At one of the St Louis Gateway shows we spent an hour on the show floor after lunch trying to "repair" someones' misfunctioning A4000D, re-seating all the boards, including the processor board, trying other keyboards, etc, and it just wouldn't take input. The place was full of Amiga experts, I think even Jens was consulted. Then the owner embarassingly remembered he had turned the switch before going to lunch to keep people from messing with his machine ...
Since then and even now, I keep a A4000D key on my keyring! You never know when you might find a locked A4000. Looking at it, this one has the number 1502 stamped on it, if that is any help.
-B