Quadrature Encoder Interface Circuit
The schematic quality is terrible but we should be able to figure it out.


The LM339 is a comparitor (not an op-amp). These are configured to convert the complementary inputs into unbalanced outputs.
The inputs are complementary and may not switch fully to V+ or GND (or may be a different voltage than the circuit in question).
The output is open-collector which can only pull low so a pull-up resistor is attached to each output. Pull up is to +15.
A 2.5 V mid-supply reference is generated by a pair of resistors, R18 and R19 and a stabilizing capacitor, C5. This provides the switching threshold for the inverters.
The conditioned outputs are generated by two more comparitors with their associated pull-up resistors and what looks like a current-limiting resistor.


The OA and OB outputs can be fed into some standard logic inputs but CMOS is most likely as it is a 15 V output.