Internal 32 kB RAM Extension for the Sinclair ZX81

Based on P. Liebert / www.zx81.de.
Translated, reviewed and reformatted by kio 2005-11-23.

You'll need a ZX81 circuit diagram, a few components and a little experience.

Remove the original RAM 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)
Find a 32 kB RAM chip, e.g. 43256, 55257 or 62256, and bend up pins 1, 2, 22, 23, and 26. They must not be connected to the pins of the socket.
Insert the RAM chip into the socket.
Connect pin 1(A14) to ground (pin 14 of IC4)
Connect pin 2 (A12) to A12 , this is the cathode of D3 on the ZX81 PCB
Connect pin 22 to ground
Connect pin 23 (A11) to A11, this is the cathode of D1 on the ZX81 PCB
Connect pin 26 (A13) to A13, this is the cathode of D5 on the ZX81 PCB
If you want to use HRG8 (suggested), connect pin 27 via a resistor of 10 k ohms to 5 volts
that's all

Remove the RAM from the socket and bend up pin 28 (only nessary for step 3)
Insert the RAM into the socket
Connect pin 28 to +5 volts
Remove the wire from pin 1 to ground and connect pin 1 (A14) with the following circuit
Build a simple circuit from two NAND gates of a 74LS00.
Mode of operation: When the ULA is requested to read data from the display file for video output, this is indicated to the ULA with /M1 = LOW and A15 = HIGH. Because the display file is located in the 16-32K area, A15 for the RAM must be forced LOW when /M1 = LOW. The two NAND gates form an AND gate.
Connect the A15' output of the circuit to pin 1 of the 32K RAM chip
Connect A15 (cathode of D8), /M1 (edge-connector 22A), 0 volts and + 5 volts to the 74LS00.

Note: this works only for the RAM from 48K to 64K, because the reset code erases the the RAM from 16K to 32K.
Replace the wire from RAM pin 28 to + 5 volts with a gemanium or a schottky diode.
Connect a backup battery of about 3 - 3.6 volts via a 100R resistor to pin 28.