Repairing the Amiga 1200 keyboard

Here’s my Amiga 1200, it’s a lot of fun. I use it mostly to play games, program in C language (sometimes 68k assembly) and to connect to the HP 1660AS logic analyzer / oscilloscope through the serial RS-232 port. The nerd specs are; CPU 68EC020 @ 14 MHz (stock), 68882 @ 40 MHz, 2MB Chip + 8MB Fast RAM, RTC, 3.1 ROMs, IDE-CF+4GB, WiFi WPA2/AES.

In 2024 I replaced my Amiga 1200 keyboard membrane because some of the keys were failing, bought it from retrofuzion.com which is a good store in my experience, highly recommended.

Then a few days ago my shift key on the A1200 keyboard started failing, after taking it apart turns out it was the key plunger (aka key post) for the shift key which was bad, it looks like this.

The black part is some kind of conductive rubber which is what makes contact with the keyboard membrane when a key is pressed, over time (decades) the rubber loses its conductivity which can be checked by measuring resistance with a multimeter.

The bad key plunger as shown is 1511 ohm which is way too high. I found working key plungers on eBay for the A1200 keyboard. After measuring a few of these by now the good ones are usually around 100 ohm, the bad ones over 500 ohm in my experience. After replacing the key plunger with a new one which is 88 ohm the shift key works again.