I have 3 ZX81 units. A few years ago one of them started exhibiting problems with reading the keyboard, which I assumed was a faulty ULA. Half a year ago, it failed ALL keyboard reads, so it was put away.

Today I decided to do a repair attempt. After some searching, I noticed the resistor pack RP3 had 3 of the 5 resistors bad in different ways. I removed it and mounted separate resistors. The keyboard attachment points still measured wrong, so more detective work.

Much searching later I had found the heat sink for the voltage regulator shorted 2 traces. Fixed with some fresh electrical tape. The keyboard still measured wrong, but not as much.

Finally found a dodgy solder joint on one of the ULA pins going to the keyboard. I reflowed the solder joint and now everything works as expected!

So happy to have a spare working ZX81, they do go bad sometime.