The lower 16k RAM is implemented across 8x "4116" DRAMs at the bottom left of the main board. (See picture above) IC: 6,7,8,9,10,11,12 and IC13 on my Issue 4S board. As you can see I've already de-soldered them! Soon as I confirmed the lower rams had failed I removed them all ready to see if I could replace them.
The removed RAM 4116 IC's:
With my spectrum sat in storage unable to use. Nearly a year later I found a suitable replacement for the failed RAM. Over on zx.zigg.net newly manufactured memory modules designed as a drop-in replacement for the lower 16k RAM are available to buy! I brought one straight away. The module is a small pcb that is soldered in where the original DRAM's sit.
The module soldered in to my spectrum board:
The DRAM's can be replaced with IC sockets then the module installed on to them, but I soldered mine directly to the board. I left a 5mm gap between the boards to avoid short circuits.
A quick power up test with the the new ram and the system booted as normal again:
Games load perfectly again!
The repaired system reassembled. Running Bomb Jack 2 from divide 2k11 interface.