HP-28S Keypad Issue

[BitWiz]

I recently acquired an HP-28S and everything is working fine except for the bottom row of the keypad. The [ON] key must be wired differently as that works no problem.

However, I am able to get the bottom row to work perfectly and even pass the keypad test if I press any where on the bottom plastic frame of the LCD.

Has anyone else experienced this issue or have any ideas. This calculator is difficult to open and service.

Thank you,

Mike

[brouhaha]

(03-13-2024 09:34 PM)BitWiz Wrote:  However, I am able to get the bottom row to work perfectly and even pass the keypad test if I press any where on the bottom plastic frame of the LCD.

The contacts between the flex circuit and the logic board are pressure-fit with a foam strip. The car on zed contacts on the flex circuit can wear, and the foam degrades over time, eventually not providing enough pressure. AFAIK the only way to fix it is to take it apart, clean or refinish the contacts, and replace the foam strip.

[Steve Simpkin]

Here are a couple of resources that show what's involved in opening and fixing a HP-28C/S.

HP 28S: shucking the clamshell, further. Book?

Open a HP 28S calculator

[vaklaff]

There's also this approach: https://www.hpmuseum.org/forum/thread-20779.html