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Rewrite from Sinclair ZX80 circuit diagram. Corrects PSU polarity (centre POSITIVE, 3.5mm jack, not 2.1mm centre-negative), regulator designator (IC22 not IC3), removes the invented capacitor table and board-revision table, removes ULA references (the ZX80 has none). Add refs, clear Unverified data
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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. 
[[File:ZX80 PCB Top.jpg|thumb|right|300px|Sinclair ZX80 main board]]
'''Always observe ESD (Electrostatic Discharge) precautions''' when handling the PCB or components—use a grounded wrist strap and work on an antistatic mat. 
Regular maintenance intervals: 
* '''Visual inspection:''' Every 12 months 
* '''Capacitor check:''' Every 2–3 years 
* '''Full electrical test:''' Every 5 years or after storage


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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.
[[File:ZX80_PCB_Top.jpg|thumb|right|300px|Sinclair ZX80 Mainboard (Issue 2)]]
 
'''Observe ESD precautions''' when the board is out of the case. Use a wrist strap and an antistatic mat.
 
== What the ZX80 actually is ==
 
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 [[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" />


== 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 — 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.
The ZX80 uses a simple linear power supply:
 
* '''PSU Part:''' Sinclair 9V DC Adapter (center negative)
== Power supply ==
* '''Specs:''' 9V DC, 700–1000mA, 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 || 22μF || 16V || Logic rail smoothing || Axial, low ESR
|-
|-
| C2 || 1μF || 16V || Video circuit || Tantalum or electrolytic
| 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" />
|-
|-
| C3 || 0.1μF || 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 || 100μF || 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 – 11 V" || <ref name="kio80" /><ref name="zx80cd" />
|-
|-
| C5 || 100μF || 16V || 7805 output || Axial, low ESR
| 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" />
|}
|}


== 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"
 
! Test Point !! IC/Pin !! Expected Value (±Tolerance)
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
|-
|-
| 5V Rail || IC3 (7805) Output || 5.00V ±0.15V
| Positive end of C8 || The incoming unregulated supply, nominally 9 V DC || <ref name="zx80cd" />
|-
|-
| 9V Input || C4 (+) || 9.0V ±0.5V
| IC22 (7805) output || +5 V || <ref name="zx80cd" />
|-
|-
| CPU Clock || IC1 (Z80) Pin 6 || 3.25MHz ±0.05MHz
| 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 || 3.25 MHz CPU clock, from the 6.5 MHz oscillator X1 || <ref name="zx80cd" />
|-
| IC1 (Z80) pin 26 || /RESET — held low briefly at switch-on by R21 (220 k) and C10 (1 µF), 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 — 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 (≥10MHz, 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 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" />
{| class="wikitable styledtable"
 
! Symptom !! Likely Cause !! Quick Check
Full detail, including replacement guidance, is in the [[Sinclair ZX80 Capacitor Replacement Guide]]. See also [[Capacitor Failure Symptoms]].
 
== Preventive maintenance ==
 
# 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 — 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 power || PSU or 7805 failure || Measure 9V in, 5V out at IC3
| 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" />
|-
|-
| 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 D3–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 — 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'''
 
## → Check PSU output (9V)
== Tools ==
## → Check 7805 output (5V) 
 
## → If 5V missing, replace 7805 and C4/C5 
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.
## → If still dead, inspect PCB for shorts
 
== Related pages ==


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


# '''Keyboard Not Responding'''
== References ==
## → Inspect membrane for cracks 
## → Clean and reseat connector 
## → Test continuity across matrix


# '''Random Freezing/Resets'''
<references />
## → Replace C1, C4, C5 
## → Test RAM ICs 
## → Check for dry joints on IC sockets


[[Category:Sinclair Computers]]
[[Category:Sinclair Computers]]
[[Category:Maintenance Guides]]
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