LowCostTuning Expand your memory up to 32K - a selfmade project in three steps
You need the schematic of ZX81, some knowhow in soldering and some parts you can
buy for pocketmoney.
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Step 1: if you only want 16K-byte it is very simple:
- remove the original 1or 2k memory chip(s)
- solder a good ic-socket with 28 pins at the place of IC4
- fit some short wire at the place of LK2 (on the right side of IC4)
- take the 38Kx8 RAM-chip and bend pins 1, 2, 22, 23, 26 up, they are not allowed to
have contact with the pins of the socket
- put the ram (43256, 55257 or 62256 e.g.) into the socket
- connect pin 1(A14) with ground (pin 14 of IC4)
- connect pin 2 (A12) with A12 , this is the cathode of D3 on the ZX81 pcb
- connect pin 22 with ground
- connect pin 23 (A11) with A11, this is the cathode of D1 on the ZX81 pcb
- connect pin 26 (A13) with A13, this is the cathode of D5 on the ZX81 pcb
- if you want to run HRG8, connect pin 27 via a resistor of 10K ohms with 5 volts
- that's all
- Step 2: full 32K-RAM from 16K to 32K and 48K to 64K
- Step 3: memory-backup