I recently was asked to help a friend with their Concertmate-1000 Keyboard. This had brought them a lot of joy over many years and they were saddened to discover one day that some keys had stopped responding. This turned into a mission to restore it to its operating glory.
Having watched a few a on-line videos and carefully marked the faulty keys, I set about dismantling it to get to the keyboard focusing on the key deck area itself as the videos had suggested this was likely to be where the fault was to be found. Underneath the keyboard was a circuit board that the keys contacted, via a rubber pad similar to a remote control. The circuit board showed some signs of corrosion with a number of damaged tracks and joints, which correlated with the dead keys. Fortunately I was able to borrow the Repair Café Gosport video microscope for a closer inspection.
I initially attempted to resolder the joints and replace broken tracks with wires. However the damage was extensive and there was no height available for overlaid wires. So I took the next step to buy a donor keyboard of the same model from eBay, sold as spares as the power supply was not working. Once the new one arranged, I plugged the keyboard from into the original and all the keys worked.
The keys on the donor were had some scratches so I opted to take the circuit board from the good key deck and replace the corroded one. This all worked and the owner was thrilled to have their keyboard back working, and they have a set of donor parts for the next fault thrown in too.