I picked up a used Sunstar KM 640 BL recently and I’m looking others who have this machine.
My choices for used machines were pretty limited, I currently live in South Korea and there are a LOT of repair shops, but no one speaks English and my Korean is not up to par with sewing machine repair.
While I’ve been able to do the basics like timing adjustment via YouTube videos, I can’t find much about this machine on the web.