Tandy TRS-80 Model 4

The keyaord is a simple matrix of switches (similar to the M3 keyoard in your manual (I assume) but with extra keys in the unused spaces of the M3 matrix. The switches can, and do, fail. What I did to my M4 was pull all the keycaps (make a diagram of where they go back), then desolder all the swtiches from the PCB (!). Take the whole lot apart, and then test the switches with an Ohmmerter. Don't expect a dead short (they're contuctive ruber contaics inside), but you can seprata the good from bad ones this way. The swtiches do come apart. Clean the conductive rubber part and the contacts and try aggain. This might get some of them going. The remaining duds come apart again, and I rubed a 6B (very soft) pencil on the conductive rubber part. That really reduced the resistance!. I then worked out the least-used keyoard locations (in my case the number pad) and put the dodgy/repaired switches there. I had enough good ones to fully populate the main area with a few dubious ones in the number pad.