Sealey mightymig 150 transformer check for power

[John Payne]

Hi , I have just put a new liner into my mightymig 150. On feeding the welding wire through using the gun trigger the welder stopped. On taking off the main cover I could see the welding wire had jammed in the liner and had found its way into and around the pcb causing a short.
I was not sure as to how much damage this may of done and looked to the internet for some answers.

I wanted to check if the transformer was still working. One feed said to use a multimeter across the rectifier where the 2 wires from the transformer joins to it and got no voltage, the third wire seems to go to a little box on the pcb . If I leave 1 multimeter lead on the rectifier and put the other to the case earth I do get a voltage.

How do I check if any other damage has been done. Many thanks John

 

[eddie49]

Hello John, and welcome to the Forum! Sorry that you have had this bad luck, but I'm sure we can help you to fix it, and then you can participate in all the great things that go on in this space.
Which parts of the machine work now, and which don't? When initially switched on, is there a power-on light, and/or a fan running?
When you press the torch trigger switch, do you hear a click inside the machine, does the wire feed, can you strike an arc with the wire at the torch tip, does the gas solenoid ( if fitted ) open?

 

[John Payne]

Hello John, and welcome to the Forum! Sorry that you have had this bad luck, but I'm sure we can help you to fix it, and then you can participate in all the great things that go on in this space.
Which parts of the machine work now, and which don't? When initially switched on, is there a power-on light, and/or a fan running?
When you press the torch trigger switch, do you hear a click inside the machine, does the wire feed, can you strike an arc with the wire at the torch tip, does the gas solenoid ( if fitted ) open?

Hi Eddie49 and thanks for the quick reply.
I have a green power light come on and the cooling fan starts to spin.
The welder will not arc up.
No click from inside the machine when the torch trigger is pressed.
I have no wire feed if connected to the pcb. If I connect the wire feed motor to a 12volt external battery it will work.
No gas solenoid.
Many thanks John

 

[eddie49]

Generally machines of this size and type have a small transformer on the PCB which provides power for the coil of a relay. This relay, which is also on the PCB and may be in a transparent plastic case, switches AC mains through to the main welding transformer. The low-voltage high-current power from the secondary of that transformer is rectified. Depending on the setting of the power range switches, the DC output from about 14v to 26v provides the welding power, and it also runs the wirefeed motor via the speed controller on the PCB.

I would start by checking that power is available to drive the relay. The small transformer on the PCB gets Live and Neutral from just after the main power-on switch. The secondary voltage is about 24v AC, and that goes to a bridge rectifier. This may be a black plastic-encapsulated circular or rectangular component, marked with two squiggles for the 24v AC input, and + and - for the DC output. This voltage is switched by the torch trigger switch to the relay coil. The rest of the components on the PCB are the motor speed controller, but that section will not be operational until the relay circuitry has powered-up the welding transformer.

If you are confident working on live mains-powered equipment it should be possible, using a multimeter, to confirm that the relay supply and control is working. The primary of the small transformer is live AC mains, and also the contacts on the relay switch AC mains through to the transformer. I am not sure what damage the welding wire did to the PCB, but the voltages and components mentioned above would be the starting point. If you would rather not work "live", some components can be checked offline for continuity and resistance, or the PCB could be replaced or sent for repair.

 

[John Payne]

Generally machines of this size and type have a small transformer on the PCB which provides power for the coil of a relay. This relay, which is also on the PCB and may be in a transparent plastic case, switches AC mains through to the main welding transformer. The low-voltage high-current power from the secondary of that transformer is rectified. Depending on the setting of the power range switches, the DC output from about 14v to 26v provides the welding power, and it also runs the wirefeed motor via the speed controller on the PCB.

I would start by checking that power is available to drive the relay. The small transformer on the PCB gets Live and Neutral from just after the main power-on switch. The secondary voltage is about 24v AC, and that goes to a bridge rectifier. This may be a black plastic-encapsulated circular or rectangular component, marked with two squiggles for the 24v AC input, and + and - for the DC output. This voltage is switched by the torch trigger switch to the relay coil. The rest of the components on the PCB are the motor speed controller, but that section will not be operational until the relay circuitry has powered-up the welding transformer.

If you are confident working on live mains-powered equipment it should be possible, using a multimeter, to confirm that the relay supply and control is working. The primary of the small transformer is live AC mains, and also the contacts on the relay switch AC mains through to the transformer. I am not sure what damage the welding wire did to the PCB, but the voltages and components mentioned above would be the starting point. If you would rather not work "live", some components can be checked offline for continuity and resistance, or the PCB could be replaced or sent for repair.

Hi Eddie, have checked out the voltages .I am not getting any output from the relay. Just phoned sealey and a new board is on its way. A total cost of 51 pound. I will let you know how things get on after I have fitted the new board. Many thanks John

 

[John Payne]

Hi Eddie, have checked out the voltages .I am not getting any output from the relay. Just phoned sealey and a new board is on its way. A total cost of 51 pound. I will let you know how things get on after I have fitted the new board. Many thanks John

Hi Eddie quick update. The new pcb came this morning. All fitted and working. Many thanks John

 

[eddie49]

John, thank you for the update. I'm glad that the problem has been solved and the machine brought back to life.

 

[Jnigel71]

Hi my welder has only a slight difference in power from min to max, been told to swap 13amp and plug with a 16 amp plug but I’ve read that the sets run great on a 13amp so do you have.an idea of the problem I have , max won’t even weld 2mm
Thanks

 

[eddie49]

Hello and welcome to the Forum !

There can be several reasons why your welder has lost power, but lack of a 16A plug is not one of them!
As a first guess, I would suggest corrosion or a loose connection at one of the joints in the low-voltage, high-current welding circuit.
What is the DC voltage between the tip of the MIG torch and the workpiece clamp at each step of the power controls? It should vary between about 15v and 30v.

 

[Jnigel71]

Thanks for your reply I will test it the weekend and let you know

 

[Jnigel71]

Hi these are the results from testing
Min 11.6
Max 17.2

 

[Jnigel71]

Hi these are the results from testing
Min 11.6
Max 17.2

Are you able to reply

 

[Jack Ryan]

Are you able to reply

What is the voltage at each step? The maximum seems a bit low.

( I assume there are more than two steps)

Jack

 

[eddie49]

Thank you for the results, and sorry for the delay in replying. The motherboard on my Research Machines PC seems to have failed, so I had to dig out another computer.
The voltages 11.6 to 17.2 are very low, and explain why it will not even weld 2mm.
This recent Sealey Mightymig 150 thread:

Wire speed

Lads. How can i speed up the wire feed on a seeley 150. It has a 1 k pot on it But even screwed right to max i havent enough wire. It just to slow throuhout its range

www.mig-welding.co.uk

states that the four voltages should be 20.5, 23.9, 28.3, and 34.8 volts. That is about double what you have, and suggest that possibly one of the four rectifier diodes has gone open-circuit. The diodes can be tested with the "Diode Test" range of a digital meter, but one of the bolted connections to the transformer secondary would have to be opened first. Please let me know if you need advice about this test, and maybe post an internal photo of the area around the rectifier in your welder.
Apart from the diodes, it is still worth looking for loose, corroded, or burnt connections, and this includes the push-on tags at the back of the "MIN/MAX" and "1/2" switches.

 

[Jnigel71]

What is the voltage at each step? The maximum seems a bit low.

( I assume there are more than two steps)

Jack

Yes there is another switch 1 and 2 sorry I will get the readings from them , all works fine just max power is very low

 

[Jack Ryan]

Yes there is another switch 1 and 2 sorry I will get the readings from them , all works fine just max power is very low

You probably don't need to worry about that now, Eddie has confirmed that the voltage is too low and made some further suggestions. I's follow those up.

Jack

 

[Jnigel71]

Thank you for the results, and sorry for the delay in replying. The motherboard on my Research Machines PC seems to have failed, so I had to dig out another computer.
The voltages 11.6 to 17.2 are very low, and explain why it will not even weld 2mm.
This recent Sealey Mightymig 150 thread:

Wire speed

Lads. How can i speed up the wire feed on a seeley 150. It has a 1 k pot on it But even screwed right to max i havent enough wire. It just to slow throuhout its range

www.mig-welding.co.uk

states that the four voltages should be 20.5, 23.9, 28.3, and 34.8 volts. That is about double what you have, and suggest that possibly one of the four rectifier diodes has gone open-circuit. The diodes can be tested with the "Diode Test" range of a digital meter, but one of the bolted connections to the transformer secondary would have to be opened first. Please let me know if you need advice about this test, and maybe post an internal photo of the area around the rectifier in your welder.
Apart from the diodes, it is still worth looking for loose, corroded, or burnt connections, and this includes the push-on tags at the back of the "MIN/MAX" and "1/2" switches.

Thanks for your reply n yes I would need advice and I will send a photo

 

[eddie49]

There are some internal pictures showing the area around the rectifier in Post #22 of this thread:

Wire speed

Thanks. Millers not in 4 t. Prob a fault in board ,I’ve got used to triggering it again after stopping, so I’ll carry on regardless, them 300 a contractors are dear just to try it, ! I’d buy one if I knew for sure, I’ll send photos of other later, but will try a separate 24v supply...

www.mig-welding.co.uk

and there is a diode test procedure written by @Seadog in Post #31 here:

Testing diodes

Yesterday I was welding up some large axel stands for use in a 4 post lift Welded up the bases on full power (7-180amps), welded fine then all of a sudden it's as if the power settings had been turned down but it obviously wasn't It's done this when I first bought it but I thought it might...

www.mig-welding.co.uk

A bridge rectifier has two AC inputs ( "~" ), 4 diodes, and two DC outputs ( "+" and "-" ).
On this machine, the two finned aluminium cooling plates contain 4 diodes each. Here, the four diodes needed to make a bridge rectifier are actually a pair of diodes in parallel, to share the high current load.
The DC output side of the rectifier connects via the inductor ( choke ) to the MIG torch and the work return cable & clamp ( the so-called "Earth" ).
The AC input side is fed by two thick solid wires ( aluminium ), with bolted connections, from the secondary of the main transformer.
To check the diodes, you will first have to open one of these two connections, otherwise the diodes will be shorted by the very low resistance of the transformer winding.

Each of the diodes conducts in one direction, and blocks the flow of current ( i.e. is open-circuit ) when the meter leads are reversed. A digital meter set to the Diode Test range indicates conduction by a reading between "400" and "700" ( or .4 to .7 ), and open-circuit by "OL" or "1.". You are looking for suspected open-circuit diodes, which would show "OL" in both directions, with no conduction.

In addition to testing the rectifier diodes ( or perhaps before testing them! ), the AC output of the transformer secondary should be measured, at each of the four power selector switch settings. To do this, set your meter to the 200v AC range and connect one test lead to the unbolted/disconnected thick wire from the transformer. Insulate that wire and position it so that it cannot touch anything. Connect the other meter test lead to the other thick wire from the transformer secondary, which is still bolted to the input of the rectifier. Plug in and switch on the welder, press the torch trigger and note the AC voltage at each of the four settings. They should be roughly the same as the original expected DC voltages, i.e. 20 to 35 volts. If the AC is low, the fault is not in the rectifier, it is the transformer itself ( unlikely ), or the primary supply or switches.