The ZX81 motherboard is quite simple by modern standards

The main chips have been highlighted:

  1. IC1 – ULA
  2. IC2 – A Zilog Z80 (or NEC clone) running at 3.5Mhz
  3. IC3 – ROM (an 8K ROM containing the BASIC interpreter)
  4. IC4 – RAM – Either a single 4118 chip in IC4 or 2 x 2114 chips in positions IC4A and IC4B

The ZX81 I own contains 2 x 2114 chips, so you will see how they fit on the board in my blog posts.

The ZX81 has two versions of the ULA:

  • 184E – with the infamous ‘back porch’ bug; may look dark on a modern TV or colour CRT
  • 210E – Interchangeable with the above; contains a fix for the back porch bug

The video circuitry on the ZX81 is quite sloppy in its implementation of a video signal and many modern LCD TVs will simply refuse to tune into it. The solution is to convert the ZX81 to output composite video; this is not quite as straightforward as the ZX Spectrum composite video modification, but well within the abilities of anyone with a basic understanding of soldering.

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