TIG welder AC broke - only outputs current one way, won't switch polarity

[SickVic]

I have a Miller Dynasty 200 DX and the AC functionality broke.
I'm not an electrical engineer myself but I have a few bright electrical friends who could help out if needed, they don't know anything about welders tho.
I'd very much like to fix this welder myself, sending it off for repairs is pretty much financially unfeasible.

So it broke right when I started an arc, I think I heard a pang (I think, couldv just been sound of the arc stopping), and the arc stopped and the machine gave some error. Unfortunately I dont remember what error, I just immediately switched the machine off and back on hoping it would work, but no error. It did weld but not as it should.
I was welding on a 1 phase outlet without ground when it happened, and wíth the HF high voltage arc start/stabilizer.

Current status:
- no error codes
- still welds on DC
- welds like standard DC electrode negative on AC setting
- so it pushes current only one way on AC, still sounds like the 150Hz it's set at, definitely some waveform, just not switching polarity

- I did visual and olfactory inspection of whole machine: everything looks fine and I dont smell anything burnt, I'v been pretty meticulous I'd say, scrutinizing all components big and small
- I measured the IGBTs (made sure I didnt charge a gate) and they should be fine
- the two ultra fast recovery diodes in secondary assembly, marked as '2', are good
- the markings on the diode thing marked as '7', I cant find it online so I dont know exactly, but it measured like a 4 diode rectifier bridge, all 4 voltage drops are the same so I guess its fine

The picture is from the 'secondary assembly' which sits at the bottom of the machine.
I'v added some pages from the manual.

What are your thoughts? Idea's? Questions? What/where should I investigate/measure? Where could the problem be?

[TERRA Operative]

Have you worked through the troubleshooting section of the manual?
The manual looks pretty good and should narrow down the fault.

The technical manual is attached below if you don't already have it, I assume this manual should be close enough to your welder to be useful. (Section 8 is troubleshooting).

[SickVic]

Oh wow thanks! I was only aware of an Owners Manual. This should be helpful, I'll get back after looking it through and some testing!

[TERRA Operative]

No probs, I spent a number of years repairing welders 10 years or so ago, so I still have my library of manuals.

Let us know how you go!

[coppercone2]

you gotta find the broken part if it went bang you can find it.

This welder was a night mare for me to fix. The main thing that will beat your ass with this one is if there is scorched PCB, particularly the one with all those little 4 terminal mosfets (cascode?). I think it arcs internally or some BS if its damaged. I replaced every single parts on that PCB and it still would not work until I replaced the PCB. I guess that if its burned, its just too hard to get all the short circuit paths off, even with hours of precision sanding , scraping, cutting and epoxy+fiberglass refills in channels.

I see your PCB are dirty. The conformal coating on that thing yellows and flakes, then you have dirt metal on the PCB. Wash all of them, bake them, possibly recoat them with MG chemicals silicone BRUSH ON coating (the spray is garbage)

I would not be suprised if its cracked conformal coating that arced with dirt some where. Its just not durable, that welder needs cleaning as part of maintence. Ignore the people that say its OK to run it when its basically covered in rust.


With what I saw going on with the conformal coating, I would remove every circuit card, do a full inspection and clean all of them thoroughly. You might find 'sunburn skin' going on.


I think that design of sucking in grinder dust through the machine with air flow over PCB is nuts, IMO the thermal design is poor. I saw alot of burning near some power resistors too. They need better signal transmission lines and filtered cabinets for the circuit cards in the type of environment it goes into. I think they got some optimisitic BS from the conformal coating distributor claiming its rated for service on venus.

[SickVic]

Thanks for the replies.
I havent had much time to work on it but I got something that might help!

I read that in stick mode the machine switches polarity, so I tried stick and it gives me a "HELP 21" error when I try to weld! It lets me select stick, but when I enable output (by pressing the torch button in lift-arc, or when I set output to ON), it gives me error 21.

The manual states: "Help 21 Display - Indicates voltage or current feedback has been detected with contactor off.". Which is confusing me because I don't get the error until I turn the 'contactor' ON? Or there's the problem: some part of the machine thinks its off when you turn it on?
Actually, its a bit unclear to me what they mean by 'contactor', most of the time its 100% clear that they mean the button/pedal to enable output, but there's also this line on lift-arc which has me a bit confused: "The solid-state output contactor does not energize until after electrode is touching workpiece.". What 'solid-state output contactor'? The IGBTs? Or they mean the torch button doesnt do anything untill you touch the workpiece, until the sensing circuitry senses loop closure between electrode and workpiece? So then gas should not start flowing when you press the button just holding the torch in the air? With my machine it does...

Actually(!), lift-arc didnt work like it should from the day I got the machine. You should touch workpiece > press button > lift > arc starts. Well my machine its like: touch > press button > short/stick/buzz. I'v been working around it by doing this: press button without touching workpiece, gas starts flowing > very quickly tap the surface > arc starts. If I tap longer than a few milliseconds it sticks. Sometimes it wouldnt work and I would have to tap and scratch up to a dozen times before it would arc. But touching the workpiece and then pulling the trigger instantly tries to weld my tungsten to the work.

Actually(!!)... I'v also had problems using HF start from the day I got it. Sometimes -actually pretty often- I would press the trigger and I would hear a single tick! sound, like an arc jumping, no HF arc between torch and work tho, and nothing, machine displayed HELP 1. I would turn the machine off and back on, the error was always gone, I would fiddle with the ground connection, and usually it would just continue to work. Note that I never had any HELP 1 error using lift-arc.
The manual states: "Help 1 Display - Indicates a malfunction in the primary power circuit caused by an overcurrent condition in the primary IGBT switching circuit. Some possible causes are: shorted output diode, shorted output IGBT, primary current transformer CT1 located on Power Interconnect board PC2, HD1, PM1, or Inverter Control board PC1"
I went back to the store some days after I bought it because it kept happening. Btw I bought it second hand (~500hours, about 14k arc starts iirc, how much or how little is that?). The arc starting wasnt strong enough they said? They adjusted some 'crystal gap'? They also mentioned the HF start was sensitive to the total resistance in the loop, so torch, torch cable, ground cable, ground connection, wiring and connectors inside the machine connecting to the HF coil. I did notice myself that it seemed to happen more when my ground connection was a bit meager. It's been better after that but still happened sometimes. I worked around it by using lift-arc more.

I don't really see what HF start has to do with "error 1: overcurrent in primary IGBT circuits". Unless that 'tick' sound I heard was actually an arc jumping inside the machine somewhere... You were saying something about internal arcing? The guys from the shop spoke of that too...
Actually.. when my AC broke I was welding with HF start and.... OOOOH! I remember, it was also HELP 1! That's why I thought little of it and just turned my machine off and back on, like I had been doing many times, only this time it didnt work fine after I turned it back on :sadge:


Man... from day one there were some things obviously wrong and I just kinda thought nothing of it. I feel so stupid writing this out. I couldv get it fixed under warranty XD

Lot of info. So what's going on here?

I should mention that:
-I have yet to completely disassemble everything so I havent inspected every single nook and cranny yet
-conformal coating is actually in pretty good shape overall, havent found any flaking or yellowing or scorching anywhere
-I actually thought the machine was very clean, the lighting/shadows really make it look bad in the photo, still tho I guess I shouldnt underestimate the grime, I want to clean it for sure
-by wash and bake, you mean bake purely for drying purposes right? Or with some other purpose as well?
-still havent found any damaged components or signs of arcing
-I have yet to go through all of the technical manual's measurements
-I did measure some stuff on the IGBT snubber board, as per manual's instructions, I believe they have you measure some gate driver components, they all measured fine
-I actually energized the IGBTs' gates while measuring continuity through them with  , not exactly switching a load, but they all switched on and off fine

[SickVic]

Ok I just disassembled further and the arc starter board is well dirty, and the coating is completely blistered 
No signs of damage after rinsing and brushing tho...
Can I remove the conformal coating with acetone? Or might that damage components?



Also.. the board you mentioned with rows of 4 pin mosfets, which would be the inverter control board, it doesnt have conformal coating like the rest of the machine, and it looks just a tiiiny bit cleaner and newer than the rest of the machine. Couldv been replaced?

[coppercone2]

I used DMF to remove conformal coating. Flux remover gets it too from a can. Its really weak.  I think IPA worked too. IDK about acetone, its pretty aggressive... I did not try that. what it does it soak up and then you brush it and peels off. It pissed me off because I could not do flux removal by any means to save the old coating, I ended up doing 100% removal (took ages) with brushing and either flux remover or IPA, and recoating them with a brush on silicone coating. It was a lost cause to salvage that coating after soldering (i initially removed it mechanically in some places with rubber abrasive)

The HF board is high voltage it should be clean as a whistle.

That machine is 750V on the DC side and like 30kV on the HV side for the spark, it needs to be clean as a whistle and the conformal coating needs to be beautiful.

For a tip, if something HV is not working partially, fully fix it before you run it. You never know what its going to do.

Inspect the spark gap board too, maybe they got rid of it on your model? mine had a big spark gap board.
ALso, inspect the plastic insulator under the PCB. They might have pin hole arc damage, look at it with a light holding it up right.


And bake it to make sure its dry if you wash it because its HV. Don't trust your oven lowest setting, at best you can heat it up, turn it off, measure the temp after a few minutes with a TC and put the boards in there, then pump the heat up a little bit after say 30 min and turn it off again



IMO based on seeing that, conductivish dust basically routed a spark some where its not supposed to go and cause chaos. I think these machines often have the HV stuff break first, then it keeps going. I say, if its not 100% intact in all functions, fix it, before it goes bad. Its like springing a water leak in your basement when the HV starts going off.


They REALLY got tight in there for the voltage level IMO. Its relying on being clean, or having good coating for being somewhat dirty. Its not gonna work in any other state IMO. With that HV board, it just takes a pinhole for a arc to escape and do whatever it does.

[coppercone2]

BTW I don't have much experience with HV boards, but people generally say HV attracts dust electostatically. So all the finest metal dust made from nearby grinding, and welding fumes (which is dust) starts coating the areas that have high potential. As soon as it cracks, you basically have a conductive shell over it for high voltages that can arc across the particles.


I got a long ass cabling for that welder so I can have it away from where I am welding. Hopefully it ingests less dust that way. I wanted to build some kind of screens for it, but thats a tough one.



IIRC, I had this french made little air purifier with HV circuit. It was built stupid (plastic weld) with no way to clean the HV part. Anyway, after a few years, I swear a saw a spark inside of the unit on the plastic shell interior that was coated with dust.


Also, wash the wiring

[coppercone2]

also inspect the TVS diodes on the front panel. Mine had a blown diode there on the MCU board. Bit of a hassle to remove that one.

and there may be a shit load of caked dust in the front and back panel honey comb. That one would just not clean, I think I ended up having to a combination of prodding , scrubbing and long ultrasonic cleans.

The amount of work that welder took to clean is preposterous. It reminds me of something that wave tek would built.


I think part of the problem is that some of the PCB are so hidden, if regular joes try to perform the maintence procedure, they just miss it. I don't think even the joes neglect it so hard, because its expensive as hell. I think they literarly just miss some of the boards that are like hidden inside of all those power components , and they accumulate dust. there is so many pictures online on forums of dirty millers. In the dynasty that arc board is like burried in a point to point network of power inductors and shit