I have no idea what you've repaired, and it is. of course, possible to use a filament lamp i such a device. However, the service manaul for the HP2631B pritner (Which uses a very similar encoder on the end of the carriage leadscrew) specfically states that the light source fro said encoder is a pair of LEDs. I beleive the LEDs are wired in series, and are IR ones. It's been a long time since I've had an HP9816 or 9826/36 keybaord apart, and when I producd scheamtics I treaded the knob encoder as a signle component, but my first thought is that the light source in there is also a pair of IR LEDs in series. If necessary I will take one or more of mine apart and check. In the early 1970s, LEDs were expensive and it made sense to use filament lamps for all sorts of applciations (for example the power-on indicator on an HP9830 is a filament lamp). in the 1980s, when the 9816, etc was produced, LEDs are a lot cheaper, The lower power consumption and increased reliability would make them an obvious choice for said optical encoders. -tony