Issue with tig welder in AC mode

[muzzarobbo]

Hi all,

I have a UNIMIG AC/DC 200 W174 tig welder and when welding in AC mode only the electrode negative half of the waveform appears to be functioning (no cleaning cleaning action and struggles to maintain an arc).

I pulled the cover off to have a look and could not find any obvious broken wires or burnt out components.

I saw a video where someone tested the mosfets with a multimeter so I decided to give that a crack and did actually find one that did not give a value, while the others next to it (in a row) al gave the same value of ~446. The video seemed to imply that a broken mosfet would show as continuity on the multimeter rather than no result however it was in Indian so I could not follow it all.

Could this single component be my only issue? is there anything else I could easily check without much risk of zapping myself.

TIA!

Murray

[muzzarobbo]

Here are some pics.

[BrokenYugo]

It's hard to say much without schematics or part numbers.

Odd man out is a good clue though, assuming they're all the same part.

You may find this video helpful, just note that a power mosfet probably won't be turned on by a multimeter like the little one shown, big power parts usually have a higher threshold voltage than the meter will provide (usually 3V max). In circuit testing can also be problematic.