I've partially disassembled the knob unit-- there's a glass/ceramic disc with radial markings, and with what appears to be an incandescent bulb on one side and a pair of sensors on the other. The bulb does not appear to be illuminating when powered up (it does seem to be getting 5V),so this seems suspect but I don't know whether it's supposed to be emitting visible light or not. I also don't know what to replace it with. Anyone repaired one of these before? I'm guessing Tony'll havesome input here :). Thanksas always, Josh