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Correct CPU clock attribution: Sinclair's ZX80 circuit diagram labels the CPU clock 3·2 MHz, not 3.25 MHz; both figures now given with the derivation
 
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[[File:ZX80.jpg|250px|A fully–assembled Sinclair ZX80]]
[[File:ZX80 PCB Top.jpg|thumb|right|300px|Sinclair ZX80 main board]]


# Sinclair ZX80 General Maintenance
The '''[[Sinclair ZX80]]''' is a simple machine to look after. It has one internal supply rail, two electrolytic capacitors, no custom silicon and no battery. Most of what goes wrong with one is mechanical — keyboard tails, edge-connector contacts, dry joints — or comes in from the external power supply.


The **Sinclair ZX80** is a pioneering home computer that requires regular maintenance to ensure longevity and functionality. This guide provides essential maintenance tips, troubleshooting advice, and preventive measures to keep your ZX80 in optimal condition.
'''Observe ESD precautions''' when the board is out of the case. Use a wrist strap and an antistatic mat.


## Identify Your Board Revision
== What the ZX80 actually is ==
Understanding your board revision is crucial for maintenance and troubleshooting. The ZX80 has several revisions, each with unique characteristics and known issues. Refer to the following table for details:


{| class="wikitable styled-table" style="width:70%; text-align:center;"
Knowing this saves you from applying ZX81 or Spectrum advice to a machine that does not work that way.
|+'''ZX80 Board Revisions'''
 
! Revision !! Description !! Key Features
* '''There is no ULA.''' The ZX80's logic is twenty standard 74LS parts. Sinclair combined them into a single ULA to make the [[Sinclair ZX81|ZX81]]; nothing on a ZX80 board can be diagnosed as "a failed ULA".<ref name="kiodiff">''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; the ZX80's display blanks while a program runs.</ref><ref name="zx80cd" />
* '''The RAM is static.''' IC3 and IC4 are 2114 (1K × 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no −5 V and no +12 V rail anywhere on the machine.<ref name="zx80cd" />
* '''The regulator is IC22''', a 7805 bolted to an aluminium heatsink. It is not "IC3" — IC3 is a RAM chip.<ref name="zx80cd" />
 
{| class="wikitable styled-table" style="width:90%; text-align:left;"
|+'''ZX80 integrated circuits, from Sinclair's circuit diagram'''<ref name="zx80cd">Sinclair Research Ltd, ''ZX80 Circuit Diagram''. Hosted on this wiki as [[:File:Sinclair ZX80 circuit diagram.png]]. Source for the IC type list, the capacitor designators and values, the supply rails and every pin number quoted on this page.</ref>
! 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
|}
 
'''Board revisions.''' No Sinclair-published list of ZX80 board issues has been found, and the unsourced revision table that used to appear here has been removed rather than guessed at. Photographs of Issue 3 and Issue 3A boards are held in the Sinclair ZX Computers Archive.<ref name="kiopcb">''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.</ref> Your board's issue number is silkscreened on it — work from what is in front of you.
 
== Power supply ==
 
The ZX80 runs from an external, unregulated DC adaptor through a '''3.5 mm mono jack'''.
 
{| class="wikitable styled-table" style="width:90%; text-align:left;"
|+'''ZX80 external supply'''
! Property !! Value !! Source
|-
| Connector || 3.5 mm mono jack — the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one || <ref name="kio80" />
|-
| Polarity || '''Centre pin positive''', shaft ground || <ref name="kio80">''Sinclair ZX80 Connectors'', The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.</ref><ref name="zx80cd" />
|-
| Voltage || Nominal 9 V DC unregulated; about 14 V open-circuit. Sinclair's circuit diagram marks the input "9 V – 11 V" || <ref name="kio80" /><ref name="zx80cd" />
|-
| Current || Sinclair published a figure for the [[Sinclair ZX81|ZX81]] — not less than 600 mA, or 1.2 A if the ZX printer shares the supply — but no equivalent published figure for the ZX80 has been found || <ref name="zx81man" />
|}
 
'''Get the polarity right.''' A supply re-terminated the wrong way round is a known killer of the 5 V regulator on this family. On these machines the regulator usually dies and saves the rest of the board, but that is luck, not protection.<ref name="tv2" />
 
The adaptor itself contains a mains transformer and rectifier. There is no reason not to open one and service it — that is ordinary repair work — but the primary side is at mains potential and the reservoir capacitor holds charge after unplugging. Unplug it, wait, and verify the reservoir is discharged before putting a hand or a probe inside. On the closely related Spectrum supplies, shorted leads have been known to set the rectifier diodes on fire, so a lead in poor condition is worth replacing on sight.<ref name="clinic">"Micro Clinic", ''Television'' magazine, 1986 — collected repair notes on Sinclair machines, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures and their consequences.</ref>
 
== Supply and clock checkpoints ==
 
{| class="wikitable styled-table" style="width:90%; text-align:left;"
! Point !! Expect !! Source
|-
| Positive end of C8 || The incoming unregulated supply, nominally 9 V DC || <ref name="zx80cd" />
|-
| IC22 (7805) output || +5 V || <ref name="zx80cd" />
|-
| IC1 (Z80) pin 11 Vcc / pin 29 ground || +5 V || <ref name="zx80cd" />
|-
| IC2 (8332 ROM) pin 24 Vcc / pin 12 ground || +5 V || <ref name="zx80cd" />
|-
| IC3, IC4 (2114 RAM) pin 18 Vcc / pin 9 ground || +5 V || <ref name="zx80cd" />
|-
|-
| **Issue 1** || Original release (Feb 1980) || No solder mask; weak video sync
| IC1 (Z80) pin 6 || 3·2 MHz, as Sinclair's diagram labels it — the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz || <ref name="zx80cd" />
|-
|-
| **Issue 2** || Late 1980 revision || Added solder mask; improved video output
| IC1 (Z80) pin 26 || /RESET — held low briefly at switch-on by R21 (220 k) and C10 (1 µF), then high || <ref name="zx80cd" />
|}
|}


## Regular Cleaning
The edge connector carries +5 V, +9 V and 0 V alongside the Z80 bus; the full pinout is printed on Sinclair's circuit diagram.<ref name="zx80cd" /> Note that /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.<ref name="kio80" />
Dust and debris can accumulate inside the ZX80, affecting performance. Follow these steps for regular cleaning:
 
1. **Power Off**: Ensure the computer is powered off and unplugged.
== Capacitors ==
2. **Open the Case**: Carefully remove the case screws and lift the top cover.
3. **Dust Removal**: Use compressed air to blow out dust from the interior and components.
4. **Surface Wipe**: Use a soft, lint-free cloth dampened with isopropyl alcohol to clean surfaces.
5. **Reassemble**: Once clean, reassemble the case and ensure all screws are tightened.


## Power Supply
There are exactly two electrolytics: '''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.<ref name="zx80cd" />
The ZX80 requires a stable power supply for optimal operation. Check the following:
- **Voltage Output**: Ensure the power supply outputs 9V DC as required.
- **Connector Integrity**: Inspect the power connector for wear or damage.
- **Replace if Necessary**: If the power supply is faulty, replace it with a compatible unit.


## Capacitor Health
Full detail, including replacement guidance, is in the [[Sinclair ZX80 Capacitor Replacement Guide]]. See also [[Capacitor Failure Symptoms]].
Aging capacitors can lead to instability and failure. Regularly inspect and replace:
- **Electrolytic Capacitors**: Check for bulging or leakage. Replace with low-ESR capacitors rated for at least 105°C.
- **Testing**: Use a multimeter to test capacitor functionality if unsure.


## Known Failure-Prone ICs
== Preventive maintenance ==
Certain integrated circuits (ICs) are known to fail over time. Monitor the following:
 
- **IC12 (74LS05)**: Weak video sync issues may arise; consider replacing if symptoms appear.
# Unplug the supply. Measure its output and check its polarity '''before''' plugging it into the machine.
- **Regulator IC5**: Overheating can occur; ensure proper heatsinking or replace with a newer model.
# Open the case and inspect the board for corrosion, cracked tracks and burnt components.
# Inspect C8 and C10 for leakage or bulging.
# Inspect the keyboard tails at the connectors. They fail by cracking where they bend. Remove them straight, never at an angle, and never fold them double — a trapped, folded tail will go open circuit sooner or later.<ref name="tv2" /><ref name="smspec">''ZX Spectrum / ZX Spectrum+ Service Manual'', Sinclair Research Ltd, section 2.2 (assembly). Hosted on this wiki as [[:File:Sinclair ZX Spectrum Service Manual.pdf]]. Cited here for Sinclair's own warning that a trapped, folded keyboard ribbon will go open circuit — the same ribbons and the same failure occur across the ZX80, ZX81 and Spectrum.</ref>
# Clean dust with a soft brush. Clean the edge connector fingers.
# Reseat any socketed ICs.
# Inspect solder joints under magnification, particularly around the power jack, the regulator fixings and the edge connector. Look for splashes as well as dry joints: a hairline solder splash measures a few ohms and produces very misleading symptoms.<ref name="tv2">"Servicing Sinclair Computers, Part 2", ''Television'' magazine, 1986. Trade servicing article on the ZX80/ZX81 family, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: keyboard tail handling, solder-splash faults, reverse-polarity damage, piggy-backing technique.</ref>
# Check the regulator is still firmly bolted to its heatsink before reassembly.
 
== Things that are not faults ==
 
* '''The picture disappears while a program runs.''' The ZX80 cannot generate a TV signal and execute a program at the same time. The screen blanks while the program works and flickers when a key is pressed. This is how the machine was designed; the ZX81's SLOW mode is what fixed it.<ref name="kiodiff" />
* '''The regulator runs warm.''' It should. On the ZX81, Sinclair told kit builders that a regulator which never warms up means the supply is not reaching it; a regulator so hot you cannot bear to touch it means it is not properly bolted to its heatsink.<ref name="zx81man">Sinclair Research Ltd, ''ZX81 Service and Assembly Manual'', section 6 (fault finding) and section 9 (power supply specification). Hosted on this wiki as [[:File:Sinclair ZX81 Service and Assembly Manual.pdf]]. Cited on this page because Sinclair published no equivalent service document for the ZX80 and the two machines share the same supply arrangement and regulator; each such use is identified as a ZX81 figure.</ref>
 
== Common faults and first checks ==
 
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Where to look first
|-
| Completely dead, regulator cold || Supply output and polarity; the power jack solder joints; C8; IC22<ref name="zx81man" /><ref name="zx80cd" />
|-
| Works then cuts out, regulator very hot || Regulator not properly bolted to the heatsink<ref name="zx81man" />
|-
| No picture at all || Tuning first — sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30–R33 network feeding it<ref name="zx81man" /><ref name="kio80" /><ref name="zx80cd" />
|-
| Cursor on screen, keyboard does nothing || Keyboard tails seated in their connectors and not twisted; the matrix diodes D3–D10; IC10 (74LS365)<ref name="zx81man" /><ref name="zx80cd" />
|-
| Screen clears but no cursor || Unplug the supply, wait a few seconds, try again — then suspect the R21/C10 reset network<ref name="zx81man" /><ref name="zx80cd" />
|-
| Horizontal black and white stripes through the picture || The external supply: insufficient smoothing, or output too low on load<ref name="zx81man" />
|-
| Random resets or lockups with a RAM pack fitted || Edge-connector contacts and mechanical movement of the pack, before anything on the board
|}


## Voltage & Clock Test Points
For a symptom-by-symptom walk-through, see the [[Sinclair ZX80 Troubleshooting Guide]].
Regularly check voltage levels and clock signals to ensure proper operation:
- **Test Points**: Use a multimeter to measure voltage at designated test points on the PCB.
- **Clock Signal**: Verify the clock signal at the CPU to ensure it operates at 3.25 MHz.


## Essential Tools
== Tools ==
To perform maintenance effectively, gather the following tools:
- **Screwdriver Set**: For opening the case and securing components.
- **Multimeter**: For testing voltages and continuity.
- **Soldering Iron**: For replacing components.
- **Compressed Air**: For cleaning dust from the interior.


## Preventive Maintenance Checklist
See [[Recommended Tools]]. The ZX80-specific additions are a multimeter (for the supply check that should precede every session) and, if you intend to chase clock or video faults, an oscilloscope.
- [ ] Inspect and clean the interior every 6 months.
- [ ] Check power supply voltage regularly.
- [ ] Replace aging capacitors as needed.
- [ ] Monitor known failure-prone ICs.
- [ ] Test voltage and clock signals periodically.


## Quick-Fix Flowcharts
== Related pages ==
For common issues, refer to the following flowcharts to diagnose and resolve problems quickly.


### Unstable Picture
* [[Sinclair ZX80]]
1. Check power supply voltage.
* [[Sinclair ZX80 Capacitor Replacement Guide]]
2. Inspect C5 capacitor.
* [[Sinclair ZX80 Troubleshooting Guide]]
3. Replace if necessary.
* [[Sinclair ZX81 General Maintenance]]
* [[Capacitor Failure Symptoms]]
* [[Recommended Tools]]


### Reboots When Touched
== References ==
1. Inspect regulator tab for shorts.
2. Insulate or replace as needed.


### RAM Pack Wobble
<references />
1. Check edge connector for poor contact.
2. Install retention bracket if necessary.


By following this maintenance guide, you can ensure the longevity and reliability of your Sinclair ZX80. Regular checks and timely repairs will keep your vintage computer running smoothly for years to come.
[[Category:Sinclair Computers]]
[[Category:Maintenance Guides]]

Latest revision as of 17:54, 22 September 2026

Sinclair ZX80 main board

The Sinclair ZX80 is a simple machine to look after. It has one internal supply rail, two electrolytic capacitors, no custom silicon and no battery. Most of what goes wrong with one is mechanical — keyboard tails, edge-connector contacts, dry joints — or comes in from the external power supply.

Observe ESD precautions when the board is out of the case. Use a wrist strap and an antistatic mat.

What the ZX80 actually is

[edit | edit source]

Knowing this saves you from applying ZX81 or Spectrum advice to a machine that does not work that way.

  • There is no ULA. The ZX80's logic is twenty standard 74LS parts. Sinclair combined them into a single ULA to make the ZX81; nothing on a ZX80 board can be diagnosed as "a failed ULA".[1][2]
  • The RAM is static. IC3 and IC4 are 2114 (1K × 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no −5 V and no +12 V rail anywhere on the machine.[2]
  • The regulator is IC22, a 7805 bolted to an aluminium heatsink. It is not "IC3" — IC3 is a RAM chip.[2]
ZX80 integrated circuits, from Sinclair's circuit diagram[2]
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

Board revisions. No Sinclair-published list of ZX80 board issues has been found, and the unsourced revision table that used to appear here has been removed rather than guessed at. Photographs of Issue 3 and Issue 3A boards are held in the Sinclair ZX Computers Archive.[3] Your board's issue number is silkscreened on it — work from what is in front of you.

Power supply

[edit | edit source]

The ZX80 runs from an external, unregulated DC adaptor through a 3.5 mm mono jack.

ZX80 external supply
Property Value Source
Connector 3.5 mm mono jack — the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one [4]
Polarity Centre pin positive, shaft ground [4][2]
Voltage Nominal 9 V DC unregulated; about 14 V open-circuit. Sinclair's circuit diagram marks the input "9 V – 11 V" [4][2]
Current Sinclair published a figure for the ZX81 — not less than 600 mA, or 1.2 A if the ZX printer shares the supply — but no equivalent published figure for the ZX80 has been found [5]

Get the polarity right. A supply re-terminated the wrong way round is a known killer of the 5 V regulator on this family. On these machines the regulator usually dies and saves the rest of the board, but that is luck, not protection.[6]

The adaptor itself contains a mains transformer and rectifier. There is no reason not to open one and service it — that is ordinary repair work — but the primary side is at mains potential and the reservoir capacitor holds charge after unplugging. Unplug it, wait, and verify the reservoir is discharged before putting a hand or a probe inside. On the closely related Spectrum supplies, shorted leads have been known to set the rectifier diodes on fire, so a lead in poor condition is worth replacing on sight.[7]

Supply and clock checkpoints

[edit | edit source]
Point Expect Source
Positive end of C8 The incoming unregulated supply, nominally 9 V DC [2]
IC22 (7805) output +5 V [2]
IC1 (Z80) pin 11 Vcc / pin 29 ground +5 V [2]
IC2 (8332 ROM) pin 24 Vcc / pin 12 ground +5 V [2]
IC3, IC4 (2114 RAM) pin 18 Vcc / pin 9 ground +5 V [2]
IC1 (Z80) pin 6 3·2 MHz, as Sinclair's diagram labels it — the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz [2]
IC1 (Z80) pin 26 /RESET — held low briefly at switch-on by R21 (220 k) and C10 (1 µF), then high [2]

The edge connector carries +5 V, +9 V and 0 V alongside the Z80 bus; the full pinout is printed on Sinclair's circuit diagram.[2] Note that /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.[4]

Capacitors

[edit | edit source]

There are exactly two electrolytics: 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.[2]

Full detail, including replacement guidance, is in the Sinclair ZX80 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

Preventive maintenance

[edit | edit source]
  1. Unplug the supply. Measure its output and check its polarity before plugging it into the machine.
  2. Open the case and inspect the board for corrosion, cracked tracks and burnt components.
  3. Inspect C8 and C10 for leakage or bulging.
  4. Inspect the keyboard tails at the connectors. They fail by cracking where they bend. Remove them straight, never at an angle, and never fold them double — a trapped, folded tail will go open circuit sooner or later.[6][8]
  5. Clean dust with a soft brush. Clean the edge connector fingers.
  6. Reseat any socketed ICs.
  7. Inspect solder joints under magnification, particularly around the power jack, the regulator fixings and the edge connector. Look for splashes as well as dry joints: a hairline solder splash measures a few ohms and produces very misleading symptoms.[6]
  8. Check the regulator is still firmly bolted to its heatsink before reassembly.

Things that are not faults

[edit | edit source]
  • The picture disappears while a program runs. The ZX80 cannot generate a TV signal and execute a program at the same time. The screen blanks while the program works and flickers when a key is pressed. This is how the machine was designed; the ZX81's SLOW mode is what fixed it.[1]
  • The regulator runs warm. It should. On the ZX81, Sinclair told kit builders that a regulator which never warms up means the supply is not reaching it; a regulator so hot you cannot bear to touch it means it is not properly bolted to its heatsink.[5]

Common faults and first checks

[edit | edit source]
Symptom Where to look first
Completely dead, regulator cold Supply output and polarity; the power jack solder joints; C8; IC22[5][2]
Works then cuts out, regulator very hot Regulator not properly bolted to the heatsink[5]
No picture at all Tuning first — sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30–R33 network feeding it[5][4][2]
Cursor on screen, keyboard does nothing Keyboard tails seated in their connectors and not twisted; the matrix diodes D3–D10; IC10 (74LS365)[5][2]
Screen clears but no cursor Unplug the supply, wait a few seconds, try again — then suspect the R21/C10 reset network[5][2]
Horizontal black and white stripes through the picture The external supply: insufficient smoothing, or output too low on load[5]
Random resets or lockups with a RAM pack fitted Edge-connector contacts and mechanical movement of the pack, before anything on the board

For a symptom-by-symptom walk-through, see the Sinclair ZX80 Troubleshooting Guide.

Tools

[edit | edit source]

See Recommended Tools. The ZX80-specific additions are a multimeter (for the supply check that should precede every session) and, if you intend to chase clock or video faults, an oscilloscope.

[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 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; the ZX80's display blanks while a program runs.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 Sinclair Research Ltd, ZX80 Circuit Diagram. Hosted on this wiki as File:Sinclair ZX80 circuit diagram.png. Source for the IC type list, the capacitor designators and values, the supply rails and every pin number quoted on this page.
  3. ↑ 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.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Sinclair ZX80 Connectors, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Sinclair Research Ltd, ZX81 Service and Assembly Manual, section 6 (fault finding) and section 9 (power supply specification). Hosted on this wiki as File:Sinclair ZX81 Service and Assembly Manual.pdf. Cited on this page because Sinclair published no equivalent service document for the ZX80 and the two machines share the same supply arrangement and regulator; each such use is identified as a ZX81 figure.
  6. ↑ 6.0 6.1 6.2 "Servicing Sinclair Computers, Part 2", Television magazine, 1986. Trade servicing article on the ZX80/ZX81 family, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: keyboard tail handling, solder-splash faults, reverse-polarity damage, piggy-backing technique.
  7. ↑ "Micro Clinic", Television magazine, 1986 — collected repair notes on Sinclair machines, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures and their consequences.
  8. ↑ ZX Spectrum / ZX Spectrum+ Service Manual, Sinclair Research Ltd, section 2.2 (assembly). Hosted on this wiki as File:Sinclair ZX Spectrum Service Manual.pdf. Cited here for Sinclair's own warning that a trapped, folded keyboard ribbon will go open circuit — the same ribbons and the same failure occur across the ZX80, ZX81 and Spectrum.
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