How a 1981 RAM Expansion Worked

Sir Clive Sinclair and his company were notorious for pushing the limits of electronic parts in search of a low price, and his ZX series 8-bit computers were fine examples …. How a 1981 RAM Expansion Worked. The ZX81 came with a meagre 1K of memory, and a popular upgrade was a 16K RAM pack.

[Happy Little Diodes] has opened one up, and to his surprise, found many more parts than expected. Inside the box is a pair of PCBs connected by ribbon cables, one of which has a selection of 74 chips and the other the 4116 RAM chips and a discrete component power circuit. This complexity comes from that cheapness, the 4116 is an inexpensive DRAM chip and requires an eclectic set of power supplies.

The functions of address selection are straightforward enough, as is the DRAM refresh circuitry. The power supply is clever in that it’s a self-oscillating switcher that provides +12 and -5 volts with a single transistor. We particularly like the quench diode in the 12 V Zener diode regulator circuit.