The knob and the numbered ring normally do move as one piece, but you can change the position of the ring. This won't fix actual tension problems, but it will change where the numbers sit. If you look at p22, figure 20 of the manual that Tammi linked to above, it shows the tension assembly "exploded". On the backside of the knob, there is a little pin, "C", that fits into one of many possible holes on the numbered ring, "D". You should be able to push on the ring to disengage it from the pin, "C". Then, you can turn the ring to the number of your choice. For example, if it is stitching medium fabric well on 7, but you think it should be number 4, then push the ring to disengage it from the knob and turn the ring to the left until the number 4 lines up with the line on the indicator. If you can't push the ring then it might be stuck to the knob and you might need to apply a drop of oil just behind the knob. Also, to disassemble the tension you have to push on the ring to free the knob.
That's great that the tension is releasing when you raise the presser foot. You might want to apply a drop of oil to the tension releasing pin, "J" (behind the faceplate, at the back of the tension assembly) to keep it un-stuck.
Don't be afraid of disassembling the tension assembly. The diagrams in the manual are really good. Also, here are a couple of very good blog posts with great pictures . They're for the 201 but it looks the same as the FW.
http://oldsingersewingmachineblog.co...sion-part-one/
http://oldsingersewingmachineblog.co...sion-part-two/
I, too am curious what your bobbins look like. Looking forward to your pictures!