and I will add the possibility of mis-assembly or bad diodes.
These are handy little machines but it took me awhile to figure it out. It kicked my chubby little butt for about a year. I was almost ready to admit I wasn't capable of using one.
These things use a pair of spiders, each containing 6 "bullet diodes" some of mine had been installed upside down during manufacture. They quickly vented their magic smoke and those remaining alive and correct felt compelled to limit the already limited power even more. Once you look into one of these things, it is easy to see how this could happen. The diodes are about the size and configuration of a thick button cell camera battery. Only a small dot of paint tells you which side is which.
Since that time, I have encountered 2 more of these machines with dead diodes. Since Lincoln bought Century, parts aren't always easy to find. EBay, however, always seems to have a cheap selection of huge diodes just begging for conversion use. So far, it seems to be a viable fix.