HP 65 - another card reader problem [Solved]

[AndiGer]

Lately I received a nice looking HP 65. Seems to work fine but the card reader.
I replaced the gummy wheel by O-rings. The card passes thru nice but with a squeezing sound (a thing I sometimes also have with a second unit that reads quite well).

But the real problem is the card in the reader seems not to be registered as the display doesn't turn dark during the card running thru.
It accidently happened to me with the second unit that there was an error when reading a card with blinking display: When I put the card into the slot during blinking display to my surprise it ran thru but no dark display as well.
So I conclude that a card is passed thru the reader when the motor switch is turned on with no regard of the calculator is in error state or not. Pure physical pass-thru of the card is just determined by the motor switch and the running motor without regard the calculator being in "ready-to-read" state.

Am I right here? Any similar observations anybody?
Any suggestions what to look for? (Currently no oszilloscope available.)

Traces of the card reader PCB are ok. Only two minor oxidations that do no harm to the traces, 0 Ohms measured.