I have a Longevity welder that was damaged by dropping, when I went to use it and hooked it up to 220 it ran ok and when I switch from one setting to another and back again I could hear a squawking ressonance in the machine, the overheat light went on and then poof, a shower of sparks.Because it is an import and I did not want to send it back to the mfg to repair, I decide to see if their ws anything to repair myself as I did take a course in electronic service technology at NAIT in the late 90's but never pursued it further and had forgotten alot of what I was taught, but am anxious to get back into the theory of how this welder works.It is a multifunction tig,stick and plasma cutter.Upon inspection of the machine their was a blown Infineon K75T60 mosfet, and I can not seem to find any other damage in the bridge rectifiers, so my question is this before I power this thing up to check and do further damage, is their a chance that their is damage to the machine in another place that I do not have the ability or education to check that someone could advise me on?Their is no schematic available, the mfg's don't want to release info on.
I attach pics of the circuit board that the damage was on I know it must be a plasma arc power board or related to it because the pilot arc connector wire comes back to this board, a small black and red wire.I desoldered the old mosfet with braid soaked in resin, applied resin and soldered the new one on without lifting any traces, then cleaned board with alchohol and appled new conductive gel behind mosfet.
Any help I would really apprecite because I could not even find a data sheet for the chip to see if their was one available locally.It cost me $35.00cdn for the chips $10.00 for the chips(2) and $15.00 for shipping from china.On the small board that the mosfet was on their are two small items that look like zener diodes in clear covering marked v1 and v2, don't know what they are but believe they may be gate resistors? Can anyone shed light on this?
thanks, appreciate any help.

 

Looks like you got yourself a chinese [ junk ] -- pun intended -- boat-anchor there.......... sorry - couldn't resist - shoot me later.

If it is typical asian made, the transformer may have either pri, sec, or both wound w/ aluminum wire, and not immobilized on the laminations - varnished - and the windings may have scuffed on the laminations or themselves as a result of the drop, in which case you definitely have a boat-anchor.

Seriously though, if you already have the correct components from china, about all you can try is replacing them tit-for-tat, though your pix show it to be quite a cobbled-up rats-nest.
Also I wonder if that heatsink is adequate for all that is on it. A little re-engineering in that area would probably help.
The left end of the grey capacitor? that the top red wire is running to, looks burnt as well, or is that just flashover ? I fail to see the diode you speak of. Is it the zener marked V21 under the red/yellow twisted pair ?

Check closely everywhere from all angles, for components touching from the impact, pinched wiring, cold solder, or fractured circuitboards w/ torn traces. repair as needed

Then if you have the means, bring the unit slowly up to voltage with a variac downstream from an appropriate isolation transformer. if it is to survive at all, it possibly will.

Best of luck for you !!

 

I have another so it is not going to kill me if I don't get it going, all the extras, regulator, pedal, torches, I can use on the other, it took a very good hit on the back, probably a four foot drop, other than the dent on the cover their is a screw missing from the board that holds the rectifiers.The repair looked to easy to get away with, and I thought their may be something to control transient voltage at the mosfets.I wish their was a good article on electronic theory of operation, component by component.Anyhow thanks for help,the other machine I have is identical, and is a very good tig, plasma cutter, so I looked at this one a being disposable as the price was about $1550.00.
I want to use this one as a portable and lender unit.
best

 

I removed the small board and tested the gate resistors to be ok and desoldered the K75T60 mosfet transistor and continued to test machine for further problems, replaced cover, plugged into wall expecting to see sparks and in all functions the machines runs well.Total cost for the infineon K75T60 was $35.00, and $15.00 was because of shipping so parts were $10.00, and I still have a spare mosfet.Thankyou for your help.
Biggest problem was finding a source for the transistor or a counterpart or crossover, would anyone know if their is one