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

[AndiGer]

Thanx Cruff,
I looked into the service manual at the mentioned page. And the electric diagram as well.
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.
Am I correct you mixed the red wire (thru C5 to ground) with the black one (negative input to U3C)?
As soon as I have time I will measure the voltages on green, black and U3 output / pin 38 of U8 and will report.