How can I fix my Hobbylock 797 to sew the chain stitch?

Hi everyone, I have just bought and cleaned up a second hand Pfaff hobbylock 797 overlocker/serger, 5 thread. The main overlocker stitching is beautiful and I am dead chuffed with it, but the single line of chain stitching just doesn't want to work. The needle and looper seem to be completely out of synchronization, although I have to say I don't fully understand how they work. Are there any internet sites that explain how to go about servicing these machines. I don't want to pay too much money for it.
Many thanks
Georgina