The ZX81 motherboard is quite simple by modern standards
The main chips have been highlighted:
- IC1 – ULA
- IC2 – A Zilog Z80 (or NEC clone) running at 3.5Mhz
- IC3 – ROM (an 8K ROM containing the BASIC interpreter)
- 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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