[personal] Shaft encoder in HP2631B printer

HP must have used several different encoders, then. I have 4 HP9000/200 machines with the twiddleknob -- an HP9826, an HP9836A, an HP9836CU and an HP9816. I also have an HP9817, but I use a standard HP46020 keyboard with that, so no know :-( Anyway, all 4 machines use the same encoder, and the light source is a pair of IR LEDs with the LED chips driectly mounted ona timy PCB on the end cover. They're wired in series to 2 little wire sockets on the board. There are 2 wire pins sticking up from the housing thac connect to these -- one is a noraml wire to ground, the other is actually a 100 Ohm (or so) resistor with one ende osldered to the +5V line, the other end goes into the +ve pin socket on the LED board. If I had to replace the light source in an encoder and I couldn't get a suitable bulb, I'd try an IR LED first. The sensors are almost vertainly sensitive to IR. Filaments bulbs give off a lot of IR (particularly if underrun). of course some is absorbed by the glass envelope [1] but enough gets out to operate the sensors. [1] I once worked on an IR spectrometer which used a Nernst glower as the light source. THis is a rod or coil of rare earth oxides, and you'd think it eas a good electircal insualtor. At room temperature it is, but get it hot, and it'll conduct (the reisstance drops with increasing temperature, in fact it can thermally 'run away'). Anyway, the nernst glower was connected in series wiht a ballast lamp across the mains. To get it started you used a bit of cototn wool soaked in ethanol and ignited to heat the glower, once it got hot, it would glow quite brightly. Of course the advantage is that the nernst glower runs in air (it can't exaclty oxidise, so you don't have a glass bulb to absorb any IR emission. The nernst lamp was actually proposed for electric lighting at one point (with an electical heater coil to start it), the advantage being that you don't have to have an evauated bulb, and you can just replace the glower when it fails, not the whole lamp. But they're less efficient than vacuum or gas-filled filament lamps for producing visible light (the golwer runs cooler than a tnngsten filament). -tony