The 3586, 8568, 8566, 8340 series units All used the RPG encoders with a lamp HP 2140-0016 (653 lamp ??). These RPG's were round flattened cylinders. if the lamps were old/darkened by age/misaligned you could get " works in one direction but not the other". The solution is to remove the RPG, pop the end cover-2 opposite tabs, use needle nose to pull the bulb from the side, check bulb for clarity, resistance-90 ohms, reinsert/replace with base of bulb flush with case of RPG, AND parallel to the optical grating inside - parallel to the PC brd. The detector diodes need to be illuminated equally.
Don B.
