Originally Posted by
Prism99
This video helped me a lot:
http://www.youtube.com/watch?v=Q1mRhcquZTM
It seems to me your bobbin tension is too tight, which is why the upper thread is unable to pull the bobbin thread up far enough. There could be lint underneath the bobbin spring (video shows you how to clean). Or, with the change of threads, you may need to follow Jamie Wallen's method of setting the bobbin thread as loose as possible first, then tightening the top tension. That is what worked for me.
This is good advice.
If what we are seeing is the bottom - to me your bottom tension is too tight (but with the towa at 170 it might not be) or your top tension too loose. Just one other thought - Rethread the top, new needle (right size for the thread if necessary) and check for "fuzz" in your tension assembly. If your tension disks are not closing correctly, it will not tighten the upper thread.
Good luck - I just solved a similar problem but it was the top thread that was too tight! Took me awhile but it is fixed. Just takes patience.