I have an SR540 and the controls are getting flaky. Same as the SR800 you make all of the adjustments by pressing and turning a single knob, but now it adjusts up when you turn it down, or it adjusts several steps when you turn it one click… It’s done this before and it got better, but it’s a strange control mechanism. I’ve had it for less than a couple of years.
You might consider easing in and getting a popcorn popper instead. That’s how I started. There are no controls or measurements, so you just roast by ear and stop relative to first or last crack.