HP 82143A / HP 82162A foto coupler (encoder) - substitute

[cruff]

According to the service manual, when the encoder pulley reflects the infrared light onto the photo transistor, the green wire (at the connection with R11 and the positive in put to U3C) will have a more positive voltage than on the red wire (at the connection with R10 and the negative input to U3C), as referenced to ground. Page 4-5 has some trouble shooting steps for determining if U3 or U8 needs to be replaced, or even if the encoder pulley needs to be cleaned. U3 is a quad comparator, which should be an LM339.