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Sinclair ZX81 modification, preventive maintenance and common fixes

Image by Evan-Amos, licensed CC-BY-SA-3.0, from https://commons.wikimedia.org/wiki/File:Sinclair-ZX81.pngSo, I got myself a Sinclair ZX81 from the UK. I got lucky and found one that came boxed, complete with 9V power supply, assembly instructions and 16 KiB RAM pack. The ZX81 was Sinclair’s 1981 follow up of the ZX80 computer (1980) and was sold as a kit or fully assembled. Like it’s predecessor it sported a Zilog Z80 CPU running at 3.25 MHz and 1 KiB of RAM. It’s main advantage was the lower cost and larger ROM (8 instead of 1 KiB) which allowed for more BASIC commands and floating point math support.

My unit was of the kit version and the original owner did a very good job of cleanly soldering it together and they even noted some errata in the assembly instructions, where a resistor had to be replaced with a different type – the schematic on the back side of the instructions did show the correct value. [Read more →]