Poorly mig, can it live again??

[flyingbenny]

Afternoon all

I found this site yesterday when searching for some answers on what to do with my poorly mig, there's a lot of knowledge on here so hopefully someone will be able to help me out.

I was given this little welder a few years ago and it has been brilliant, I've used it restoring a mk1 escort van and other small jobs and it has never let me down. Not until a couple of days ago anyway, I was doing some repairs to a trailer so swapped from 0.6 to 0.8mm wire and had it turned up higher than usual, again it was welding a treat until it just stopped. The fan still runs but nothing else.

This is the welder, I have no idea what make it is and before everyone starts I know its probably only a cheapo thing that is not worth doing anything with but as I said it was free, it's done all that I've needed it to and for bodywork its been great. I cant afford to change it for 'one of these' or 'one of those' so if there is a way of getting it up and running again that would be great and I would be a happy man again.

And this is what the back of the pcb looks like, I'm mechanically minded but not electrically but even I can see that doesn't look right and guessing this is what's causing my problems??

That is on the back of this black part circled:

I know it's a bit heath robinson but could I repair the damaged section by soldering in a link of wire to bridge the gap? Is the black thingy (relay??) likely to be fried? Does anyone fix these? Would a board out of something more common fit??

As I said I'm not very electronically minded but I am practical and if someone could say try this or try that then I would have a go.

Apologies for the miss match picture orientations, they were all ok on the pc

Thanks in advance guys, I hope I can get it running again, although the guy who gave it me did say 'I guess you'll want my Kemppi now then?' so perhaps I'm better off leaving it broken!!

Cheers
Ben

 

[rtbcomp]

I've managed to screw the pictures up whilst trying to orientate them properly - I'll get them back ASAP

 

[cliff366]

I've managed to screw the pictures up whilst trying to orientate them properly - I'll get them back ASAP

Go stand in the corner you naughty mod

 

[skotl]

While we wait for the pictures () you won't be able to fix it by just bridging a blown piece of track; that track will have blown because one or more components has failed.
A relay is always a good candidate, and relatively inexpensive (£5 to £10 from Maplins, cheaper from ebay) to source.

Looking forward to seeing the pics!

 

[flyingbenny]

Thanks rtbcomp for trying to fix my pictures, what is the best way to insert them? I just uploaded the pics, right click, 'Copy Link Address' and paste it into the insert image url box?

skotl hopefully there will now be pics

 

[RonA]

While we wait for the pictures () you won't be able to fix it by just bridging a blown piece of track; that track will have blown because one or more components has failed.
A relay is always a good candidate, and relatively inexpensive (£5 to £10 from Maplins, cheaper from ebay) to source.

Looking forward to seeing the pics!

I concur with your diagnosis, the blackening below the board seems to line up with the black box (relay?) on the top of the board. But as you quite rightly point out there may be other components that have failed.
RonA

 

[rtbcomp]

...what is the best way to insert them? I just uploaded the pics, right click, 'Copy Link Address' and paste it into the insert image url box?

skotl hopefully there will now be pics

The easiest way is to copy drag and paste drop them from your hard disk.

 

[skotl]

Now that we've got the pics (cheers,RTB ) I would definitely start with that relay.

Do you have a soldering iron and desolder pump? You'll need the pump as you'll end up doing more damage trying to remove the components without one.

 

[AdrianH]

It could be possible that the rectifier diodes have gone and put a short across the transformer, so it draws a lot of current and burns out track when energized by the relay.

Very hard to tell with the pics but where does the thick brown wire and while covered cables go to/from in the bottom of the 5th pic?

Adrian

 

[the snooper]

It could be possible that the rectifier diodes have gone and put a short across the transformer, so it draws a lot of current ad burns out track when energized by the transformer.

Very hard to tell with the pics but where does the thick brown wire and while covered cables go to/from in the bottom of the 5th pic?

Adrian

I would agree, if the track that has burnt is connected to the switch in the relay then something else has gone

 

[flyingbenny]

Thanks gents

Adrian - I've just checked, the brown wire goes the the middle of the main on/off switch and the two white wires go to the trigger switch in the torch.

Keep the info coming guys, any pointers on what to check and how to check it would be greatly appreciated.

Cheers
Ben

 

[the snooper]

check the diode is ok where the red arrow points and not short circuit

 

[the snooper]

these are the three wires that you need to be concerned with the white will probably go to the transformer, or power selector, thick brown is live into the relay where does the thinner brown wire go to

 

[AdrianH]

Thanks gents

Adrian - I've just checked, the brown wire goes the the middle of the main on/off switch and the two white wires go to the trigger switch in the torch.
Ben

Nope

You got the brown wire, but the white covered wire you got the wrong ones. I mean this one




What the relay I believe is doing is switching the mains to the main welding transformer, and I think there is a short on either the primary side, or more likely the secondary side.

But with out a diagram it is difficult to know where things go. the bit of track that has burnt out is the smallest section of that track, so has acted like a fuse and blown.

I would suggest the white wire will go off to the selector switch first and select windings on the primary side, so always worth checking the switch is not breaking up.

Looking at it I would suggest it would be very similar to the SIP type welders, but with a control board of it's own.

I will post again in a few mins.

Adrian

 

[AdrianH]

check the diode is ok where the red arrow points and not short circuit

If the diode was short and causing an issue then the thin track next to your arrow would have vaporised.

• 

It is going to be similar input to this for the sip, where 'T' is the output from the relay to the switching and then the main transformer.

Adrian

 

[the snooper]

If the diode was short and causing an issue then the thin track next to your arrow would have vaporised.

• 

It is going to be similar input to this for the sip, where 'T' is the output from the relay to the switching and then the main transformer.

Adrian

yeah should have realised that, if it is his welding rectifier that's has gone I would have expected quite a lot of really bad buzzing before that track went pop

 

[the snooper]

its hard to tell from this picture but is this wire damaged or just the picture ?

 

[flyingbenny]

Wow for you guys to come up with stuff like that from my dodgy pictures is ace, I wish I understood what was going on in there. It helps if i think of it as hydraulics with valves, pipes and restrictors rather than switches, wires and resitstors, it makes more sense then . I will have another look tomorrow and answer the questions on what goes where, bare with me as there will be more dumb questions from my end, when you say test this or that it doesn't necessarily mean I know how to do it.

Right, money wise I know its probably not worth spending much time on especially as I found out tonight from the guy who gave it me that its 20 odd years old but I think that if you guys could help a complete numpty novice to understand a bit more of how it works and get it up and running just by pics etc that would be amazing.

Thinking back to when it died, the last couple of runs just before it gave up totally it was welding really badly like the wire speed was turned up too much or welding dirty metal, it just wasn't smooth like it had been all day.

Cheers
Ben

 

[Wedg1e]

I wish I understood what was going on in there.
Ben

• 

MIG welders at this level are pretty simple things; referring to the diagram above which won't be a kick in the pants of the arrangement of yours you have the Mains coming in at left, through a double-pole switch and then one side of that switch (we'll call that Neutral ) is connected permanently to the main transformer (sat at the bottom of the chassis). A Neutral feed is also taken to the control PCB to allow the electronics to function (bit like having only one side of the battery connected on your car, it won't do much).
The Live pole is also fed to the PCB and is rectified into DC and dropped to a voltage that the control circuit can cope with.
The Live is also fed to the on-board Relay which doesn't do anything until you press the torch trigger, when it switches over to supply Live to the main transformer - VIA the welding output selector switch(es). In the diagram shown the Neutral is also switched to provide Hi/Low with the Live switching between Max/Med/Min, but the principle's the same; what's happening is that different 'taps' of the transformer's primary winding are being fed with 240V. The other side, or secondary winding of the the transformer has a fixed number of turns (of copper wire) in relation to the primary so depending on how many turns of the primary are fed 240V determines what voltage appears across the output of the transformer. At High/Max setting all the 'turns ratio' is at its highest so the secondary generates its maximum voltage.
The secondary voltage is now passed through a pair of diodes configured as a 'half-wave' rectifier; this produces a DC voltage albeit not a perfectly smooth one (for which you'd want a full-wave rectifier) and this voltage is taken up to the torch tip. Note that's it's a low-ish voltage, probably not more than about 30V on these sets so can't hurt you; the main point is that owing to the way transformers work, dropping the voltage massively increases the amount of current that can be generated. So although you're welding in Amps it's the Volts being driven through the arc that makes that much current flow.
At the same time, the rectified voltage is taken back to the control PCB and supplies the wire speed control part of the circuit (the big silver transistor with heatsink fins around it is probably doing all the work here); as you vary the wire speed control that transistor is switched progressively harder 'on' (think of a light dimmer switch) to allow more current to drive the wire feed motor. A slight benefit of tapping the weld voltage this way is that as you go up the output range, the wire speed 'automatically' increases due to the higher voltage reaching the motor. It's beneficial because you normally want more wire feeding in to go with the higher current!
There's a thermal trip slipped into the transformer windings that goes open-circuit if the transformer gets too hot; in the diagram above it breaks the supply to the torch trigger so the set stops working until the transformer cools down and the trip resets itself.
Finally there's a 'choke' - essentially just a coil of heavy-gauge wire wound on a suitable former that modifies the behaviour of the current flow to enhance the weld... some MIGs also have a bank of smoothing capacitors here to further improve the arc.
As I said, simple stuff... now if only I'd got to grips with hydraulics

Incidentally there should be enough makers' marks and numbers on that relay to help find a replacement if one is needed.

 

[Wallace]

Some info from a previous post on here identifying the machine: Link