Welder for repair

[ThereWhere aboutsof thesmok]

Hi folks, been given a welder for repair, and I found a single IGBT had failed in the powersupply. The current being pulled had raised the resistance of the PTC's and so all was good.
Now with each end of the primary consisting of 3 IGBT's in parallel, I thought I would fire it up with the two remaining IGBT's.

Now the next one has failed!

Is it possible there might be a short in the coil or is it worth trying a fresh set to see if all is good?

Also the IGBT's seem to be old, G4PC50W and I figured a newer replacement might be as good if not better?

Thanks for any hints..

Oh service manual is here....

https://www.manualslib.com/manual/1986588/Kemppi-Minarc-150.html?page=4#manual

[ThereWhere aboutsof thesmok]

hmm, tried with two fets and now I am getting about 390 on the bridge, but this slowly lowers as the varistors heat up, so clearly there is another short somewhere! The transformer resistance is very low but not knowing what it should be it's difficult to tell. Could do with a Q tester I guess?

[xavier60]

Ignoring why the IGBTs failed for now, 390V is enough for the control supply to become active and pull in a relay that is supposed to bypass the thermistor. This is a precharge function that just about all inverter welders have.
Have all power diodes been checked for shorts?

[MikeK]

Friendly note: Maybe reconsider your username choice?  It's quite long and overlaps some of the post header info.  It's also misspelled.

[ThereWhere aboutsof thesmok]

Yeah I have checked two primary and all the secondaries and they seem ok, reading 0.35V or so forward bias.

There did seem to be nothing on the output.

I have removed the transformer for now, I once built a ring tester and wander if I still have it somewhere....

[pcprogrammer]

Friendly note: Maybe reconsider your username choice?  It's quite long and overlaps some of the post header info.  It's also misspelled.

I second that, but this is something you can't do yourself. You can contact a moderator to do this for you. Simplest way is to use the "report to moderator" button on the right of every post and write that you need your display name changed.

[ThereWhere aboutsof thesmok]

Thanks, sent a message
I was rushed lol

[ThereWhere aboutsof thesmok]

ok so found my ring tester, after a quick repair (it is 20 years old lol!) I tested the primary and secondary and they show green, secondary 'auxilary' shows red, which seems a bit suss to me... Looking at the diagram, I am wondering if I could inject 24v or so into the aux supply and leave the winding disconnected... The feedback does seem to come from the main secondary...

 

[ThereWhere aboutsof thesmok]

After being reminded on how to use my ring tester    I did so again. Shorting the aux coil reduced the all greens to reds so I think it's safe to say that it is ok. Just gotta stump £50 for a load of new IGBT's though now :-(