Hi,

Thanks for all your help guys.

I managed to get hold of a mig welder, and can now replicate the problems here in the workshop. I have looked at the mains inputs to the pic's in relation to when the thyristors are firing, and there is no change with or without the fault, so i feel that the reference signals are not the cause of the problems. This is a big releif since using opto's would have meant a redesign of the pcb.
I have connected the star point of the welding transformer secondary through a 1k resistor to 0V on the pcb. I have also fitted a capacitor to the welding earth where it comes in to the machine, and to mains earth, and also fitted ferrite's to the welding earth cable. This seems to have cured the problems. I can hardly measure any interference coming into the machine now. I'm now off to weld hundreds of studs with the mig running. Hopefully i will not get any bad welds. How things will work out on site with 20 or so mig's running is another matter.

Once again thank you all for your help, & advice

Regards
Bob...