Sinclair ZX80 General Maintenance
The Sinclair ZX80 General Maintenance page provides essential information for maintaining and troubleshooting the Sinclair ZX80 home computer. This guide covers various aspects of the ZX80, including architecture, storage, I/O, and common maintenance practices.
| Specifications | |
|---|---|
| Manufacturer | |
| Type | |
| CPU | |
| Memory | |
| Storage | |
| Display | |
| Sound | |
| OS / Firmware | |
🧠 Architecture
The ZX80 is built around a Z80A-class microprocessor, clocked at 3.25 MHz, and utilizes a simple architecture that allows for efficient operation despite its limitations.
Video generation
The video output is generated by the CPU itself, which can lead to flickering during BASIC program execution due to timing issues.
💾 Storage
The ZX80 primarily uses cassette tapes for data storage, with a baud rate of 300, making it relatively slow compared to modern standards.
🔌 I/O and Expansion
The ZX80 features an edge connector for expansion, allowing users to connect various peripherals and memory packs.
🔧 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).
- 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:
| Key | F mode |
K mode |
L mode
|
|---|---|---|---|
| P | PRINT |
P |
(
|
| R | RETURN |
R |
)
|
| G | GOTO |
G |
>
|
🔗 Edge-Connector Pin-out
| Pin | Signal | Description | Pin | Signal | Description |
|---|---|---|---|---|---|
| 1 | A15 | Address bus MSB | 23 | D4 | Data bit 4 |
| 2 | A14 | 24 | D5 | ||
| 3 | A13 | 25 | D6 | ||
| 4 | A12 | 26 | D7 | ||
| 5 | A11 | 27 | BUSACK | Bus acknowledge | |
| 6 | A10 | 28 | BUSRQ | Bus request (NC on ZX80) | |
| 7 | A9 | 29 | WAIT | Insert wait-states | |
| 8 | A8 | 30 | HALT | ||
| 9 | A7 | 31 | INT | ||
| 10 | A6 | 32 | NMI | ||
| 11 | A5 | 33 | RESET | ||
| 12 | A4 | 34 | M1 | Machine-cycle 1 | |
| 13 | A3 | 35 | RFSH | ||
| 14 | A2 | 36 | CLK | CPU clock (3.25 MHz) | |
| 15 | A1 | 37 | GND | ||
| 16 | A0 | 38 | +5 V | ||
| 17 | D0 | Data bit 0 | 39 | +9 V DC | |
| 18 | D1 | 40 | /MREQ | ||
| 19 | D2 |