LK401: Some keys have died.

[Professor Fate]

Hi folks,

I've run into a serious problem with the LK401 of my VT420: Some keys in the left upper corner (F1 thru F5, caret, number '1') lo longer emit any keystrokes after not having been used for a couple of years. As some of you may know, F3 gives access to the set-up menu of the terminal. I've opened the case of the keyboard and removed the keycaps in question, but can't find anything strange, not even on the PCB:




See https://photos.app.goo.gl/NJFCjiokFc5zxukR6 for images in full resolution.

Now: How can I bring these keys back to life? In know the LK401 is a rubberdome board with conductive strips printed on plastic sheets, and I've already run into a similar issue with a Model M (which turnd out to be lethal), I really wanna scrap another keyboard. Any hints?

[Professor Fate]

OK, I disassembled the board and found out that some of the conductive strips on the lower of the two plastic sheets are corroded:



So I applied some conductive varnish to the strips, reassembled and connected the board…and suddenly found out that the keys do work, but emit the wrong keycodes:

F1 → s
F2 → e
F3 → w
1 → a

What's wrong here? Did I apply too much varnish, or did it flow over an adjacent strip when reassembling the keyboard?