Lear Siegler ADM-3A Replacement Keyboard
Like many owners of the venerable Lear Siegler ADM-3A, my lovable 'dumb' terminal was showing it's age with its CRT cataracts and failing Hi-Tek keyboard contacts. The CRT problem had leaked brown corrosive liquid down between the keyboard and PCB for many years, causing eventual failure of the Hi-Tek contacts. The CRT issue was relatively straightforward to resolve with plenty of heat and acetone, but sourcing 40+ year old precision switch contacts is difficult.
Instead I designed a replacement keyboard with the help of www.Keyboard-Layout-Editor.com, Plate & Case Builder, and EasyEDA. This replacement keyboard is meant to use Cherry/Gateron MX switch footprints and match the Row/Column wiring of the original keyboard matrix as detailed on the maintenance manual schematic. It's designed so that a simple 40 Pin IDC cable connection can connect to the keyboard, then the individual wires can be connected to the terminal motherboard itself. It's a PCB that is meant to be supported as a daughterboard using standoffs from the original PCB after removal of the original keyboard. Extensive support holes are added for use as a generic retro keyboard by enthusiasts.
Project files can be found at this link.
Pictures of the completed version 2 keyboard installed in the ADM-3A are below. Version 3 fixes a missing trace (bodge wire shown below), corrects the spacing of the 'user' keys next to the spacebar to fill out the empty space better, moves the position of some mounting holes to better align with the ADM-3A main PCB, and alters the PCB footprint slightly to not cover up the ADM's optional rotary baud rate switch, if installed.