Been doing some more testing with the scope this lunchtime, concentrating on the RAM. One of the 4116 chips in the bank IC6-13 has a much lower voltage (and different shaped trace) on the DOUT pin and one of the RAM chips in the bank IC15-22 has no voltage at all.
If the bottom 16K of RAM is faulty then that would explain the unsuccessful boot.
I’ve got no reference trace for a working 4116 so will power up my ZX-81 over the weekend to scope it out, but all evidence points towards two faulty RAM chips at the moment.
Unfortunately my ZX-81 has 2 x 2114 RAM chips in (4 bit DRAM) so I was unable to compare the trace against that on the Spectrum. I think this trace I scoped out earlier from the 7 (probably) working 4116‘s looks credible though:
All chips are getting the correct voltages. The only issues are with chips IC7 and IC16, both of which have anomalies on their DIN/DOUT pins; IC7 has a different trace and IC16 has practically no trace at all.
If the chip numbers are to be believed then it is possible both chips represent the same bit. I’m not a man who believes in coincidences so this may turn out to be a fault somewhere else.
A quick glance at the ZX Spectrum schematic indicates that both these RAM chips are on D1 on the data bus. I’ll check out D1 on the ULA, Z80 and ROM next to see if any (or all) of those are the culprits.