Need tools? Here's a video showing what I use (with links!): https://youtu.be/MLK649c1_fM
Many times I've seen these receivers with broken potentiometer shafts, and unfortunately the best fix is to replace the entire thing. In this video I show what it takes to do so!
If you use this video to work on your receiver, perform all work at your own risk. I am not responsible for any injury or damage.