Sinclair ZX80
The Sinclair ZX80 is an 8-bit home computer sold from January 1980 by Science of Cambridge, the company that became Sinclair Research. It was sold as a kit at £79.95 and ready-built at £99.95, and it was the first computer most people in Britain could imagine buying.
 | |
| Sinclair ZX80 logic board. Twenty-two ICs, all standard parts — there is no ULA on this machine | |
| Specifications | |
|---|---|
| Manufacturer | Science of Cambridge Ltd (later Sinclair Research Ltd) |
| Type | Home computer, sold as a kit or ready-built |
| Released | January 1980 |
| Discontinued | 1981 (superseded by the ZX81) |
| Intro price | £79.95 kit, £99.95 ready-built |
| CPU | Z80-class processor (Sinclair's diagram marks IC1 "780-1"), clocked at 3.25 MHz — the 6.5 MHz oscillator X1 divided by two |
| Memory | 1 KB static RAM — two 2114 chips at IC3 and IC4. Expandable through the rear edge connector |
| Storage | External cassette recorder through 3.5 mm EAR and MIC jacks |
| Display | Monochrome RF to a television. The display blanks while a program runs — this is normal, not a fault |
| Sound | None |
| Power | External unregulated DC adaptor through a 3.5 mm mono jack, centre pin positive. Nominal 9 V; Sinclair's diagram marks the input "9 V – 11 V" |
| OS / Firmware | 4 KB Sinclair integer BASIC in mask ROM (IC2, an 8332) |
| Successor | Sinclair ZX81 |
| Model no. | ZX80 |
Sinclair published no service manual for it. What exists is the circuit diagram, which is hosted on this wiki, plus trade servicing articles from the period and the community archives. Every figure on the ZX80 pages here is traced to one of those; where nothing could be traced, the figure has been removed rather than guessed at.[1]
For servicing see Sinclair ZX80 General Maintenance, Sinclair ZX80 Troubleshooting Guide and Sinclair ZX80 Capacitor Replacement Guide.
What the ZX80 actually is
editThree things distinguish it from the machines that followed, and all three matter when repairing one:[1][2]
- There is no ULA. The ZX80's logic is ordinary 74LS parts. Sinclair combined them into a single ULA to make the ZX81 — so nothing on a ZX80 board can be diagnosed as "a failed ULA", and ZX81 repair advice does not transfer.
- The RAM is static, two 2114 chips giving 1 KB. Because there is no DRAM there is no −5 V and no +12 V rail anywhere on the machine — the only rails are the incoming unregulated supply and the +5 V the regulator makes from it.
- The video is generated by the processor. There is no display hardware in the modern sense; the Z80 emits sync and picture itself, which is why the screen blanks whenever BASIC is actually doing something.
The IC complement
editFrom Sinclair's own circuit diagram:[1]
| Ref | Type | Ref | Type |
|---|---|---|---|
| IC1 | 780-1 (Z80 CPU) | IC12 | 74LS00 |
| IC2 | 8332 — 4 KB mask ROM | IC13 | 74LS04 |
| IC3 | 2114 RAM | IC14 | 74LS05 |
| IC4 | 2114-1 RAM | IC15 | 74LS05 |
| IC5 | 74LS373 | IC16 | 74LS10 |
| IC6 | 74LS157 | IC17 | 74LS32 |
| IC7 | 74LS157 | IC18 | 74LS74 |
| IC8 | 74LS157 | IC19 | 74LS74 |
| IC9 | 74LS166 | IC20 | 74LS86 |
| IC10 | 74LS365 | IC21 | 74LS93 |
| IC11 | 74LS00 | IC22 | 7805 regulator |
The regulator is IC22, not IC3. IC3 is a RAM chip. Getting this wrong is how people end up replacing memory on a machine with a power fault.[1]
Power
editAn external unregulated adaptor feeds a 3.5 mm mono jack, centre pin positive. It is the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one. Nominal 9 V; about 14 V open-circuit; Sinclair's diagram marks the input "9 V – 11 V".[3][1]
Get the polarity right. A lead re-terminated the wrong way round is a known killer of the regulator on this family.[4]
Full detail, including what to check and in what order, is on the Sinclair ZX80 General Maintenance page.
Storage
editCassette, through 3.5 mm EAR and MIC jacks. There is no disc interface and Sinclair sold none for this machine.
Expansion
editA rear edge connector carries the Z80 bus along with +5 V, +9 V and 0 V. The full pinout is printed on Sinclair's circuit diagram, and that is where to read it from — an earlier version of this page carried a pin-by-pin table that was not Sinclair's and is now removed.[1]
One difference from the ZX81 is worth knowing before you plug anything in: /ROMCS does not exist on the ZX80 bus. That input was added for the ZX81, so ZX81-era expansion hardware that relies on it will not behave the same way on a ZX80.[3]
Board revisions
editNo Sinclair-published list of ZX80 board issues has been found. Photographs of Issue 3 and Issue 3A boards are held in the Sinclair ZX Computers Archive, and that is the only board-issue evidence located.[5] Your board's issue number is silkscreened on it — work from what is in front of you.
An earlier version of this page carried a table of "Issue 1 (February 1980)" and "Issue 2 (late 1980)" boards with specific changes attributed to each: missing pull-ups on IC12, a resistor pack RP1 added, the board edge widened by 1 mm to fit Timex card guides. None of it could be traced to any source, and the IC12 claim contradicts Sinclair's own diagram, which shows IC12 as a 74LS00 rather than a video driver. The table has been removed.
Capacitors
editThere are exactly two electrolytics on the board: C8, 22 µF 16 V, across the unregulated input, and C10, 1 µF, in the power-on reset network. Everything else is ceramic and is not a service item.[1]
An earlier version of this page named "ageing electrolytic C5 (47 µF) on the video rail" as a cause of unstable picture. There is no C5 electrolytic, no 47 µF part and no video rail on this machine. See the Sinclair ZX80 Capacitor Replacement Guide.
Things that were on this page and should not have been
editThis page was rebuilt during a verification drive. The following were removed because they could not be traced to any source, and in several cases are contradicted by Sinclair's own circuit diagram:
- A memory map giving the ROM as 8 KB at 0000–1FFF. The ZX80's ROM is 4 KB — IC2 is an 8332 — and the page's own infobox said so. The 8 KB figure belongs to the ZX81.[1]
- A table of ROM entry points including a floating-point calculator at 03B5. The ZX80 runs integer BASIC and has no floating-point arithmetic at all. A "reset" vector at 1C00 is equally impossible: a Z80 resets to 0000.
- A 44-way edge connector pinout allocating signals in a tidy sequential run. Read the pinout off Sinclair's diagram instead. The table also gave /ROMCS on pin 21, and /ROMCS does not exist on this machine's bus.[3]
- "CPU clock 3.25 MHz, ÷13 from a 13 MHz crystal" and "half the UK colour burst". The oscillator is 6.5 MHz and it is divided by two; the PAL colour burst is 4.43 MHz and has nothing to do with it.[1]
- A "turbo crystal" modification claiming a 10 per cent speed gain from fitting a 3.58 MHz part. On a machine whose video timing is generated by the processor itself, changing the clock does not make it faster — it stops the television synchronising.
- "Drill discreet vents" in the case as a remedy for overheating. Do not drill holes in a forty-five-year-old computer. If the regulator runs hot, that is what its heatsink is for; if it is too hot, look for a fault.
- A "Woolmark-approved polyester" keyboard supplier story, a Guinness World Record, and a "SLOW token present in the ZX80 ROM". None of these could be traced anywhere.
Related Pages
edit- Sinclair ZX80 General Maintenance · Sinclair ZX80 Troubleshooting Guide · Sinclair ZX80 Capacitor Replacement Guide
- Sinclair ZX81 — the successor, where the TTL became a ULA
- Capacitor Failure Symptoms · Recommended Tools
References
edit- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Sinclair Research Ltd, ZX80 Circuit Diagram. Hosted on this wiki as File:Sinclair ZX80 circuit diagram.png. Source for the IC type list, the 4 KB 8332 ROM, the 2114 static RAM, the 7805 at IC22, the 6.5 MHz oscillator X1 divided by two, the supply rails, the two electrolytics C8 and C10, and the edge-connector pinout. This is the only Sinclair-published technical document located for the machine; Sinclair issued no service manual for the ZX80.
- ↑ Differences between ZX80, ZX81 and Timex 1000, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: the ZX80's standard TTL logic was combined into one ULA for the ZX81, and the ZX80's display blanks while a program runs.
- ↑ 3.0 3.1 3.2 Sinclair ZX80 Connectors, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage, and the absence of /ROMCS from the ZX80 bus.
- ↑ "Servicing Sinclair Computers, Part 2", Television magazine, 1986. Trade servicing article covering the ZX80 and ZX81 family, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: reverse-polarity damage and handling practice.
- ↑ Sinclair ZX80 — PCB, The Sinclair ZX Computers Archive, k1.spdns.de: photographs of ZX80 Issue 3 and Issue 3A boards. Secondary source; the only board-issue evidence located.