probably, like many early monos, there are two switch contacts on each key: one is a trigger, the other provides the pitch information . the pitch contacts usualy have a string of resistors that divide the voltage to make a scale. if a wire has come loose from one end or the other of the keyboard, each key would get about the same voltage and therefor play (close to) the same pitch.
at the same time, it was common to have the preset tabs/buttons feed voltage to a diode matrix to select the correct functions/modulations. these switches were usualy in series...that is, only one can be 'on' at a time. if the wire feeding voltage to them were off, you could loose the signal to change 'tone'.... so all presets AFTER that bad point would not work.
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