for salvaging hi-res encoder wheels and detectors from deskjet printers, you can use this website to check if the particular printer has an encoder disk or not as not all of them have it.
just bought a deskjet 930C from an ad in gumtree for $5.
before buying, i had a look here,
http://h20141.www2.hp.com/HPPARTS/De...33D66A25949059
chose the model, click on the keyword search dropdown and choose encoder. that'll show that it has a paper motor encoder disk.
to see where it is, choose one of the print mechanism diagrams.
this disk is the 1800line or 7200 ppr (quadrature) disk.
the disk is just stuck on the gear with adhesive, you can pry it out as in the pic.
you can remove the detector with the pcb and solder wires to the detector pcb.