ZX81 Internal 16KB Memory Expansion
This module is for the ZX81 Computer and expands the RAM from 1KB to 16KB.
There is no need to cut tracks on the ZX81 motherboard, or having to bend any pins on the new SRAM.
To upgrade the RAM some soldering is required. If you are not skilled at soldering then do not attempt this yourself as you risk damaging the ZX81 motherboard as the PCB tracks are easily ripped.
Installation
– Desolder the 2 x 4118 RAM chips from the ZX81 (IC4a/IC4b) Solder in SIL sockets if you want to revert to the original chips.
– Clear out the solder holes and solder in 2 x 14 pin SIL sockets at IC4. Or use a 28 pin socket if you are not going to solder in the SIL sockets for IC4a/IC4b. (See photo for end result)
– Insert 16KB module into IC4 sockets. Take note of the orientation.
– Solder wires from:
D1 pin on the module to D1 on the ZX81
D3 pin on the module to D3 on the ZX81
D5 pin on the module to D5 on the ZX81
– Solder a jumper link on the ZX81 motherboard – set to LK2 (L2 on Issue 1 PCB’s).
To test the memory expansion, run the following command:
PRINT PEEK 16388+256*PEEK 16389
Result should be 32768.
The SIL sockets and wiring is not included (The are just shown as an example in the photos).
You will need to source this yourself.
Only the 16KB RAM Upgrade module is included.
To revert to 1KB remove the jumper, wires and 16Kb module and re-insert the 2 x 4118 chips.
This module has been tested on a ZX81, and also stress tested on a SRAM testing jig.
