Hello, and Welcome !
The voltage that you measured - 40 volts - I assume that is DC across the output from the PCB to the motor? That is high, I would have expected only 24v. Was that with the speed pot turned up full? Does the voltage vary if you turn the speed control?
Have you tried the motor on it's own - it should spin up on a 12v or 24v battery.
This earlier thread may help, it could be the same control board on your machine:
Have retired and want to do a bit of welding Got given a Murex Tradesmig 251 but it has a problem with the wire feed Have a bit of experience with electronics and have traced the fault to the PCB All pots and switches are OK and I have power to the PCB Contactor clicks but no feed Motor is OK...