In the past, I had a perplexing problem where the main tuning knob
would not stay put. After tuning, it would usually begin to rotate as
much as several Khz in the opposite direction.
I've recently found the fix for this which is quite simple:
1. Remove the rubber skirt on the knob.
2. Loosen the set-screw using an allen wrench and remove the knob.
3. There is a spiral spring wrapped around 2 parts - one being a
small plastic washer.
4. Remove the spring by carefully rotating it counter clockwise - it
should unscrew and easily slip off of the assembly.
5. Now, make sure the plastic washer is snugly against the part in
back of it on the shaft.
6. Next, carefully screw the spring back on by rotating it clockwise
as it is being lightly
7. Rotate the spring clockwise, as far as you're able to with your
fingers (no tools!).
8. Replace the knob, tighten the set screw, and put the rubber skirt