Kemppi Minarc Evo 150 Only Welding at Full Current Regardless of Potentiometer Setting

Hi there. New member here. I have a Kemppi Minarc Evo 150 with a small problem. I’ve seen some different problems with those but not quite like this.

It does start and weld, the potentiometer for output current is showing on display and can be changed
- but it only welds on full current (140a) no matter what I set it to. Any advice where to start looking? What is controlling current output?

Regards ftar

To start with check the potentiometer in the evening I'll see if I have the schematic I'll send you. Waldemar

Added after 11 [minutes]:


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Thanks a bunch. I think pot is fine.. I have a handheld potentiometer too and it does the same. Looks like my problem is IGBT module VS-150MT060WDF.. which can’t be bought afaik.

Firstly, this diagram is not from this machine. Secondly, if the module was faulty, the welder would not weld at all. The current control circuit is faulty.

If you have the schematic diagram at your disposal, please let a colleague know that the fault is on the control system's side. As I wrote, I have hard copies of the documentation in my quarters. I work in Norway and have limited access to a scanner.

Added after 54 [minutes]:

Hello I am sending a document

I can't help you right now because I'm at work, we can talk tomorrow afternoon PS unplug all the plugs from their sockets and reinsert them, check the connections of the potentiometer (encoder) for cold solder joints or gaps on the PCB greetings

For the Minarc EVO 150, the schematic was not published.

>>21694512 thanks. I did check the modules and they seem ok. Current control circuit you say. Would be nice with an electric diagram or such to source the current control circuit

Added after 4 [minutes]:

>>21694587 done it 3 times.. and tested. Still full current when set on 13a. Can't really find anything wrong with solderings so far. Imma get the IGBT modules back and get on the current control circuit and check and see if I can find anything.

Take a photo of the motherboard, the model I sent you shouldn't differ too much, I spoke to a Norwegian guy who says he also has something, from what I found out it only differs in the display and that there is an encoder instead of a potentiometer.

It differs, Waldemar, and in a fundamental way!
EVO 150 has PFC 150 does not. EVO dual forward power amplifier on 50A module, 150 - six transistors in two groups.

The reference for output current measurement is two or three parallel connected 0.05Ω constantan resistors! Next to the output rectifier on five diodes in TO247 housings. They go to the corner of the board (near the ribbon cable to the galvanic separation circuits and the amplifier. The rest of the circuit is on the control panel.

Thanks for the information, we'll see something tomorrow mate Jon promised to bring again thank you for your attention greetings I'll write something tomorrow

Buddy waldek63, as I wrote that this is not the diagram, do not undermine the opinion of people who, like me, have repaired hundreds of such machines and ordinary and evo. And in addition I did not write about the control , but about the current control . There was a problem with cracking resistors in these machines , so that you never knew which one. And the schematic of the board , as has already been said here the manufacturer has never made available.
I am attaching the service manual because I don't know if it is in the base, and I don't want to look for it. There is a schematic where the current control comes to the panel and by the paths look for components that may be faulty.

Did you get to the bottom of this problem? Mine is doing the same thing.
Thanks.

I did not.. ended up parting it out.

>>21848753 Try to go 'thread to thread', Read start with the constantan resistors on the board near the output rectifier, along the paths reach the amplifier/signal converter and onto the flat cable and control panel. Do a waveform and then voltage measurement with an ocyloscope and see if it goes to the module. I suspect that one of the SMD resistors has cracked which is blocking proper matching of the signal flowing current. And use a 0.1-0.3 ohm 4-5 kW resistor for the measurements.

Thank you for getting back to me. I have bought a new machine, but being new to testing PCBs I wanted to try give this a go after having success with a gym stair master which ended up being a bad MOSFET, making the machine hold the brake on max power always, similar to this thing being stuck on max amps always? If it was a cracked resistor, would I be able to visually find that? Is it worth taking the heatsink off and testing the MOSFETs, or am I barking up the wrong tree? This is a bit more complicated than my first play on a PCB, and I've only got a multimeter for testing
Any suggestions welcome, but will need to be dumbed down for me, sorry
Thanks

Added after 6 [minutes]:

Also just to let you know, I sent this machine back to a dealer who repairs them and I can clearly see they have cleaned all the test points to test the board but they said I needed a new board and with that cost plus labor I may as well get a new machine