A single key plays at maximum velocity regardless of how hard you hit it. The Manual's Fix: Turn to page 42 (Keyboard Scanner Logic). The manual explains that each key has a dedicated photo-transistor. A stuck note usually means the comparator LM339 on the scanner board has failed. The manual provides the test points (TP6 and TP7) to check the square wave output.
: A digital copy of the Kurzweil MIDIBOARD 1986 Service Manual is hosted here, which includes essential schematics for troubleshooting the internal electronics. Kurzweil midiboard service manual