Toyota Expert 9000 Tension Knob

I own a single head Toyota Expert 9000 that is always kept in clean and in good working order. Recently, I started having a problem with the upper thread tension on one position. When the knob in this position is turned, it has almost no resistance and no additional tension is placed on the thread. I've re-threaded with no improvement. I replaced the tension spring with no change. I also swapped the knob another that I know is good - no change. Is it possible the problem could be with the tension stud? I know it sounds odd, but I'm stumped.

Location: 
United States

I'd be surprised if the stud threads were stripped. More likely the knob but you say you've swapped knobs already. Does the knob tighten down on the stud and compress the spring or does it keep slipping and never compress the spring? If it's compressing the spring and pushing on the disc, check between the discs and make sure there's no buildup holding them apart. Always good to clean between the discs thoroughly. If all else fails, swap out known good parts of the tension setup one piece at a time until you find the piece that's causing the problem.

Good luck,
Rob