I know this is an old thread, but for those who discover it after trying to solve their own tensioner release issue on their 206rb(like myself), I wanted to post what i have found out and maybe help someone along the way.
I purchased a 206rb-1 and shortly thereafter realized the tension release wasn't working. Pretty obvious because when you raise the presser foot and try and remove your work you usually shred you thread or spend extra time trying to pull thread through the tension discs and release your work. You can still sew just fine, but it is a complete pain to actually get any work done. I watched the majority of these amazing consew 206rb maintenance videos (Part 1: https://www.youtube.com/watch?v=y4ACNMX65zE Part 2: https://www.youtube.com/watch?v=8ROVK2bKObU) and it part 2 at 26:59 I discovered what I think is a major issue for many people with this problem.
In the thread tensioner assembly there is a tiny piece called the tension release plunger-25 (10793) that is easily misplaced. Many people complain that the tension release lever rod-11(10720) doesn’t seem to be long enough(and this is possible) but it is likely due to this little plunger not being inside the tension stud-22(10789). You can get a replacement from south star supply (http://southstarsupply.com/product/26-tension-release-plunger-10686-10793/)
Another thing that could be interfering with the tension release is that tension release lever-13(10678) could be bent and not transferring the action of the rod to the discs. You can either get a replacement, or try bending it back into shape to confirm this is the problem before ordering another.
Both of these are super easy to remedy, but in my case the tension release slide-11(10546) was broken(see photo) and needed to be replaced. This actually isn’t too difficult if you are mechanically inclined and enjoy a project. Also gave me a great opportunity to clean up lots of grime in my machine. I also discovered that the plunger was indeed missing from my tension assembly so I am happy I checked!
Anyways, I hope this helps someone in the future!
Link for parts diagrams I am referencing in my post
http://www.supsew.com/download/Consew/Consew%20204RB-1,%20206RB-1,%20208RB-1.pdf