Most often keybed and painel parts need replacement.
Here the service manual pages with part numbers and the how-to´s.
Front Panel Switches Parts list
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Image by mike.partington How to remove individual keys
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Image by sensitivedragon How to install individual keys
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Image by sensitivedragon Parts list and diagram of individual keyboard keys
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Image by sensitivedragon The complete manual, with all part numbers are here: Manuals: Owner and Service - JD800 and JD990
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