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

[teenix]

The manual mentions having some special stuff (Magna-See) to see the length of the magnetic data tracks to determine which resistor to use to trim the motor speed, so that the track length is within tolerances. The faster the card passes the longer the data track appears on the card therefore it will probably have missing data.

Slow card feeders have tracks that are too short will probably have data bits too distorted to be read properly by the calculator hardware.

The card is supposed to move at a nominal 6cm per second +/- 5%, so you might be able to time the passing of the card and adjust the resistor to suit.

A smaller resistor value should slow the motor down and shorten the data tracks.

The minimum specified is around 2800 ohm (2K8).

The median resistance for the adjustment resistor is around 4700 ohm (4K7)

As mentioned in another post a variable resistor would help. A miniature Bournes type multi-turn 100Kohm would do with a 6800ohm (6K8) resistor in series to limit the minimum resistance. Both components are commonly available. Connect the 6K8 resistor to one of the outer connectors on the Bournes. Then make connections from the other end of series resistor and the wiper of the Bournes (Centre connector).

If you clip these 2 wires directly across the speed resistor, that will give a range of around 2K7ohm to 4K5ohm. This will avoid having to desolder the original part for testing.

If this doesn't work, then there might be other issues.

cheers

Tony