Wolf MIG 140X - Lost 99% of welding power

[SplatterMaster]

See attached table for voltage readings

With a rectifier input cable disconnected the fuse doesn't blow

The bulkhead fittingd are really tight and have no corrosion present

 

[Hutcho]

When you tested the diodes did you do both plates of the rectifier as you only gave four readings

 

[eddie49]

Thank you for the voltage test results. With from 20 to 41 volts, the AC outputs from the transformer secondary look OK, if a little high. However, the DC outputs from the rectifier are way down - 7v to 12v ! At first sight, this means that the rectifier has failed, and this happened by coincidence when you were fixing the gas valve in the torch.....

Maybe you could unbolt both the positive and negative output wires directly at the rectifier, and check if the voltages are still so low there?

 

[SplatterMaster]

When you tested the diodes did you do both plates of the rectifier as you only gave four readings

The diodes are press fit into the plates, in 4 parrellel pairs and soldered together where they join the 2 inputs from the secondary coil. I didn't want to destroy anything in testing so tested the 4 paths through the diodes leaving them paired up.

The 485 forward, 1(inf) backward values are the 2 routes to the negative plate

The 486 forward, 1(inf) backward values are the 2 routes to the positive plate.

 

[eddie49]

Maybe you could unbolt both the positive and negative output wires directly at the rectifier, and check if the voltages are still so low there?

There are probably thin red & black wires with push-on tags taking the rectifier output to the wirefeed motor via the speed controller. To fully isolate the rectifier output, one of these wires should also be disconnected.

This is puzzling, because your test of the rectifier diodes shows they are all OK .....

 

[SplatterMaster]

Thank you for the voltage test results. With from 20 to 41 volts, the AC outputs from the transformer secondary look OK, if a little high. However, the DC outputs from the rectifier are way down - 7v to 12v ! At first sight, this means that the rectifier has failed, and this happened by coincidence when you were fixing the gas valve in the torch.....

Maybe you could unbolt both the positive and negative output wires directly at the rectifier, and check if the voltages are still so low there?

Will unbolt the rectifier output wires and test again.

I was wondering if the diodes could pass muster with the test power from the multimeter but maybe short open under the power from the welder in operation? Is that a thing?!...


I was also wondering if the filter could be at fault.


There's a small transformer on the control board with output to the motor positive (red) wire, and a white wire to the motor negative, this also T's off and goes to the rectifier... I have no idea what that game is all about?!

 

[a111r]

Will unbolt the rectifier output wires and test again.

I was wondering if the diodes could pass muster with the test power from the multimeter but maybe short open under the power from the welder in operation? Is that a thing?!...


I was also wondering if the filter could be at fault.

Yes, that is a 'thing', as the meter's inputting only 9 volts. I have had that diode failure mode before, once or twice.

Keep at it... it's only a '2 pager', thus far....

 

[SplatterMaster]

I'm tempted to bypass the filter for a welding test,

 

[SplatterMaster]

Yes, that is a 'thing', as the meter's inputting only 9 volts. I have had that diode failure mode before, once or twice.

Keep at it... it's only a '2 pager', thus far....

Haha I no stranger to seeing long forum fault finding missions to the end :-)

 

[a111r]

You're doing well.
So the next step could be repairing a cheap, 'semi- decent machine'?

 

[SplatterMaster]

You're doing well.
So the next step could be repairing a cheap, 'semi- decent machine'?

Haha, I thought someone might say that!

To be fair to the little 'Wolf' it has done all I've needed it too until this small discretion.

Many years ago while still a student I worked at a race car preparation business, they had lovely Lincoln welders Tig & Mig with which I did 90% of my previous welding experience.

Then with a decade break from fabrication and the purchase of this little hobby welder I wasn't expecting much in the way of quality of weld but it's honestly been ok.

You always need a very straight torch cable/hose or the wire feed rate is a bit random, and the duty cycle is quite low before it overheats. But otherwise for light guage fabrication down the shed it's ok.

 

[SplatterMaster]

Crude filter bypass:

No difference

 

[SplatterMaster]

If nothing else my label maker is getting a workout.

Removed the rectifier completely for test, similar values to before.

 

[SplatterMaster]

The blue Varistor doesn't look damaged / scorched or anything but could that be at fault?

Does anyone know what it's purpose is here?

Other than buy a new one and solder it it to test I dunno how I'd test a varistor practically at home!


Or do I need to gamble.on a new rectifier / hot rodding this one with new/upgraded diodes?

 

[eddie49]

There's a small transformer on the control board with output to the motor positive (red) wire, and a white wire to the motor negative, this also T's off and goes to the rectifier... I have no idea what that game is all about?!

The small PCB-mounted encapsulated transformer gets AC power when the welder is first switched on. The output, usually 12v or 24v, is rectified and fed to the coil of the on-board relay via the torch trigger switch. The contacts of that relay send AC mains to the main transformer. It is basically an auxiliary power supply, rated at 1 or 2 Watts, responsible for powering-up the main welding output. That is all it does.

I'm tempted to bypass the filter for a welding test

The inductor/choke/filter is just a coil of thick solid wire on a laminated core. They almost never fail, and could not cause the faults seen.

 

[SplatterMaster]

Thanks Eddie that makes sense now

 

[eddie49]

The blue Varistor doesn't look damaged / scorched or anything but could that be at fault?
Does anyone know what it's purpose is here?

The varistor ( or Voltage Dependent Resistor VDR or Metal Oxide Varistor MOV ) is used to suppress high-voltage spikes, caused by the inductance of the transformer windings, and prevent them from damaging the diodes. It offers a high resistance to normal ( low ) voltages, but the resistance drops when it sees a high-voltage spike ( e.g. 100 - 200v ) and it shorts the spike.
If yours was faulty, due to the high available current it would be visibly damaged - burnt or split. It looks fines, as does the rest of the rectifier.

 

[eddie49]

The DC voltages measured at the rectifier output and listed in post #28 - from 11.6 to 22.9 volts with the wirefeed motor disconnected, and from 10.3 to 18.6 volts when it was re-connected - are much higher than the torch tip voltages shown in post #21 - from 7v to 12v. The former are still low, but enough to weld with.
Did something change, or is voltage being dropped on the path to the torch?

Digital meters will register voltages whilst drawing very low currents - picoamps. Measuring the AC secondary and DC rectifier voltages across a load resistor ( such as 3 x 12v 60W car headlamp bulbs in series ) may help to clarify what is happening. The load current will only be 5 Amps, but that may be enough to show if voltage is being lost across a high resistance somewhere.

 

[SplatterMaster]

The DC voltages measured at the rectifier output and listed in post #28 - from 11.6 to 22.9 volts with the wirefeed motor disconnected, and from 10.3 to 18.6 volts when it was re-connected - are much higher than the torch tip voltages shown in post #21 - from 7v to 12v. The former are still low, but enough to weld with.
Did something change, or is voltage being dropped on the path to the torch?

Digital meters will register voltages whilst drawing very low currents - picoamps. Measuring the AC secondary and DC rectifier voltages across a load resistor ( such as 3 x 12v 60W car headlamp bulbs in series ) may help to clarify what is happening. The load current will only be 5 Amps, but that may be enough to show if voltage is being lost across a high resistance somewhere.

I can't see how the voltage can drop on the way to the torch:
>Torch/wore tip to rectifier +ve is only 0.2ohm.
>EarthClamp to rectifier -ve (through filter) is only 0.3ohm.

I will see what I can scrounge to make a controlled test load of some kind :-)


I have a friend with the same welder, realistically is there much risk to his components if I borrowed his machine to swap parts across to find the fault a less intelligent way? I was thinking of starting with the rectifier...

 

[eddie49]

I can't see how the voltage can drop on the way to the torch:
>Torch/wore tip to rectifier +ve is only 0.2ohm.
>EarthClamp to rectifier -ve (through filter) is only 0.3ohm.

With very thick cables and solid bolted connections, the high-current secondary circuit should be showing resistances of milliOhms. From Ohm's Law, V=IR, so if the welding current is 30 Amps, a 0.2 Ohm resistance will drop 6 volts, and 0.3 Ohms will cause 9 volts to be lost.

I have a friend with the same welder, realistically is there much risk to his components if I borrowed his machine to swap parts across

When I worked as a computer field engineer, I found that the most likely result of swapping components between a bad and a good machine was that you finished up with two bad machines.