If it is just the UHF that is out, as Riley Casey said, it’s not the crappy rotary switch on the back of the speaker. However, it is highly likely you have an oxidized spot on the UHF potentiometer. When the speaker is playing, and you rotate the level adjustments on the front panel for each of the Midrange, HF, and UHF drivers, does the sound cut in and out, or is there smooth attenuation? My project for the last two weeks have been getting 43xx monitors out of storage and ready for listening, and I found problems with all of them, mainly scratchy pots, though there was an open capacitor on one of them causing no output from one driver. If you are still using the original crossovers, an open cap is definitely a possibility, but I’d clean the pot first.
Here’s how I did it. Get a can of De-oxit, a flashlight, and a small handheld mirror. Pull the woofer from the speaker, also the 2121, and the acoustic lens off the front of the speaker. Put a utility table (not a fancy dining room table with a center pedestal leg) against a wall so it wont move, then put a few thick towels or blankets on the table. Put the speaker face maybe a foot back from the table, facing the table, with some of the padding between it and the table, and maybe another foot of padding hanging down past the table edge. Then, lean the speaker forward against the table, and pick up the back of the speaker and shove it up onto the table, face down, but leave the woofer hole hanging off the edge of the table. Now you can sit under the speaker, use the mirror and flashlight to see the back of the potentiometers, and when you spray the De-oxit into the back vents on the pots, it will dribble down onto the contacts, instead of just out of the case. I try to give each pot two sprays in different holes, about 1sec spray each, then tip the speaker up and work the pots all the way back and forth about 5 times, then give each one one more shot, and rotate them again. I had pots that were completely dead and now work fine. Out of the 18 pots I’ve done this week, only one still has a spot where it mutes, but it’s all the way up, where I won’t use the pot. BTW, since you’re in there, clean all the pots. Also, De-oxit is not good for you, so make sure you ventilate the room. If the driver still doesn’t output at any level position, you’re going to have to crack open the crossover and test caps.
Btw, I am not keeping any of my 43xx monitors, but if I was, I would buy/build new crossovers for them - I’ve listened to all the drivers in a 4343 in project speakers with active crossovers for testing, and they are capable of much better sound than what I’m getting with the stock crossovers.
Good luck