How A 1981 RAM Expansion Worked

A teardown video by Happy Little Diodes reveals more complexity than expected inside a 1981 Sinclair ZX81 16K RAM pack. The upgrade used two PCBs connected by ribbon cables: one packed with 74-series logic chips, the other holding 4116 DRAM chips and a discrete power supply circuit. Because the 4116 DRAM needed multiple voltages, the pack included a clever self-oscillating single-transistor switcher generating +12V and -5V, complete with a quench diode in its 12V Zener regulator. The piece also notes the RAM pack's infamous instability, since it was designed on a machine clamped to a desk, so any jostling in real use would trigger a reset. Later versions consolidated the logic into a single custom chip.

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