I have a C64 that has recently stopped responding to two 'columns' of keys, namely '9,0' 'i,o' 'j,k' and 'n,m' - I've swapped the CIA chips (U1 and U2) to no effect, so I pulled a keyboard from a spare C64 and tried it and it worked perfectly. This leads me to conclude there is something wrong with the actual keyboard.
I found this document: Commodore 64 keyboard connector and keyboard matrix and it appears all the keys I am having difficulty with are related to (I think) Port A Bit 4, which is pin 16 on the keyboard header.
I'm uncertain what to do next. Does this sound like a simple broken wire inside the keyboard? Or is there any active electronics inside the keyboard? - To open the keyboard further would require me to de-soldier the Shift Lock key and I suspect the keyboard internals are nothing more than the typical flat membrane affair.
Note, While obviously highly related, I don't think this question is a duplicate of Commodore 64 with partly working keyboard because that question was answered by swapping the CIA chips and does not provide any troubleshooting guidance beyond a bad CIA and/or inspecting the keyboard header.