I fixed a similar problem on a TDS340 that turned out to be contamination on the PCB that was shorting one of the buttons, the encoders and buttons are multiplexed so a fault with one can cause others to not work. Check for SMT electrolytic capacitors on the front panel board, I know the TDS400's have them, I don't recall whether the 300 series do or not. Those are known to leak and make a mess. Fortunately these are easy to take apart, just don't lose any of the buttons when you remove the front panel bezel. The knobs pull straight out and will come out with the shaft. The front panel PCB snaps on to the back and I think has a couple screws. This is a good time to give the panel and buttons a good scrub, it will look like new.