I have a 464 Plus and it has an issue whereby upon switching it on you can hear a continuous key tone on the menu. F2 and F1 both work and you can open Basic where you see this: 7833333333333333333 (recurring). Pressing Del or Clr stops this. However, the 3 key is non-functional and every letter on the first QWERTY row gives a double press (aside from Q). So we get IO from pressing just I etc.
On Burnin' Rubber the left and right on the D-pad don't work but once the lower keyboard connector marked CP03 is unplugged their function returns. The upper keyboard ribbon seems to have no effect on the D-pad.
If you boot up with both keyboard ribbons disconnected then you hear no key tone, so I'm guessing the problem may lie with the keyboard rather than the AY chip.
After this testing I'm thinking that the keyboard is outputting a continuous 3 signal and confusing the rest of the system but I'm no expert and I'd be happy to hear any suggestions as to just what might be going on here and how it could be fixed.
Have you opened it up to take a look at the keyboard membrane. Is sounds like perhaps it just needs realigning (hopefully).
My 6128+ is incredibly fussy about its keyboard, after a few weeks I'm guaranteed to lose functionality of some keys - I usually need to open it up and have a fiddle, which after a couple of attempts tends to restore normal functionality.
Thanks for pointing me toward the membrane. It was covered in dust and dead moths. I wiped it down then separated the two parts and reseated them. 464 now works perfectly!