Hi @jaby,
I don’t know music so I’m not sure what keys (wires) you would need to look at. I only know how the notes appear as the notes or sharps and then what octave the particular note is and then work it out where the key is on the keyboard to find the wire.
Basically +3.3V is fed to the CPU on the control board and to the keys via a resistor and then a wiring / diode matrix and then when particular key(s) (if more than 1 key) are operated this voltage is fed back to the decoder on a particular wire(s) (if more than 1 key operated). This causes the CPU to detect a voltage change and also then the decoder knows which note needs to be played.
You would need to know the wires that the particular notes use (it may be common to several notes - that’s what the diodes are for - to separate the keys on the same wire) and then check that the +3.3V is being supplied via the resistor.
Here’s an image of the relevant part of the circuit diagram where I have highlighted the power for the keys (you can see the resistors there on the circuit) and where it goes to the CPU and then down through the connectors to the keys ans then back to the decoder.
Given that it is not all keys (notes) but only some don’t work it ''may be'' something common like the +3.3V power supply to only some of the resistors. As can be seen there are 3 x +3.3V power feeds that feed 4 resistors each for all the keys (notes). If one feed for example was missing then this would affect more notes than if only the one resistor was faulty as this may only affect say two notes over all the octaves. Alternatively the problem may be the connector at the decoder end of the circuit board.
Hopefully this all makes some sense to you. You said that you cleaned all the sockets so presumably this means the ones on the DMLCD board as well as the 61L and 61H boards.
[image|2159752]
(click on image to enlarge for better viewing)