I have a Fluke 97 Scopemeter which have some defective keys. My specific unit is an early first revision one. I’ll quote an image from EEVBlog here.
The keypad is membrane type and some traces are not connecting anymore. I used conductive silver epoxy to connect broken traces. It’s necessary to scratch off the oxide layer of the silver trace and/or draw the epoxy onto the key contact part which is always conductive.
I used Mechanic brand silver epoxy. It’s a heat set type. I baked it in the oven at 60 degrees Celsius top and bottom, with fan on, for 5 minutes after each application. The baking temperature should ideally reach at least 135 degrees Celsius but it’s not possible with this type of membrane. Low temperature baking or hot air is also effective, as long as most of the solvents are driven out.
After all broken traces are repaired, all keys returned to working condition. Before final assembly I cleaned all key pads and conductive rubber with isopropyl alcohol.
My previous attempt failed because the adhesive reacted poorly to my specific humidity levels. That particular environment changed the outcome significantly.