I was wondering that it is possible we may have skipped a few stages.
Looking at the circuit the motor does use the welding voltage via a control circuit on the PCB to vary the wire speed, but the PCB it self is still powered from the small transformer down there at the bottom of the welder.
The circuit shows a few things such as a fuse in line or a breaker?, not really clear on the main circuit. But could I suggest a check for volts on the Red circled diode on the control board. It should have 24 Volts across it and if there is then it would quickly show the small transformer/fuse etc is all OK.
The control board does seem to be the same as Snooper posted and is also the same as the circuit I posted earlier.
So here we are:
View attachment 82143
View attachment 82145
The small transformer provides AC to the board. Half wave rectified by D1 (bottom right) R10 is a 680 OHm resister provides current to the diode Z1 Zener diode at 24 Volts and smoothed by C9 a 10 uF capacitor.
Just checked voltage on diode and if ive done it right it measures 52 volts!! i dont know if this is relevent but it will not trigger at all on the first two settings of the three position switch. it only works when its turned to the broken line position. i thought that was for when you are spot welding? it allways worked on the first position before i.e. continuous weld.
The bottom half of the circuit seems to be the control for continuous welding spot and stitch welding, so if these are not working correctly could point to an issue on this side of the PCB. It looks as though T7 the BC237 has to switch on to allow the motor speed control to work.
If no 24 Volts across the diode, could you check for approx 27 volts AC across 2-6 and 5-9 of the connector.
Again just my thoughts
Adrian