potentiometers get bad in that the extra resistance between the wiper and the track gets very large and unstable. How can we counter that? well, connecting it to an amplifier with very high input impedance may help. So that's what i did.

This is the original schematic:

And this is my modification:

I used an OPA2344 opamp, which is a rail-to-rail dual thing with cmos inputs. CMOS inputs have extremely low bias currents, making the circuit tolerate incredibly high source impedances. Any other rail-to-rail opamp with cmos/jfet inputs and working off of 2.7 volts should do.

Here's how it looks:

It was one of the most delicate pcb surgery i've ever done. Cutting, stripping and soldering to hair-thin pcb tracks is tricky. Note that there are a lot of mechanical parts right above the pcb, finding a place to put the amplifier so that it doesn't interfere took some effort.

The thumbstick works perfectly now. But i have to admit, the thumbstick kinda fixed itself by when i took apart the controller to probe things. So, my fix might not be the fix. I'll report how it holds up.

Update: as of 2022-04-06 (4 months since the mod), no issues.

Update: 2022-04-11 i'm noticing that both left and right thumbsticks are slightly misbehaving. I'll let them get worse a bit...