Start by checking there is address bus data on all 32 lines at U890 (Ramsey).
At any one of the fast memory SIMM sockets, check there is multiplexed address data (row/column) on all 10 of the fast RAM address lines. There should always be activity on these lines due to the constant refresh cycles, regardless of any system activity. Looking at the signals with a 1uSec/division timebase, you should see something like 4 active low strobes on each line.
Would also be worth checking that all 4 column address strobes at the SIMM sockets have activity. Those 4 signals are common to each SIMM socket, so doesn't matter which socket you check.
Could also be a defective U890, though it wouldn't be my first guess.
What behaviour do you see when running the system normally, with standard ROMs? Are you seeing something like it running OK with chip memory only, but you get some kind of failure (e.g. yellow screen) when trying to boot with any fast memory fitted? If you try to boot with no startup and report the system memory (using 'avail'), do you see the correct amount of fast memory reported, even if it isn't usable?
