Sinclair ZX80 General Maintenance
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.
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| A fully–assembled Sinclair ZX80 | |
| 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| Discontinued | March 1981 (superseded by ZX81) |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 |
The ZX80 arrived in February 1980—just months before the BBC Micro project was even conceived—and became the first consumer microcomputer to retail for under £100 in the United Kingdom.<ref>Mail-order advert, Practical Computing, March 1980.
- Launch prices (1980): £79.95 in kit form, £99.95 fully assembled (plus £4 P&P).
- Sinclair’s glossy national-press adverts boasted “The computer you can afford to own” and invited readers to “own a real computer for less than the price of a colour TV.”
- Production was handled by Timex’s Scottish plant in Dundee; the first 10 000 boards were delivered in six weeks.
- Estimated lifetime sales: contemporary interviews with Clive Sinclair quote “50–70 000 kits” and around “30 000 ready-built” units, giving an aggregate of ≈ 100 000 ZX80s worldwide before the ZX81 superseded it in March 1981.
🧠 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.
🔧 Known PCB Revisions & Errata
| Issue | Visible Clues | Key Fixes / Changes |
|---|---|---|
| Issue 1 (Feb 1980) | “ZX80 Issue 1” silkscreen; no solder mask between edge pads. | • Missing pull-ups on IC12 (74LS05) video drivers caused weak sync on some TVs.
• Regulator IC5 runs hot—Sinclair added stick-on heatsink in later batches. |
| Issue 2 (late 1980) | Added green solder mask; extra wire-link near IC12. | • 4× 1 kΩ SIL resistor pack RP1 installed for stronger TTL drive.
• Board edge widened 1 mm to fit Timex card guides. |
🛠️ 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
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🔧 Common Modifications
- 8 KB ZX81 ROM upgrade – drops-in to give floating-point BASIC, SAVE/LOAD VERIFY and better editing (still no SLOW mode).