Which resistance controls the card reader speed on a HP-97

[Kees Bouw]

@AndiGer, thank you for your tip regarding testing the card reader without disassembling the calculator. This made it a lot easier. I have attached a picture of my setup with the failing card reader :-)

I have tried reading a card with all resistor values listed in the service manual, I have tried 4 different rollers: square o-rings, round o-rings and two sizes of silicon tubing, I have also tried several positions of the eccentric cam. But nothing worked. At best, I could read one particular card (diagnostics) some of the time. All other cards resulted invariably in "Error". So I gave up - for now. Fortunately I have another HP-97, also original and not restored. I decided to restore the card reader from that calculator in exactly the same way and I attached this card reader to the calculator I was working on. From the first try it worked, and it still works, flawlessly. I have not seen a single card that gave an error. Writing also works fine. The motor current is with slightly over 180 mA within range. It still has the original resistance. This encourages my opinion that the problem with the first card reader is not the resistance or the roller or the cam, it must be something else. Maybe there is still some tiny piece of dirt on the magnetic head, or maybe there is something wrong with the flat cable (it is in poor condition, the flat cable of the second card reader seems to be of better quality), or maybe the motor does not give a constant speed, or maybe there is a weak component on the pcb. Just some thoughts. I will try again later. In the service manual there are some pictures of scope signals, maybe I should get a scope and see if that helps to pinpoint the problem. For now, I have one fully working HP-97 and I am very happy with it!