The chap who sold me some of the PDP parts was kind enough to let me try another KDJ11-D CPU board that he had, since I thought that the RAM was dead on the first one. Here it is:
This one works! Well, in reality it also has issues ... if I try to use all 1.5 Mb of RAM it also fails the self-test on startup. So I tried setting the W25 jumper as documented here. With that jumper in place it seems to work fine, although it means that I'm limited to 512k of memory. So I imagine that the problem must be in the upper megabyte. It would seem that these old RAM chips have a tendency to die. However, it should be good enough for me to continue moving forward. Although at some point it would be nice to have a fully working KDJ11-D (M7554) in my machine. I must keep an eye out for that.