Dan-Mig 140 Mono suddenly no arc

[Darrell Dieringer]

I've been using my buddy's Dan-Mig 140 Mono. I used it earlier today and it worked fine. Until it stopped making an arc - mid project of course.

I managed to find a number of posts on these forums regarding the Dan-Mig 140 Mono (aka Migatronic 140 Mono). This post (https://www.mig-welding.co.uk/forum/threads/dan-mig-140-mono.98734/) and others it lead me to helped me troubleshoot.

My buddy is the original owner of the machine and I know he has made no modifications to it.


Here's what I've checked:

• contactor clicks (and plastic button moves) when trigger is pulled
• wire feed works normally when trigger is pulled
• contactor has continuity when energized
• wall plug is wired correctly and wires are properly secured within it
• cord from wall plug is properly connected to terminal block within welder chasis
• welding ground (earth) cord has continuity and is in good condition
• both fuses are correctly sized and have continuity
• both fuse holders have proper continuity when tested from the terminals on their backsides

Still, I can't strike an arc. Changing temp setting has no effect.


Here's what happened:

• The machine started welding fine today. It worked for a few welds quite well
• Then, I started getting a weaker arc.
• Then the arc "sputtered".
• Then it failed.
• The "weaker-sputter-fail" timeline was about five minutes of normal intermittent use.
• I hadn't moved the machine (so no sudden impacts or bumps or knocks), nor had I changed any settings.

I know enough about electronics to follow a schematic, locate components, and operate a multi meter safely. As far as failed circuit board component diagnosis, I'll need some hand holding.

I've identified the triac on the control board appearing as TXD10K40 in the schematic appearing in https://www.mig-welding.co.uk/forum/threads/dan-mig-140-mono.98734/.

I know it's an AC switching component. In a couple of posts on these forums that triac is mentioned in discussion of wire feed problems, though the consensus suggested the wire feed motor is likely a 24V DC motor and that an AC switching component probably doesn't drive it.

I haven't found a definitive reference regarding what that triac does control, so I thought maybe it is causing my "no arc" problem.

On my welder, however, that triac is a "Q2006L4" (well, i'm reasonably sure the last digit is a "4"). With the help of googling "how to test a triac with an ohm meter" and my best amateur skills, it does seem to test fine (though I could be wrong).

I'd appreciate any advice you can offer regarding further troubleshooting. I'm happy to post photos of anything you think might be helpful.

Thank you in advance.

 

[eddie49]

Hi Darrell, welcome to the Forum!
This PCB schematic is for the Migatronic 180, hopefully similar to your machine:

It shows that the TXD10K40 Triac is used to switch the gas valve and the Contactor. Since your Contactor pulls in when pressing the torch trigger, that Triac must be OK, and the logic on the PCB that triggers the Triac, and the auxiliary 24v power for the PCB, must also be OK.
I would suggest checking for correct flow of AC mains power after the Contactor, via the power level selector switch, up to the primary of the main welding transformer. Then AC out of the secondary, which should vary smoothly from about 15v to maybe 35v AC, as you step through the selector switch positions. Finally confirm that you're seeing the equivalent DC after the bridge rectifier.
There are full assembly schematics of various Migatronic welders, which may be helpful, here:
https://www.migatronic.com/media/1685/50111801_spare-parts-list_automigxe.pdf

 

[Darrell Dieringer]

I would suggest checking for correct flow of AC mains power after the Contactor, via the power level selector switch, up to the primary of the main welding transformer. Then AC out of the secondary, which should vary smoothly from about 15v to maybe 35v AC, as you step through the selector switch positions. Finally confirm that you're seeing the equivalent DC after the bridge rectifier.

Thank you for those troubleshooting steps.

• Confirmed: Full mains voltage through energized contactor. (In the US we have split single-phase; 120V to ground for each leg; 240V between legs)
• Confirmed: Full mains voltage through the selector switch
• Confirmed: 17VAC - 31VAC at the input to the bridge rectifier (steps up with each of the 6 selector positions: 17VAC, 19, 20, 22, 28, 31)
• FAILED: no voltage (AC nor DC) after bridge rectifier

Well, the bridge rectifier appears to be my problem. I presume the green plates (see pic below) are the bridge rectifier. It says "Diodensaule" on it, which at least resembles "diode", which sounds like a necessary part of a bridge rectifier.

The tests, above, were performed with the guts reinstalled. This (unfortunately blurry) photo was taken earlier while I was removing ~40 years of shop grime. I figured I might as well clean it in an effort to make it magically work.

AC Voltage (17V-31V) is present between the large lugs to the far right of the green plates. No voltage is present whatsoever on the other lugs.

Is there a more modern component that replaces the four large green bridge rectifier plates? What would the knowledgable among you do next?

 

[Darrell Dieringer]

I found the cooked component. I like electrical failures to make sense.

What is the best practice for a repair? I suspect that if the component burns its own leg off, it's probably dead.
Source and replace the single fried component? Replace the entire rectifier assembly with something modern?

 

[eddie49]

Great progress with the troubleshooting! And your machine cleaned up so well. It looks better than new. Did you use some kind of cleaning solvent?

The Migatronic assembly is so weird, with a flat bar "spaceframe" holding the components, rather than just mounting them to the casing of the welder as usual.
It was brave of you to dismantle the rectifier. When I tried this, the shouldered ceramic insulators disintegrated. I hope you made careful notes of how it all went together.
At least one of the diodes, which are paralleled in threes on each plate to share the current, has gone short-circuit, but you need to test all of them. A diode will pass current in one direction only. Most digital voltmeters have a Diode Test function. With the leads connected to each diode one way there will be no conduction - infinity reading - the other way should show "400" to "700". A failed diode will be either open-circuit - infinity Ohms - both ways, or short-circuit - zero Ohms- both ways.
The terminal posts of the diodes are spot-welded to thin brass shims, to connect to the next green plate. There is a small hole through the brass tab, as if this is an intentional weak point to act as a "fuse" in case the diode goes short-circuit.

This earlier Forum thread describes a similar failure on a Migatronic:
https://www.mig-welding.co.uk/forum/threads/wire-feed-failure.92868/
Post #22 describes the press-fit 300v 25Amp Siemens E1220 diodes.

This type of diode is often used in car alternators:
https://www.ebay.co.uk/itm/Diode-Po...-With-Bosch-Generator-Alternator/123683282295

I think that diodes labelled in red ( if you have any ) are reverse-polarity compared to the ones marked in black ( i.e. stud-anode vs. stud-cathode ).

Whether to find and fit new diodes or replace the whole rectifier depends on:
- how many have failed
- can you source them, or an equivalent, in USA
- do you want to retain the original appearance inside the machine

For a complete replacement rectifier, search for "mig welder rectifier", such as:
https://www.ebay.com/itm/Clarke-180EN-Mig-Welder-Rectifier-Weld-Parts-WE-6916/143507737578

 

[a111r]

It's a shame to replace old, quality components with cheap ones but that is likely the easiest and cheapest route with bridge rectifiers.
I like to repair, rather than replace. I found I could use my automotive ball joint removal tool to push out press fit diodes, easily.

From what I've read, replacing those that have failed with the same type may work. They have to be balanced out, or more will fail.
Replacing all twelve would be better regarding that issue but then you're easily in the same ball park as a new one, like this for $50.
https://www.ebay.co.uk/itm/1-DQFI20...488462?hash=item3b452a92ce:g:jHAAAOSwuQFejTzQ

 

[Darrell Dieringer]

Great progress with the troubleshooting! And your machine cleaned up so well. It looks better than new. Did you use some kind of cleaning solvent?

Thank you! I used a common household cleaner called "409" - at least it's common in the US. Normally it's used on kitchen surfaces - it's great for general cleaning and mild degreasing.

At least one of the diodes, which are paralleled in threes on each plate to share the current, has gone short-circuit, but you need to test all of them.

Turns out six of the diodes are dead - the one with the obvious failure is short circuit thus no longer a diode. Its five friends (also with the red part numbers) are open ciruit. (The six diodes with the black part numbers - the ones on the front two plates - all tested okay.)

I ordered a replacement mig welder rectifier avaiable from a UK seller on ebay. Their components looked to be much more robust than the direct-from-China counterparts.

It should arrive in about a week or so. I'll let you know when I have the welder working again.

I'm thrilled to be making progress on the repair. I couldn't have done it without "eddi49" and "a11r". Thank you.

 

[a111r]

Good stuff!
Sounds like you've bought one from Technical Arc's Ebay shop, Yorweldsupplies?

 

[eddie49]

Thank you for your kind words, and I'm glad that things are progressing well.
The new rectifier may arrive with documentation, but in case it does not, the two AC inputs will probably be two metal "ears" to the top right, the Negative output will be a U-shaped loop that joins the two outermost plates, and the Positive will be a single tag in the exact centre of the stack of plates. There will be a spike-suppressor ( blue ) soldered between the pos & neg outputs.
The rectifier can only be mounted by the two isolated threaded rods. No other part of the aluminium plates or the connectors can come into contact with the chassis or case of the welder.
Since the machine has a large bank of polarised electrolytic capacitors, which tend to go off like shotgun shells if abused, I would suggest confirming the correct polarity of the rectifier output by initially powering-up without the capacitors in the circuit.

 

[Darrell Dieringer]

The new bridge rectifier (indeed from "yorweldsupplies" on ebay) arrived yesterday. They packaged it with great care - and a lifetime supply of bubble wrap.

There was no documentation, however the leads were labeled (AC, DC Neg, DC Pos). That was pretty straightforward. It is incredibly well built.

The area within the welder chasis for mounting the bridge rectifier required the new unit to be thinner by about 2cm. So I swapped in the thinner leads and some thinner spacer washers and thinner ceramic insulators from the old rectifier. I discovered that the new rectifier uses three different thicknesses of spacer washer to maintain even spacing between each plate. I thought that was excellent attention to detail. I was able to do all the swapping one piece at a time as to avoid complete disassembly.

The thinner leads are also longer, so they have the added benefit of reaching the electrical conductors without having to re-drill mounting holes for the rectifier mounting posts. There was still the matter of getting those mouting posts to line up with the mounting holes in the funky vertical space frame. That wasn't so bad, either. I used another part from the old rectifer - the one that already spanned the width of the original mounting posts/holes. One new hole in that and I was in business.

I did confirm proper output polarity prior to connecting the capacitor bank. I have DC voltage on the output side, AC on the input (of course), and no shorts to the chasis. I haven't welded anything with it yet - simply ran out of time yesterday. I'm really looking forward to it!

Thank you again for all the troubleshooting advice.

 

[a111r]

Good work, another nice old machine saved!

One week for the postage to, that's very impressive. UK domestic postage was taking twice as long, recently.

 

[eddie49]

You've done a great job, especially slimming-down the plate stack. Does the welder have a fan, I cannot see one? If not, perhaps consider adding a fan, although there is not much space for one.

[TecArc are a well-respected UK manufacturer, especially for their traditional Oxford and Portamig ranges, often mentioned on this Forum.]