The ZX Spectrum motherboard is fairly straightforward by modern standards:
The main chips have been highlighted:
- IC1 – ULA
- IC2 – Zilog Z80A Processor
- IC3, IC4, IC23 to IC26 – Decoder / Multiplexor chips
- IC14 – LM1889N TV Video Modulator
- IC5 – HN613128P 16K ROM chip containing BASIC interpreter (&0000 – &3FFF)
- IC6 to IC13 – 4116 RAM chips for the lower 16K of memory (&4000 – &7FFF)
- IC15 to IC22 – 4532 RAM chips for the upper 32K of memory (&8000 – &FFFF)
The board pictured (an Issue 2, 48K) is the model that I currently own. The transistor (TR6) soldered over the Z80 processor is a patch to fix a bug in the Issue 2 ULA. TR6 is a ZTX313 and is connected to pins 11 and 30 of IC2 (the Z80) and pin 33 of IC1 (the ULA).
Note that there are other variants with slightly different board layouts but the general principle of operation is the same.