I have 2 sergers...a Brother & a Singer. Both are from the mid/late 80's & both are 3/4 thread. Mine both use industrial needles that are currently being made....I buy mine on ebay & get 100 needles for around $15-$20. I think most (if not all) new sergers use 15x1s. Knives are still being made for both of mine too...again, easy to find & cheaper on ebay. Knives last a long time if you don't hit a pin with them.
Neither of mine have ever been in a shop for repairs....& they get VERY heavy use! They're really not hard to work on. KenmoreRulesAll, you probably just need to move the looper a tiny bit...& then turn the wheel by hand to make sure that it's not hitting the other looper & to make sure that it's forming a nice stitch. Tolerances between needles/loopers & between the upper/lower loopers are VERY slight but nothing should actually be rubbing/hitting anything else. Tolerances are SO close that most sergers can only use size 11- 14 needles....smaller are too far from the loopers & larger ones hit the loopers.