Wow , the 500 is a antique in the plasma world. 20+ years ago we ran a couple of the 500 and there big brother the 1500.
If your torch is still the miller factory unit and has not been changed ,then it's most likely a OTC APT 5000, made in Japan for miller.
Go to the miller web site, miller welds. go to the bottom of that page ,there is a option for manuals, open and on the right there is a search option , type in APT 5000.
It will take you to the torches parts and service section, it's also down loadable. That said, don't put much money into this guy, if there are other problems with it,.
Parts are getting obsolete and very expensive.
