HP 41c card reader repair

[Johnboy]

Since my original post I have successfully repaired 6 out of the 7 card readers I have. The repaired ones all read/write and respect the protected cards.

The one remaining card reader is problematic, giving me "card err" and "checksum" error messages when reading a card. By swapping parts with one of the good working card readers, I have deduced that the problem is not with the electronics (bottom half of the card reader), but with the motor / card receiver unit. I have changed the roller and given it a good clean, and run the head cleaning card through a few times but still the same errors occur. I just happen to have a spare motor/card receiver unit (condition unknown) and when swapping that one in, I get same/similar errors.

I find it strange that known good electronics doesn't work with the both of the motor/card receiver units I have.

Unless anyone has any suggestions on what to do next, I am going to disassemble the card receiver units and check the switch contacts and pcb tracks. The only other component on that unit is the IR led receiver, can they go bad ? and I thought that was to do with "protected" card detection anyway, shouldn't affect reading.

Any further information/suggestions appreciated.

Thanks
John