HP 98203 keyboard "knob" debugging

Thanks to a tip froma list member, I managed to snag a compatible keyboard for my HP 9816 -- a "Nimitz" class keyboard, the 98203B(http://www.hpmuseum.net/display_item.php?hw=682). It works fine (though it dwarfs the machine it's connected to) except that the "knob" in the upper-left does not appear to function. I've looked for service manuals but I can't find anythingthat's particularly detailed. I've partially disassembled the knob unit-- there's a glass/ceramic disc with radial markings, and with what appearsto 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