Zx Spectrum Plus does not respond to keyboard commands

[Ast A. Moore]

Just for kicks and giggles, test TR3 (above the KB1 connector, in the middle of a bunch of resistors and capacitors). Also check C67. Oh, and I’m sure you already did, but just to be sure, check that there’s 5V on R25’s lead that’s opposite the one connected to the TR3’s collector.

[balford]

This is going to be a ULA problem, unfortunately. What happens is that one of the KBx lines in the ULA gets stuck low, so the ROM code thinks multiple keys are permanently held down, therefore no keypresses can be registered. If you run a diagnostic program that displays the status of port 0xFE then you can actually see the stuck bit associated with the broken line.

Issue 3 machines with 6C001E-6 ULA's are especially prone to such issues as these ULA's are quite fragile - I've got a broken one with exactly the same fault in my spares box.