the footpedal

Ok, that's a common method, still could the footpedal pot be dirty giving you full current intermittently? It would be super easy to check with a ohm meter and rule that out. Something like this is a process of elimination to find the source of the problem.

You said it seems like you are at full power X3. It would take quite a bit of heat (amps) to blow a hole like that which again points to checking the foot pedal.

As a test, you could turn down the front panel amp knob way down and see what effect that has. Even with a dirty/bad pot on the pedal the current wouldn't go above what the panel control is set at. If the "full current X3" symptom disappears then the foot pedal is definitely suspect. Test is thje same idea as welding without a foot pedal, which is done by some people on occasion for special circumstances, set the heat on the panel and go. (Exhaust tubing while on the car would be a example)

On my TIG, I set the panel control just a little higher than the max heat I need and use the pedal during the weld, it makes fine control with the pedal easier and less sensitive. For example, If I expect to need about 50 amps I set the panel at maybe 70 and start welding. If I left the panel at the max 200 amps the pedal control would be extremely touchy. The concept would help in troubleshooting your problem.

[ken]

My miller 185 tig welder of the same vintage did the same thing and it was the Hall sensor ken

[BWS]

Clean and re gap the contact points.

[nickel-city-fab]

Gas lens old and full of junk? That'll do it too if it flashes over.