The problem is only there sometimes…

usually measure 82-86v open voltage on ac and dc settings.

But after leaving on or using for some time, the ‘low input voltage’ warning light comes on.

I suspect its also the inverter board. One of the large capacitors had blown a hole in it, so i replaced all 4 (bright red in photo).

next step to check all transistors? (Need to find out how!)

 

Maybe loss in contact between components in control board or inverter, sometimes connecting when work fine, and not when there is a problem
Look for bad contact or bad solder joints in the PCB

 

Maybe loss in contact between components in control board or inverter, sometimes connecting when work fine, and not when there is a problem
Look for bad contact or bad solder joints in the PCB

Ok will check - thanks for all your tips, super helpful

 

Only today I found your answer. It's better to use the "quote" mode .

So, the issue is related to inverter . Found the pulse transformer (-s), the IGBT power modules or the IGBT power transistors are driven from those transformers. Check pulses on the primaries of those transformers, amplitude is about 15-20V, frequency is about 20-50kHz

getting back to this after some delay…

i measured the pulses on primary of the transformer and i i get just below 20v. I cant get a good frequency reading , i am using two high voltage probes as a sort of differential probe using math function of oscilloscope (with isolating transformer and dim light bulb connected to welder for safety). Frequency reading by was going up and down around 1-2mhz which i dont believe is accurate). (I should also mention my isolation transformer also eeduces voltage to 120v whilst this is a 240v machine so may affect some of the internal voltage levels?)

I dont however get any output from this pcb… i did apot one of the mosfet resistors was bad so will change that. Other things to check / try?? Appreciate any more tips!