Sinclair ZX80

The Sinclair ZX80 is an 8-bit home computer marketed by Sir Clive Sinclair’s Science of Cambridge (soon renamed Sinclair Research) as *“the first personal computer for under one hundred pounds.”* Released in January 1980, it ignited the UK micro-boom and sold about 100 000 units worldwide despite numerous limitations – most famously its blanking video that disappears while BASIC is running code.

Sinclair ZX80

A fully–assembled ZX80 with 9 V PSU and UHF TV lead
Specifications
Manufacturer	Science of Cambridge Ltd (later Sinclair Research Ltd)
Type	Home computer / hobbyist kit
Released	29 January 1980
Discontinued	March 1981 (superseded by ZX81)
Intro price	£79 .95 kit / £99 .95 ready-built (≈US$199/249 in 1980)
CPU	Zilog Z80A-compatible (NEC µPD780C-1) @ 3.25 MHz
Memory	1 KB static RAM on-board (expandable to 16 KB, 48 KB with mods)
Storage	External cassette tape (300 baud, EAR/MIC jacks)
Display	Monochrome RF to TV – 32 × 24 text or 64 × 48 block-graphics
Sound	None (no audio hardware)
Dimensions	195 mm × 175 mm × 50 mm (7.7 ″ × 6.9 ″ × 2.0 ″)
Weight	≈ 340 g (0.75 lb)
OS / Firmware	4 KB Sinclair Integer BASIC in ROM
Predecessor	Science of Cambridge MK14
Successor	Sinclair ZX81
Codename	“ZX80 project”
Model no.	ZX80

🧠 Architecture

• CPU – Z80A-class microprocessor clocked at 3.25 MHz (half the UK colour-burst for simple TV timing).
• ROM – 4 KB mask ROM containing Sinclair Integer BASIC, line-editor and I/O routines.
• RAM – 1 KB of 2114 static RAM; addressable to 48 KB via the rear expansion bus (commonly 16 KB DRAM pack).
• Logic – Built entirely from 18 inexpensive 74-series TTL ICs; no custom chips – video, keyboard scan and cassette I/O are all done in firmware.

Video generation

The Z80 itself bit-bangs UHF video: during the FAST display loop it outputs a sync pulse then immediately fetches the next character row from RAM, interleaving code and picture. When the CPU is diverted to BASIC interpretation the timing breaks, so the TV loses sync – producing the ZX80’s trademark flicker. (The later ZX81 added a SLOW mode with hardware assistance to cure this.)

💾 Storage

Load / SAVE uses the 300 baud Kansas City Standard with simple square waves on 3.5 mm EAR/MIC sockets. Program listings average ~4 seconds per KB.

🔌 I/O and Expansion

• Edge connector (44-way) – exposes full Z80 bus, power and video lines.
• 9 V DC input – un-regulated (internally regulated to 5 V).
• RF modulator – channel 36 PAL TV.

Commercial add-ons included:

*1–3 KB SRAM packs, 16 KB DRAM pack*

*ZX Printer, joystick adapters, floppy-drive interfaces, composite video mods.*

🛠️ General Maintenance

Issue	Cause	Remedy
Unstable picture / snow	Ageing electrolytic C5 (47 µF) on video rail	Replace with low-ESR 105 °C part
Reboots when touched	Loose 7805 regulator tab shorts to case	Insulate tab / fit TO-220 spacer
RAM pack “wobble” resets	Leverage on edge bus ⇒ poor contact	Fit retention bracket or right-angle header
Overheating	No ventilation slots (black “stripes” are cosmetic)	Drill discreet vents or run uncased board

Full recapping, regulator heatsinking and replacement of the brittle membrane keyboard are popular reliability upgrades.

📜 Schematic & PCB

The complete single-layer PCB schematic is freely available (public-domain scan of Sinclair service sheet 322-10).

Sinclair ZX80 circuit diagram

Original Issue 1 board – note hand-drawn track work

🔧 Common Modifications

• 8 KB ZX81 ROM upgrade – drops-in to give floating-point BASIC, SAVE/LOAD VERIFY and better editing (still no SLOW mode).
• Composite video mod – bypass RF can for crisp LCD/monitor output.
• Full-travel keyboard – external matrix wired to KB pins 8-15.
• Internal 16 KB SRAM – replaces external pack, eliminating “wobble.”
• Turbo crystal (3.58 MHz) – simple Xtal swap for ≈10 % speed gain (requires retune TV).

🔢 ROM Keyword Map

Each key performs multiple functions depending on mode :

Example ZX80 keyboard layer

Key	F mode	K mode	L mode
P	PRINT	P	(
R	RETURN	R	)
G	GOTO	G	>

🗂️ Related Pages

• Sinclair ZX81 – successor with improved video logic.
• Sinclair ZX Spectrum – coloured 1982 evolution of the Sinclair line.
• Sinclair Computers General Maintenance