Repairing a broken key of a Yamaha DX11. The key did not spring back up when pushed. This was due to the plastic in the key having snapped and the steel spring had detached.
It is quite common that the plastic in these old keyboards becomes brittle over time. If the issue in the video happens to you, check if you can find the little part that has broken off inside the case. If you cannot find it you might be able to attach the spring back if you modify the key slightly. Another option is to buy a new key