HP 67 Circuit Board

[teenix]

(07-15-2022 09:36 PM)metawops Wrote:  Now, first question: am I allowed to use the card reader under this (lab power supply, no batteries) circumstances? I hope so, because I did insert a magnetic card. ?

Yes, as long as the supply can provide 500mA or so comfortably. CAUTION: Do not accidently reverse the power polarity and switch on or you will most likely destroy some internal parts. Some (like the sense chip) are connected direct to battery with no protection. The card motor should draw about 180mA when pulling a card.

(07-15-2022 09:36 PM)metawops Wrote:  Maybe the silicon tube is too thick? I measured an outer diameter to be 6.57mm which should be fine, right? (I have another tube that has an OD of 6.08mm – both measured when applied onto the axle.)

I'd use the smaller diameter. Also if the tube is too hard, especially on cold days, then the pressure on the card reader becomes greater and could cause damage to the mechanical parts, and also overload the motor and sense chip motor driver.

I could be wrong but I feel tubing of 1/4" or greater is slightly too large. !/4" o-rings only have a point contact on the card and probably squash up a bit and the diameter reduces a bit in use, plus in practice, there is only one o-ring in contact with the card. With tubing this effect lessens. I've used silicon tubing that feels quite soft, slightly less than 1/4" OD and it works fine in some readers but not all. With the motor removed you should be able to manually feed in a card back and forth but with not too much force - hard to quantify this though. It would be nice to dream and have an original drive wheel in mint condition to make some measurements. I am looking for o-rings with a 50 buna n rating (softer than normal) but I haven't find them in Oz yet.

(07-15-2022 09:36 PM)metawops Wrote:  In another attempt (before I saw that the screw was missing) I connected the motor directly to the lab power supply (card reader fully assembled) and set the power supply to app. 1.8V – can I supply more voltage here?.

I would avoid doing this as you will be feeding power back into the sense chip. I don't know if this will damage it and other components or not but it is not worth the risk. Unsolder the +ve wire from the motor, or unplug it from the sense board if it is the type with wire sockets fitted, then connect the motor direct to the supply for testing.

The sense chip seems to output about 2.4V when running the motor - details in Classic Notes - page 32.

cheers

Tony