Older 67 card readers and 65 card readers

[teenix]

The only thing I can think of is poor power flow between the boards through the connecting pins or a dirty power switch connection. Probably corrosion or dirt on the circuit board holes or poor contact tension, and maybe old or poor quality lubricating paste on the switch contact.

The 97 service manual mentions that some sense ICs did run marginally close to their tolerance limits causing intermittent read errors and I guess slightly reduced supply voltages through the boards may occasionally push the chips of this type into these error conditions.

As far as I can see, the key/display board has nothing else to do with the card reader.

cheers

Tony