[HP48GX] Testing repaired keyboard

[mfleming]

Hi,

Here's a link to an article on the keyboard schematic of the Pioneer style calculator. Although not the same as that of the HP48 series, the key matrix arrangement follows the same pattern.

http://www.hpmuseum.org/cgi-sys/cgiwrap/...i?read=451

Have a look through some of the old articles in the link below to see if you might find the same information for the HP48. I think your testing approach would be to identify a column or row trace, then measure the drop in resistance on the other traces as you press keys. As the above note says, resistance should drop from 10 Mohm to under 5 Kohm.

http://www.hpmuseum.org/cgi-sys/cgiwrap/...ticles.cgi

Getting a good contact with the flex circuit is the tough part - you don't want to hold two probes against the circuit while trying to press a key with a third hand! To arrange semi-permanent contacts with the flex circuit, try taking an appropriately sized double-row header and bending the pins on the long side together to form something like a clamp. Then you can slide the flex circuit between the pairs of pins that are touching each other.

The old Pioneer flex traces were 0.1 inches apart, so you could use double-row headers like those for an Arduino. Not sure of the HP48 series though, they could have a metric pitch.

Good luck!
~ Mark