I have disassembled HP DeskJet 3000 j310a. I've removed paper feeder encoder board (shown on image). The problem is that when I connected it to 3V3 supply (it has builtin resistor on LED side), it always has 1.60-1.63V on output A or B no matter whether the gap is cutted or not. I'm sure sensor is not damaged because the printer was fully working (was a donor for a damaged j310a printer in better condition). What might it be?
I see that this post is very old, but I will still answer it.
There is a difference between a light gate (the one you have on the other side of the board) and an encoder (the one you are trying to test). A light gate simply tells you if there is an object blocking the light, but an encoder is used to track motion in both directions using an encoder strip or disk (clear plastic with black lines).
They are identical on one side as they both have an IR LED emitting light. The main difference is that an encoder has 2 outputs (A and B in your case) which alternate on/off when the black lines of an encoder passes through giving an offset square wave pattern that controllers use to track the motion of a motor or print head.
I advise you watch this video to see the output signal and understand how to use it: https://www.youtube.com/watch?v=0QLZCfqUeg4
I also worked with similar printer parts, and found that they easily burn out even at 3.3v power (these components usually run on around 1.5v) so be careful.