The slot in the bobbin basket is too narrow to accept bigger needles above a #150. That is why the thread is fraying because the needle is getting caught in the slot. You have the parts list for the 669? If not, go to Durkopp Adler website and then to products. Go under 669Eco and down load the operations manual, parts list and mechanics manual. Page number 12 of parts list shows the breakdown of the hook. The bobbin basket is part #0367156020. That has the slot in it that the needle goes into when it descends to do a stitch. I put a bubble on the right side of the slot so it will accommodate a #160 and #180 needle. Right side because that is the side that the thread come out of the eye of the needle for the hook to pic up.
I used a Dremel with a carbide bit and polished it out with flat carbide Mitchell cord. Why the Eco can only use up tp a #150 needle (which they really no longer make) and the 669 Silverline model with toys can use up to #180 is beyond me. Durkopp Adler is aware of the problem because I talked to Ed Atkinson, the head of Durkopp Adler America in Georgia 2 weeks ago. I know they used the same hook from the 367-373 flatbed on the 669 Eco. The 367 had the same problem with larger needles. Send me your phone number and a good time to call.
glenn