Sinclair ZX80 · ZX81 · QL

The Sinclairs other than the Spectrum: the ultra-cheap pioneers ZX80 (1980) and ZX81 (1981, 1 KB of RAM!), and the pro QL (1984, 68008, Microdrive). Standard Sinclair repair: a hot regulator, RAM, membrane keyboard.

Sinclair ZX81 — photo by Rama, CC BY-SA 2.0 fr (Wikimedia Commons).

Identification

Model	Year	CPU	Note
ZX80	1980	Z80 @ 3.25 MHz	all white, screen flickers while computing ("FAST/SLOW")
ZX81	1981	Z80	1 KB RAM, single ULA, 16 KB "wobble" pack
QL (Quantum Leap)	1984	68008	128 KB, two endless-loop Microdrives

Repair (technician)

• 7805 regulator: runs very hot (ZX81) → heatsink / replace (the "unstable/dies when hot" fault).
• ULA (ZX81): sensitive custom chip → substitution (modern reissues exist).
• "RAM pack wobble" (ZX81 + 16 KB): bad contact at the rear connector → clean, brace, redo joints.
• Membrane keyboard: broken flex → membrane replacement (a common remade part).
• QL — Microdrive: degraded tapes and pressure pads → clean head, replace pad; sound-ULA caps.
• Light recap + check the external PSU → Recap.

Procedure — a dead computer, step by step ✅

What this adds

Ordered for a home computer: what kills these is its power supply, its battery, its capacitors, its socketed RAM and its keyboard — roughly in that order, and well before the CPU. → PCB diagnosis

1. The power supply — measured before it is plugged in. This is the one step not to swap around. On a machine of this age a supply does not simply stop: a 5 V regulator that fails high puts 7, 9, 12 V onto the bus and takes the RAM and the custom chips with it. Measure every rail off the machine, under a dummy load if you can, and only then plug it in.

2. The battery, if it has one. Any RTC or backup cell soldered to a board is a future leak. If yours has one and it has never been changed, take it out before it takes the tracks with it. → Backup batteries

3. Capacitors. Recap is the default on these machines, not a last resort — and on the supply first, since that is what protects everything else. → Recap

4. RAM, on its sockets. These machines were built to be repaired: the RAM, and often the logic, sit in sockets. Reseat everything first — thirty years of oxide is a classic no-boot. Then swap chips: a bad RAM chip usually gives a repeatable pattern of garbage or a screen frozen at a fixed character.

5. The keyboard. A console does not have one; a computer's is a wear part. A membrane keyboard fails as whole dead rows or columns — that is the membrane, not the machine. A mechanical one wants its contacts cleaning, not replacing. Check this page: the failure mode is often documented per model.

6. Storage — tape or disk. Both often come down to the same things: a dirty head, and for tape the azimuth and a belt turned to tar. ⚠️ On the floppy side, check whether the mechanism actually has a belt before looking for one: many do, but some drives are direct drive, and hunting a belt that isn't there means stripping a drive for nothing. A machine that boots but loads nothing is a drive fault, not a logic fault — do not go looking on the board for it.

7. Video output. RF, composite, RGB or a dedicated monitor, depending on the machine (see Identification). Test with something else before you conclude: a period modulator and a modern TV disagree far more often than either is broken.

8. Still nothing. Now the logic: rails on the board, reset, clock, bus. → PCB diagnosis

Common faults

Symptom	Likely cause	Lead
Crashes when the machine is moved	RAM pack wobble	Clean/brace the connector
Unstable / dies when hot	7805 regulator	Heatsink / replace
No picture	ULA	Substitute
Dead keys	membrane	Replace the membrane
Microdrive unreadable (QL)	tape / pad	Clean, replace pad

Modernisation & mods — what to buy, how

Fragile parts: ZX81 ULA, membrane, QL Microdrives

ZX81: a dead ULA is replaced by a reissue ("vLA/ZX8x"); remade membrane keyboard. QL: Minerva ROM (fixes bugs), revised 7805 regulator + heatsink; the Microdrives (endless-loop tapes) are the weak point → move to SD.

Modern storage - ZX81: ZXpand 🟢 (edge-connector board) = SD reader (loads .p), 32/56 KB RAM and sometimes AY sound → kills the cassette and the famous "RAM pack wobble". Otherwise audio from a PC/phone 🟢 (.p → WAV). - QL: QL-SD 🟢 (SD card in the ROM port), QUBIDE 🟡 (IDE/CF), or vDriveQL 🟢 which emulates the Microdrives; Gotek + FlashFloppy 🟡 via a disk interface.

Video - ZX81: UHF output → composite mod 🟡 (sharp on a modern display), then RGB2HDMI downstream. - QL: RGB output 🟢 → SCART then OSSC / RetroTINK or RGB2HDMI.

⚠️ Do not host ROMs/games — method only.

Archived technical documentation ✅

Manufacturer documentation archived locally: schematics, block diagrams, expected voltages, parts lists. The reference when a symptom matches no known fault.

• ZX81 — PDF
• Timex Sinclair 1000 — PDF
• Sinclair QL — PDF
• ZX80 — PDF

Status

Manufacturer documents kept for preservation (machines out of production for decades), with no explicit licence — the same position as for the cabinet manuals already archived here.

ZX81 schematics and pinouts ✅

Machine schematics and pinouts for the RAMs (2114, 4118/4801, 4816) and the ULA.

Provenance

Schematics of undocumented origin in the collection they came from: no author and no licence is stated there.

See also

• ZX Spectrum (the big brother) · Oric · Recap.

Sources & attribution

• Photo Sinclair ZX81 — Rama, CC BY-SA 2.0 fr, Wikimedia Commons — https://commons.wikimedia.org/wiki/File:Sinclair-ZX81-Rama.jpg
• Wikipedia — ZX80 / ZX81 / Sinclair QL (Z80/68008, ULA, Microdrive) — CC-BY-SA — https://en.wikipedia.org/wiki/ZX81
• Established Sinclair community knowledge (RAM pack wobble, 7805, membrane) — synthesis.

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.