06.04.2018 Page 5 of 16
232KB internal RAM extension
2.1 Short description
This little PCB board gives you full 32K-RA internal memory from 16K to 32K and from 48K to 64K.
The schematic is shown at the end of this document… while here’s a small description:
IC1 is a 74LS00 circuit (4 NAND gates in a S D SOIC housing), while we only use 2 of the NAND gates as follows:
When the ULA is forced to output the display-file to your screen, this is indicated to the ULA with the signal / 1 =
LOW and A15 = HIGH. Because the display-file is located in the 16-32K area of your ZX81, A15 for the RA must be
LOW when / 1 = LOW. The two NAND-gates take care about this situation. As we are using 2 NANDs this will form an
AND at the end.
Resistor R1 is used optionally if you like to use HRG8 screen mode. I’ll recommend adding this component.
IC2 is a H 62256-70 SRA PDIP housing IC (32Kx8 RA -chip) all these parts enables your ZX81 to have at least 2 x
16KB of RA available. So let’s start to upgrade your ZX81.
First work items to be done is, to open your ZX81 and to carefully remove the PCB board to do some reworks which
allows the usage of the new 32KB internal RA board. Having the ZX81 board carefully detached from the keyboard
flat cables, please start locating the existing RA chip(s).
•Depending on your PCB issue, remove the original 1 or 2k memory chip(s) from your ZX81 PCB. If the memory
chip(s) is/are soldered, I recommend to cut the IC leads on top of the PCB with a side-cutting tool and to
unsolder the remaining leads afterwards.
oThen use a solder sucker or some solderwick to get the remaining solder out of the solder pads
(example picture ZX81 Issue 3 board)
