Casio CTK-611 some keys not working

[mygrove]

I have acquired an (old) Casio CTK-611. All seems to work as expected, except that 8 keys in the centre of the keyboard do not work. (the keys are G#3- D#3 and A3-D3). In fact, when you first switch on they can be used once but only sound very faintly, then nothing. I have had a look under the keys to see if it was perhaps a dust issue - it wasn't. Looking inside it is the first 8 notes of the second half of the keyboard pcb KY2M. I understand this board was a common component of many Casio products.

But is it this pcb that is likely to be the problem or something else? Any clues on fault finding are appreciated.

Thanks.