Liquid Arc MIG welder repair guidance - Page 1
Forum section: electronics / repair

author: Tricon
author_rank: Newbie
timestamp: December 26, 2024, 08:54:47 pm
content: Hello, I have a faulty Liquid Arc (Lincoln Electric) (MAXMIG 200i) transformer-based MIG welder. It failed suddenly after recent use, with the failure being the inability to strike an arc. There are a few tiny sparks when grounded, but no arc. I measured the voltage at the tip (28VDC) and voltage at the transformer output (22VAC), which seem to be reasonable readings (though I am not sure).From my research, the inability to strike an arc appears to mean a high resistance between the torch (-) and the return (+). I measured the resistor across the capacitors, and it is within specification. The thyristor is an AEG M-12283-5. I cannot find a datasheet to test it properly. Using a multimeter, the device works as when I power the gate (using the diode test function), power passes through it (the test process I used is herehttps://fastron.com.au/blogs/power-semicondcutor-testing/failed-scr-diode-testing-using-a-standard-multimeter).While I don't think it is the cause of the fault (though I will stand corrected), the power to the torch, due to a loose nut, touched the chassis. Additionally, I noticed that the pulse potentiometer is bent. The fuse, I believe, controls the wire feed, and that function works correctly.An additional fault I noticed is that the gas supply solenoid starts and stops quickly (after a ~second). I think this has to do with the fact that an arc is not struck.The board appears to be working, as I saw a pulse of about 10ms with a 1.1Vp-p across the gate(58) and the cathode (52). I cannot read the part number on the transistor that drives the Thyristor / SCR; however, it looks serviceable.What additional troubleshooting tasks can I perform to determine the fault and repair the unit?I searched for a replacement circuit board (or additional information) but found nothing.Thanks for any suggestions.

author: ComfyChair
author_rank: Contributor
timestamp: December 27, 2024, 02:50:38 pm
content: According to the owner's manual (https://www.lincolnelectric.com/assets/ap/operatormanuals/ima578rev0b.pdf), have you verified the following?

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 28, 2024, 05:40:12 am
content: The Maxmig 210i and 210r is a tapped transformer design controlled by a contactor, it doesn't use an SCR.Unfortunately I don't have much in the way of Liquidarc manuals in my repository, and not the 200i...Can you post a picture of the waveform to the SCR?What voltage is being supplied to the SCR?What voltage is coming from the SCR when the trigger is pulled?

author: Tricon
author_rank: Newbie
timestamp: December 28, 2024, 09:00:57 am
content: Thank you. The manual linked by ComfyChair is for a different machine. I have a 200i and not a 210i or 210R. However, I took some of the tips in that document and can confirm that The main transformer is fine as I measured resistance, around 0.5 ohms on the coils. The rectifier is OK (well, as far as I can tell) as DC is supplied to the output. The choke is OK, and I measured it, too, with a similar reading to the transformer of 0.5 ohms.The rotary switch is OK, as I can see varying resistance when I rotate it.I have attached an image of the trace to the gate when the trigger is pulled.I did not measure the direct output of the SCR when the trigger was pulled; however, at the torch tip to the return, it was 28VDC.I'll take some other measurements and advise.I also see that the schematic is missing from my first post. I have attached it.Thank you.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 28, 2024, 11:16:35 am
content: Check the rectifier using the diode test function on your multimeter, it's just a beefy bridge rectifier, so can be tested as such.The inductor is unlikely to be bad, besides maybe burnt connections, so check the actual mechanical joints for signs of corrosion or burning.The capacitors are generally obviously popped when they fail due to the high currents, but you can just disconnect them for testing if you want, they are only there to smooth the output voltage for smoother welds.If there are any burnt connections, I used to use electrical jointing compound when remaking connections (Nickel anti-seize grease can work too, don't use copper anti-seize on aluminium!). I'd file/sand the surface flat and clean and immediately smear with the jointing compound and make the joint nice and tight and I never had a joint burn a second time. This is especially important where aluminium conductors are present. Check ALL connections in the welding current path, as well as all other electrical connections around the place.Also, check the main switch for voltage drop across the closed terminals for each power setting (set your multimeter to volts and test across the closed terminals, so you are looking for any voltage drop across the 'shorted' terminals), you should have a few 10's of millivolts max when pulling full current during welding as well as idle. If you see a couple volts or more, you have a faulty switch.Do this check as described and let us know the results, a normal ohms test can show a good connection under no-load but the contacts can fail when pulling current.Itseemsthe signal to the SCR is what would be expected. The ripple is at 100Hz, which is expected from a bridge rectifier running at 50Hz and the 11V seems about the right level for a good turn on voltage.It looks like wires 54 and 55 are the AC power to the control board, this would be rectified for the control circuitry on the board itself. This appears to be working ok as the circuit does function somewhat.Wire 53 is probably what is detecting the presence of a struck arc so maybe trace that around the PCB and check for any busted components.