I donated my 778 to the school. It's never been a great machine. It's always had too much pressure on the presser foot. I've got it turned all the way down and it still skews the fabric on long runs. It's still in good condition, though. I'm donating the 758 to them, too. It has a cracked cam stack. It still works, though. I figured it was best if they had two alike, since they are just learning to use a sewing machine. I've told them not to have the machines repaired when they die.
On the 600, the needle bar is loose. According to Roy, "If it's freely swinging, something in the linkage isn't properly connected."
I need to download the service manual and see what I can do. I'm getting handier with a screwdriver after a few small repairs. Hopefully, I can fix this. I have been watching for a 600 or 603. Maybe this is it.
bkay