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Joined: Aug 2007
It could be the adjustment on one or more of the reader switch contacts. These are the small gold leaf springs in the reader mechanism. They are activated by the card pressing on little nylon roller balls that then press the leaf springs against contacts on the back of the keyboard circuit board. There are very small screws on each spring contact that adjust the contacts. Setting them properly on an HP67 is a very tedious and touch process since you can't get to the adjustments while the reader is put together.
I adjust mine by tracing the switch contacts to the CPU board contacts and monitoring them there with an ohm meter. While insering a card, the three switches make contact in sequence. The three switches are the motor start, head enable, and write protect. If the head enable switch is wrong, you may not be able to read cards and they won't write properly. If the write protect is wrong, attempts to write to any card will show error. If the motor never starts or always runs the motor switch is wrong. The switches may never be closing or stuck always closed. Sometimes they are real flakey. Also the adjustment of one switch can affect some of the others.