The other minor repairs that needed to be done to be properly playable and servicable.
One of the main issues was that the center manual couldn't be played properly - the keys only sat a few mm above the felt rest and therefore wouldn't activate properly when pressed down. In tearing the manuals apart, I thought it be best if I cleaned and re-worked the manuals to have better connectors to the keyboard scanner (at the time, they were still on horrible header cable to DIP socket things)
but also to re-adjust the home position of all the keys to level and fix the wonkiness.
Inside the manuals, you can see two solid core copper wires used as contacts that touch the back part of the metal spring when a key is pressed. Some of these contacts were very corroded, or not even anywhere near the metal bar! From the construction of the diode matrix, it was very clear it was replaced at some point and bodged in to form a proper matrix scanner, rather than a much wider single row keyboard scanned in parallel (or forming some analog VCO control more likely).
All of these keys were taken out, cleaned, contacts sanded, and then adjusted to sit at the same height across all manuals to be properly playable. I added locking IDC connectors to have as a better bridge to the horrible connections on the existing keyboard scanner, and moved the piston buttons to a DB15 connector (which is where I found out that the center buttons were double-touch and still haven't added on the extra cores to fix this).