Osborne 1 keyboard repair?

[Tom Hunter]

I am trying to figure out if it is possible to repair a Osborne 1 keyboard.
The keyboard is made by "Oak Switch Systems" and the type is FTM or "Full
Travel Membrane".
The problem I am seeing is that 3 keys ("h", "j" and "y") are permanently
pressed.
I did some experiments with the "h" key.
I measured about 20 ohm across the matrix pins for the "h" key.
I pulled off the "h" key keycap and the white plastic plunger with the both
large and small spring - no change in resistance.
I cleaned the now exposed membrane using Isopropyl alcohol - no change in
resistance.
I applied moderate heat using an electric hair dryer to both sides of the
area around the "h" key - no change in resistance.
I then used a lab power supply set to current limit hoping to zap whatever
is causing the partial short (20 ohm). I slowly increased the voltage and
current limit in short bursts until I hit 100 mA before I gave up the
experiment not wanting to destroy the two flex PCBs feeding into the
membrane. They did get warm but not hot.
I have run out of ideas of what else to try. I still measure 20 ohm across
the two keyboard matrix pins associated with the "h" key.

Has anyone got experience repairing or restoring this type of membrane
keyboard mechanism used in the Osborne 1 and probably in other keyboards
too?

Here are some good and detailed photos of keyboard mechanism:

https://www.flickr.com/photos/123564336@N03/sets/72157644113347562/

Thanks
Tom Hunter