This article is a port from a thread on the Apple II Enthusiast Facebook group. The author of the original thread is John Keoni Morris. The thread is transcribed here as is.
A very common problem with the Apple IIc is having the volume knob/potentiometer fall apart. Here is a quick tutorial on how to put these back together, at least if there is no major damage.
If yours looks like this, then you are unfortunately screwed. Here there is obvious physical damage and the resistance strip is missing.
A good one should look like this. It will need to be desoldered from the motherboard. Clean up the black strip with a bit of Deoxit.
These are the 3 parts of the assembly. The knob, potentiometer and a little circular clip.
What typically happens is that the clip slips off the end of the knob and allows the know to fall off.
So, after cleaning up the electrical portion, we need to reassemble. This involves simply laying the ring over the end of the knob with the curved portion facing upwards.
Then using a 4mm socket, we push the ring back down onto the shaft. No too hard, but it should click in place.
Then we use our soldering iron to melt the end of the shaft just a bit to pin it back in place.
Finally, we simply solder the entire assembly back into place! We are done!