Chinese Super 200P MOSFET welder repair - Manufacterer: Yangzhou DJ ?

[poxxxy]

Hi guys,

I bought this 3-1 unit from ebay faulty, thinking it might be an easy repair. However the unit outputs nothing at the terminals in form of an arc, also no HF start noise coming from the unit or spark at the spark gap.

I've had the PSU board out and it all looks ok, I attempted swapping out an LM324N Op amp on the front control board after seeing someone had a similar problem and claimed this fixed it... no dice .

I wish I had the knowledge to reverse engineer the unit and get it back up and running...

I've heard of other problems with these units where the OC current light is illuminated, however in this case the unit appears to power on, fans run, relays appear to click when switching between TIG/MMA/Plasma... but the ammeter output reading remains at 000 on the front display. I'm a bit lost on where to even look in the unit at the moment...

I'm wondering if the machine is basically junk now really as I can't find a similar unit to test/compare against. Perhaps I could construct a new machine out of some of the components in there but it certainly sounds daunting .

Some pics of the unit here: http://s1344.photobucket.com/user/poxxxy/library/LTS%20UK%20Super%20200P%20Multi%20Purpose%20Welder?sort=3&page=1
Video of the fault here:

[poot36]

I would start by checking that the power supply is supplying the correct voltages to all of the components in the welder.  Try and find the chip that drives the MOSFETs for the HV section and see if it is getting power and if it is check that the safety shutdown inputs on it are not been activated.  Check all MOSFETs for shorts with your multimeter on ohms range and then if that passes test each MOSFET on the diode range and see if you get numbers that make sense based on the internal body diode (if they have one) and if your meter can supply enough voltage to turn on the gate of the MOSFET you should be able to measure a very low resistance or diode check number between source and drain of the MOSFET.  This is assuming that the gate terminal does not have a very strong pulldown or pullup resistor attached to it.