My Dodgy Mig Welder

[DieselBoy]

I have had this Uni Mig 240 for a quite a few years. It has never gone particularly well. Its always been gutless, and the wire feed has stopped and started. You can watch the wire feed motor slow down and speed up, and stop and start. Its ok on low settings for welding panel steel which I do heaps of, but crank it up to weld some 3mm plate or heavier and its shocking

Scenario: welding with it on setting over 3. Welds ok for first 3 seconds, then starts to burn back up the wire. Ok, so you turn the wire speed up a shade, and it welds ok for a few seconds than all of a sudden it spits the wire out fast. Keep you finger on the trigger, and the feed slows down again. It might speed back up, or some times even stop altogether. Its all over the place.

Its had a new liner in case it was that. (I'm sure it wasn't, as I could see the wire feed motor stopping and starting.

Its had a new PCB in case it was that.

I have stripped and tested the wire feed motor. Its ok.

I have taken it to work and tried it on the work power. Still much the same.

Have brought the work mig home (so I can actually weld stuff ) and its fine on the house power.

Its been to the shop twice, they tell me its all good, we have checked/replaced XYZ as above and its sweet.

Its not. Far from it in fact.

So I need a new repair shop, can any one suggest a place other than Arc Welding Supplies to try in Hamilton or Auckland??

Anyone have any thoughts on what Gremlins or Trolls might be lurking behind the covers causing these symptoms??

It should be an awesome machine. Its so gutting that's its not playing the game!!!

[3VILC]

Maybe a long shot, but microswitch in the torch playing up..or wires to said switch?
Mines playing the same game, but I know what it is, wire liner has deformed 1 single coil small enough to jam the wire..but noone seems to carry a liner with the same fitting..teach me for buying chinese crap haha..good little machine normally tho..might dodgy it up with an offcut from my choke cable sleeve joined on, probably just mean I'll have to dismantle the torch to manually feed the wire past that point, but at the rate I use a spool of wire it wont be much hassle really

[DieselBoy]

Yeah, it's not the switch as it maintains an arc until the wire burns up to the tip.

The powe is there at the tip, just the wire feed rate is all up the shit.

I should video what the feed motor does!!!!

[Crash bandicoot]

The arc is travelling back up to the tip because the wire feed is slowing down whilst the amperage is still trvelling through the wire.

The wire feed drive still works, but all over he place which says it is the potentometer used to set the speed is fooked.

[klompy]

Neville at IC Electrical in Rotorua is a Legend on welders.

[mudlva]

just a though...
when it welds good is the hose nice and straight or nice even curve?

is the wire nice and clean?

is the wire going through the right wire feed groove' ie not feeding 0.8 through the 1.0 groove?

feed spring tension correct pressure?

if the wire is a bit dull or rusty tinged and the hose has quite tight bends the fiction on moving the wire through the inner creates problems with the wire feed

to check, first turn the gas off.

hose perfectly straight pull the trigger. if the wires nice and shinny then it should slide through nice and consistent, if it still plays up, shoot it

if its still playing up then after shooting it take it to a doctor

[De-Ranged]

My gut suspicion is the something to do with the power feed to the drive motor either the dial or the wires
But first off I'd try a new liner and tip, if you have a worn tip the liner (if its a wound wire one) or the hand piece try and make the circuit this causes the wire to weld to this stopping the wire till the feed brakes it loose, you will notice this more with greater voltage settings it wont be a gradual thing its a stuttering pronounced pulsing

A long shot but one I had recently the end cap on the euro fitting that holds the liner in had come undone and pushed back into the welder jamming the wire as it came out of the front feed tube from the drive rollers this made the speed slow down depending on how the torch cable was laid out.... took me a while to find that, it wasn't till it pushed the feed tube back into the rollers hard enough it started to slice a side off the wire that I traced it to this lol

[chrome]

the liner may be arcing out on the torch head causing a dead short, any burn marks on the liner?
even if you re[placed it and the sheathing in the gun is koozed and it will kill the new liner in seconds

[churchill]

Stick a voltmeter on the motor and see what happens, I suspect the voltage should stay constant.

[DieselBoy]

Yeah, just to clarify, liner, tip, feed rollers, wire etc are all in tip top shape. Been through it all to eliminate that as the issues, as like you guys have pointed out, it's the first place you go looking when you have feed issues.

As Crash and De-Ranged mentioned, next point of call was the potentiometer. That's why I replaced the Printed Circut Board, it's got the potentiometer on it for the wire speed.

I have tested the voltage to the wire feed motor under no load, i.e not welding, just pumping wire out of the torch. The voltage increases as you turn the speed up right up to 24v as it should.

I suppose I could have a crack at getting someone to watch the voltage at the feed motor whilst welding when it's playing its games.

I was wondering if one of the little transformers could be playing up when the welder comes under load?????

[Team Tonka]

The wire feed motor circuit takes its feed from the main transformer secondaries. ie the weld current, so a drop in input voltage will slow it down. Usual culprit is dust and dirt in voltage selector switch which burns out the contacts. Worn reduction gears in feed motor is another possibility, although I'm sure your set has the full size feed motor so should be ok. If you do have to replace the switch fit it with a plastic bag pulled over it so the square pulls through the bottom of the bag and then tie wrap the open end around the leads. Easy to bypass the switch to test. Cheers Andy.

[Team Tonka]

And despite being Italian its actually quite a good set and worth sorting.

[oldblue]

I have one of these Welders and if it hasn't been going for a while, I need to wind the Pot switch quickly up and down the range ( slow to fast ) a few times, then it goes OK.

[churchill]

Test the voltage on the motor whilst your welding as this is when the issues occur.

[DieselBoy]

Cheers for tips

It's got a plastic cover over the back of the voltage switch. So if I bypass, it would rule out the switch as being the problem??

If the voltage is dropping at the motor while welding, what will this indicate?? Will test that tonight!!!!

[churchill]

If the voltage is varying i'd be looking upstream of the motor for the fault. If the voltage is constant check the motor and everything downstream of it.

[Team Tonka]

Ok, so lets start at the beginning.
Check the mains cord for damage and that's is big enough. Min 2.5mm2
Start at the plug and check and clean all electrical connections as far as the main contactor. Both live and neutral.
Pull the trigger and check for 240v at the other side of the contactor, then the voltage switch.
Still with ac put the meter across the main transformer outputs. Turn the voltage switch and watch the volts notch up.
Visually check the rectifier block. Look at the diode tops and check non have burnt off leads. Feel the connections and be happy they are clean and not loose.
Put the meter on dc and check the output volts from the rectifier. Should be as noted on rear panel.
There should be 2 smaller leads from the rectifier block to feed the wire feed motor. Make sure that connection is ok
Connect the wire feed motor to a 12v car battery and see if its runs constant. Leave it going for a couple of minutes and see if it gets hot. On 12v it shouldn't.
Connect the volt meter again across the output from the main contactor, pull the trigger, start welding and see if the voltage drops out. If it drops below 200v watch the contactor for excessive sparking and (when turned off) check to see if contactor or voltage switch is getting hot.

That should keep you quiet his evening. And be careful, 240v is enough potential to do you some harm.

[DieselBoy]

Roger that