CN21 No sound from any key - Druckversion

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CN21 No sound from any key - tonydix - 17.09.2020

I. am trying to help a friend with a repair to this instrument. The symptoms are as follows :-
Instrument powers up and plays sounds when either lessons or concert music is selected. The music can be stopped and started with the start stop key. The metronome produces sound. The volume slider changes the volume. So all seems to be working except no key produces any sound. I suspected that power to the keyboard had been lost so I removed the keyboard and attached connectors to the pins 8 and 16 of the 74HC138 chips and found a good solid 5 volts.
Now I am at a bit of a loss. What other faults could cause these symptoms ?
Can the instrument or keyboard be muted ?  Can the microprocessor be reset ?
Any advice or help would be much appreciated.  Tony