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

[cruff]

(01-02-2017 05:18 PM)AndiGer Wrote:  Do you think that one of the integrated circuits is bad? U3 would be replacable, ok. Why do you think the printer works properly when I switch the photo coupler with the other one from the working printer? Vice versa the other printer runs into the left side wall with the supposed to be defective photo coupler.

I don't know myself, just mentioned the troubleshooting table, but it does seem that from your testing, the issue is with the problematic encoder. If the window of the phototransistor is dirty, or the phototransistor has degraded, it would result in a lower voltage reaching the + input of U3, which would prevent the comparator from triggering. Take a look for debris covering the phototransistor. Alternatively, the output of the IR LED could be degraded, or the LED current could have changed, which would affect the comparator threshold.
Also, if the values of resistors R10 and R11 are not correct, which seems unlikely, this would affect the signal comparision.

(01-02-2017 05:18 PM)AndiGer Wrote:  Am I correct you mixed the red wire (thru C5 to ground) with the black one (negative input to U3C)?

No, the red wire supplies +5V (Vp) to both the IR LED and the photodiode. C5 is a bypass capacitor, which is present to help prevent noise from affecting the phototransistor output to U3. If C5 is shorted, which seems unlikely, it would prevent the circuit from functioning properly.