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Correct CPU clock attribution: Sinclair's ZX80 circuit diagram labels the CPU clock 3ยท2 MHz, not 3.25 MHz; both figures now given with the derivation
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The Sinclair ZX80 is a classic home computer, and with careful maintenance, it can remain reliable for decades. This guide covers essential care, preventive steps, and troubleshooting for all ZX80 board revisions.ย 
[[File:ZX80 PCB Top.jpg|thumb|right|300px|Sinclair ZX80 main board]]
'''Always observe ESD (Electrostatic Discharge) precautions''' when handling the PCB or componentsโ€”use a grounded wrist strap and work on an antistatic mat.ย 
Regular maintenance intervals:ย 
* '''Visual inspection:''' Every 12 monthsย 
* '''Capacitor check:''' Every 2โ€“3 yearsย 
* '''Full electrical test:''' Every 5 years or after storage


<templatestyles src="Template:StyledTable/styles.css" />
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 โ€” keyboard tails, edge-connector contacts, dry joints โ€” or comes in from the external power supply.
[[File:ZX80_PCB_Top.jpg|thumb|right|300px|Sinclair ZX80 Mainboard (Issue 2)]]
ย 
'''Observe ESD precautions''' when the board is out of the case. Use a wrist strap and an antistatic mat.
ย 
== What the ZX80 actually is ==
ย 
Knowing this saves you from applying ZX81 or Spectrum advice to a machine that does not work that way.
ย 
* '''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 ร— 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no โˆ’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" โ€” IC3 is a RAM chip.<ref name="zx80cd" />


== Board Revisions ==
{| class="wikitable styled-table" style="width:90%; text-align:left;"
{| class="wikitable styledtable"
|+'''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>
! Revision !! PCB Markings !! Notable Differences !! Common Issues
! 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
|-
|-
| Issue 1 || "ZX80 ISSUE ONE" silk || Early production, hand-wired mods || Poor video sync, regulator overheating
| IC5 || 74LS373 || IC16 || 74LS10
|-
|-
| Issue 2 || "ZX80 ISSUE TWO" silk || Improved video, revised regulator layout || Keyboard connector wear, RAM instability
| 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
|}
|}


== Power Supply & Regulator Care ==
'''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 โ€” 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 โ€” work from what is in front of you.
The ZX80 uses a simple linear power supply:
ย 
* '''PSU Part:''' Sinclair 9V DC Adapter (center negative)
== Power supply ==
* '''Specs:''' 9V DC, 700โ€“1000mA, 2.1mm barrel
* '''On-board Regulator:''' 7805 (IC3) 5V linear
* '''Common Failure Modes:'''
** PSU cable splits or shorts
** 7805 overheating or output drift
** Capacitor (C4, C5) leakage or bulging


'''On-board Capacitor Designators:'''
The ZX80 runs from an external, unregulated DC adaptor through a '''3.5 mm mono jack'''.
* '''C4, C5:''' Smoothing and decoupling for 7805
* '''C1, C2, C3:''' Logic supply filtering


== Capacitor Replacement Table ==
{| class="wikitable styled-table" style="width:90%; text-align:left;"
{| class="wikitable styledtable"
|+'''ZX80 external supply'''
! Position !! Value !! Voltage !! Notes !! Series/Type
! Property !! Value !! Source
|-
| C1 || 22ฮผF || 16V || Logic rail smoothing || Axial, low ESR
|-
|-
| C2 || 1ฮผF || 16V || Video circuit || Tantalum or electrolytic
| Connector || 3.5 mm mono jack โ€” the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one || <ref name="kio80" />
|-
|-
| C3 || 0.1ฮผF || 50V || Noise bypass || Ceramic disc
| 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" />
|-
|-
| C4 || 100ฮผF || 16V || 7805 input || Axial, low ESR
| Voltage || Nominal 9 V DC unregulated; about 14 V open-circuit. Sinclair's circuit diagram marks the input "9 V โ€“ 11 V" || <ref name="kio80" /><ref name="zx80cd" />
|-
|-
| C5 || 100ฮผF || 16V || 7805 output || Axial, low ESR
| Current || Sinclair published a figure for the [[Sinclair ZX81|ZX81]] โ€” not less than 600 mA, or 1.2 A if the ZX printer shares the supply โ€” but no equivalent published figure for the ZX80 has been found || <ref name="zx81man" />
|}
|}


== Voltage & Clock Test Points ==
'''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" />
{| class="wikitable styledtable"
ย 
! Test Point !! IC/Pin !! Expected Value (ยฑTolerance)
The adaptor itself contains a mains transformer and rectifier. There is no reason not to open one and service it โ€” that is ordinary repair work โ€” 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 โ€” 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>
ย 
== Supply and clock checkpoints ==
ย 
{| class="wikitable styled-table" style="width:90%; text-align:left;"
! Point !! Expect !! Source
|-
|-
| 5V Rail || IC3 (7805) Output || 5.00V ยฑ0.15V
| Positive end of C8 || The incoming unregulated supply, nominally 9 V DC || <ref name="zx80cd" />
|-
|-
| 9V Input || C4 (+) || 9.0V ยฑ0.5V
| IC22 (7805) output || +5 V || <ref name="zx80cd" />
|-
|-
| CPU Clock || IC1 (Z80) Pin 6 || 3.25MHz ยฑ0.05MHz
| IC1 (Z80) pin 11 Vcc / pin 29 ground || +5 V || <ref name="zx80cd" />
|-
|-
| Video Sync || IC5 Pin 3 || 5Vpp square wave
| 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" />
|-
| IC1 (Z80) pin 6 || 3ยท2 MHz, as Sinclair's diagram labels it โ€” the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz || <ref name="zx80cd" />
|-
| IC1 (Z80) pin 26 || /RESET โ€” held low briefly at switch-on by R21 (220 k) and C10 (1 ยตF), then high || <ref name="zx80cd" />
|}
|}


== Essential Tools ==
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 โ€” 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" />
* Antistatic wrist strap & mat
* Multimeter (with continuity and voltage)
* Oscilloscope (โ‰ฅ10MHz, for clock/video)
* Soldering iron (fine tip, ESD safe)
* Solder sucker or wick
* Small flat and Phillips screwdrivers
* IC extractor
* Fine tweezers
* Isopropyl alcohol & brush (for cleaning)


== Preventive Maintenance Checklist ==
== Capacitors ==
# Power off and unplug the unit.
# Discharge static and open the case.
# Inspect PCB for corrosion, broken traces, or burnt components.
# Check all capacitors for bulging or leakage.
# Verify keyboard membrane and connector for cracks.
# Clean dust and debris with a soft brush.
# Test PSU output voltage before connecting.
# Reseat all socketed ICs gently.
# Inspect solder joints for cracks ("dry joints").
# Reassemble and test operation.


== Common Faults & Quick Checks ==
There are exactly two electrolytics: '''C8''' (22 ยตF, 16 V) across the unregulated input, and '''C10''' (1 ยตF) in the power-on reset network. Everything else is ceramic and is not a service item.<ref name="zx80cd" />
{| class="wikitable styledtable"
ย 
! Symptom !! Likely Cause !! Quick Check
Full detail, including replacement guidance, is in the [[Sinclair ZX80 Capacitor Replacement Guide]]. See also [[Capacitor Failure Symptoms]].
ย 
== Preventive maintenance ==
ย 
# Unplug the supply. Measure its output and check its polarity '''before''' plugging it into the machine.
# 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 โ€” 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 โ€” 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.
ย 
== Things that are not faults ==
ย 
* '''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>
ย 
== Common faults and first checks ==
ย 
{| 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 power || PSU or 7805 failure || Measure 9V in, 5V out at IC3
| No picture at all || Tuning first โ€” sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30โ€“R33 network feeding it<ref name="zx81man" /><ref name="kio80" /><ref name="zx80cd" />
|-
|-
| No video || C2, IC5, or ULA fault || Scope video out, check C2
| Cursor on screen, keyboard does nothing || Keyboard tails seated in their connectors and not twisted; the matrix diodes D3โ€“D10; IC10 (74LS365)<ref name="zx81man" /><ref name="zx80cd" />
|-
|-
| Keyboard dead || Membrane or connector || Inspect for cracks, reseat
| Screen clears but no cursor || Unplug the supply, wait a few seconds, try again โ€” then suspect the R21/C10 reset network<ref name="zx81man" /><ref name="zx80cd" />
|-
|-
| Random resets || C1, C4, or RAM || Replace caps, test RAM
| Horizontal black and white stripes through the picture || The external supply: insufficient smoothing, or output too low on load<ref name="zx81man" />
|-
|-
| Overheating || 7805 or shorted cap || Check C4/C5, IC3 temp
| Random resets or lockups with a RAM pack fitted || Edge-connector contacts and mechanical movement of the pack, before anything on the board
|}
|}


== Troubleshooting Flowcharts ==
For a symptom-by-symptom walk-through, see the [[Sinclair ZX80 Troubleshooting Guide]].
# '''No Power'''
ย 
## โ†’ Check PSU output (9V)
== Tools ==
## โ†’ Check 7805 output (5V)ย 
ย 
## โ†’ If 5V missing, replace 7805 and C4/C5ย 
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.
## โ†’ If still dead, inspect PCB for shorts
ย 
== Related pages ==


# '''No Video Output'''
* [[Sinclair ZX80]]
## โ†’ Confirm 5V presentย 
* [[Sinclair ZX80 Capacitor Replacement Guide]]
## โ†’ Test C2 and IC5 (video circuit)ย 
* [[Sinclair ZX80 Troubleshooting Guide]]
## โ†’ Swap ULA if availableย 
* [[Sinclair ZX81 General Maintenance]]
## โ†’ Inspect video connector and cable
* [[Capacitor Failure Symptoms]]
* [[Recommended Tools]]


# '''Keyboard Not Responding'''
== References ==
## โ†’ Inspect membrane for cracksย 
## โ†’ Clean and reseat connectorย 
## โ†’ Test continuity across matrix


# '''Random Freezing/Resets'''
<references />
## โ†’ Replace C1, C4, C5ย 
## โ†’ Test RAM ICsย 
## โ†’ Check for dry joints on IC sockets


[[Category:Sinclair Computers]]
[[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 โ€” keyboard tails, edge-connector contacts, dry joints โ€” 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 ร— 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no โˆ’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" โ€” 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 โ€” 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 โ€” 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 โ€“ 11 V" [4][2]
Current Sinclair published a figure for the ZX81 โ€” not less than 600 mA, or 1.2 A if the ZX printer shares the supply โ€” 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 โ€” that is ordinary repair work โ€” 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 3ยท2 MHz, as Sinclair's diagram labels it โ€” the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz [2]
IC1 (Z80) pin 26 /RESET โ€” held low briefly at switch-on by R21 (220 k) and C10 (1 ยตF), 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 โ€” 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 ยตF, 16 V) across the unregulated input, and C10 (1 ยตF) 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 โ€” 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 โ€” sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30โ€“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 D3โ€“D10; IC10 (74LS365)[5][2]
Screen clears but no cursor Unplug the supply, wait a few seconds, try again โ€” 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. โ†‘ 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. โ†‘ 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. โ†‘ Sinclair ZX80 โ€” 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. โ†‘ 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. โ†‘ 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. โ†‘ 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. โ†‘ "Micro Clinic", Television magazine, 1986 โ€” 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. โ†‘ 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 โ€” the same ribbons and the same failure occur across the ZX80, ZX81 and Spectrum.
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