Hello, I had the exact same problem with my fireface uc. I also saw a person selling his ucx for 500 € on willhaben because of the broken selector knob. The knob looked exactly like in the photo, with one part broken off. What I did, was the following:
Purchase a replacement encoder on rme.
https://www.rme-shop.com/acatalog/RME-- … -3534.html
plus the cap, because it was glued and i could not get it out.
https://www.rme-shop.com/acatalog/RME-- … ml#SID=113
THis is for UC, for UCX:
https://www.rme-shop.com/acatalog/RME-- … ml#SID=113
The parts came with PostNL out of the Netherlands, the ratings are very bad for this carrier, but I had absolutely no problems. Only thing is the tracking link does not work sometimes, so you have to enter your barcode on postnl tracking page
Then I saw, how the status changed. Can take up to 2 weeks, but better than no knob. Patience is key!
So then, the encoder and the cap came. The encoder normally gets soldered on the board but it still worked with the old knob (very loose and wobbly), so I though why not only put the one black part in. So I disassembled the encoder. I bent away the metal pins, disassembled everything. I cut the plastic (yes, you heard right. I cut it open with a knife) and got to the part I only needed. Then I put the new cap onto this part and put it into the hole and voila!!!! It worked. Now I have a working knob again and I am very happy not to have to pay repair costs which probably are high. I´d love to post some pictures of the process, but I am no admin. I also hope in the future RME will make solid knob solutions and find a better solution for this, because this knob breaks off a lot it seems. Especially with such an expensive device, I am disappointed that the knob breaks off so easily (I broke it off with my spinning chair) When I look at the audient id14 I had, never would there have been the possibility that the knob breaks off.