I searched the web and did not find any specific best way to handle keys that have no response at all given my described situation. Most places say "clean everything"....OK tried that. What does one do specifically (materials and procedure) to clean the black carbon contact points to ensure conductivity? This is not the same issue as the SOL/Tandy-II keyboard problem with the dried-out pads. The PET has springs, and this particular keyboard has no rust and very little crud. There must be something microscopic like a thin film that needs to be cleared. I believe the pads are extending and contacting the control board. I tried using deoxit and isopropyl alcohol. Hope I did not ruin. I was able to get a few keys to work but there are a lot more to fix.
I use alcohol (91%) and a qtip on both the contact points and the carbon pads. Have you tried using a pencil eraser on the board contact points? Might be worth it on the carbon points as well (pencil eraser) and then clean again with alcohol.