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{{Infobox computer
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. 
| name        = Sinclair ZX80 General Maintenance
'''Always observe ESD (Electrostatic Discharge) precautions''' when handling the PCB or components—use a grounded wrist strap and work on an antistatic mat. 
| image        =
Regular maintenance intervals: 
| caption      =
* '''Visual inspection:''' Every 12 months 
| manufacturer =
* '''Capacitor check:''' Every 2–3 years 
| type        =
* '''Full electrical test:''' Every 5 years or after storage
| release date =
| discontinued =
| price        =
| cpu          =
| memory      =
| storage     =
| display      =
| sound        =
| dimensions  =
| weight      =
| os          =
| predecessor  =
| successor    =
| codename    =
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| logo        =
}}


The '''Sinclair ZX80 General Maintenance''' page provides essential information for maintaining and troubleshooting the Sinclair ZX80 home computer. This guide covers various aspects of the ZX80, including architecture, storage, I/O, and common maintenance practices.
<templatestyles src="Template:StyledTable/styles.css" />
[[File:ZX80_PCB_Top.jpg|thumb|right|300px|Sinclair ZX80 Mainboard (Issue 2)]]


== 🧠 Architecture ==
== Board Revisions ==
The ZX80 is built around a Z80A-class microprocessor, clocked at 3.25 MHz, and utilizes a simple architecture that allows for efficient operation despite its limitations.
{| class="wikitable styledtable"
! Revision !! PCB Markings !! Notable Differences !! Common Issues
|-
| Issue 1 || "ZX80 ISSUE ONE" silk || Early production, hand-wired mods || Poor video sync, regulator overheating
|-
| Issue 2 || "ZX80 ISSUE TWO" silk || Improved video, revised regulator layout || Keyboard connector wear, RAM instability
|}


=== Video generation ===
== Power Supply & Regulator Care ==
The video output is generated by the CPU itself, which can lead to flickering during BASIC program execution due to timing issues.
The ZX80 uses a simple linear power supply:
* '''PSU Part:''' Sinclair 9V DC Adapter (center negative)
* '''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


== 💾 Storage ==
'''On-board Capacitor Designators:'''
The ZX80 primarily uses cassette tapes for data storage, with a baud rate of 300, making it relatively slow compared to modern standards.
* '''C4, C5:''' Smoothing and decoupling for 7805
* '''C1, C2, C3:''' Logic supply filtering


== 🔌 I/O and Expansion ==
== Capacitor Replacement Table ==
The ZX80 features an edge connector for expansion, allowing users to connect various peripherals and memory packs.
{| class="wikitable styledtable"
 
! Position !! Value !! Voltage !! Notes !! Series/Type
== 🔧 Known PCB Revisions & Errata ==
|-
{| class="wikitable styled-table" style="width:70%; text-align:center;"
| C1 || 22μF || 16V || Logic rail smoothing || Axial, low ESR
|+'''ZX80 Logic-Board Issues'''
|-
! Issue !! Visible Clues !! Key Fixes / Changes
| C2 || 1μF || 16V || Video circuit || Tantalum or electrolytic
|-
| C3 || 0.1μF || 50V || Noise bypass || Ceramic disc
|-
|-
| '''Issue 1''' (Feb 1980) || “ZX80 Issue 1” silkscreen; no solder mask between edge pads. || • Missing pull-ups on IC12 (74LS05) video drivers caused weak sync on some TVs. 
| C4 || 100μF || 16V || 7805 input || Axial, low ESR
• Regulator IC5 runs hot—Sinclair added stick-on heatsink in later batches.
|-
|-
| '''Issue 2''' (late 1980) || Added green solder mask; extra wire-link near IC12. || • 4× 1 kΩ SIL resistor pack RP1 installed for stronger TTL drive. 
| C5 || 100μF || 16V || 7805 output || Axial, low ESR
• Board edge widened 1 mm to fit Timex card guides.
|}
|}


== 🛠️ General Maintenance ==
== Voltage & Clock Test Points ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
{| class="wikitable styledtable"
! Issue !! Cause !! Remedy
! Test Point !! IC/Pin !! Expected Value (±Tolerance)
|-
|-
| Unstable picture / snow || Ageing electrolytic C5 (47 µF) on video rail || Replace with low-ESR 105 °C part
| 5V Rail || IC3 (7805) Output || 5.00V ±0.15V
|-
|-
| Reboots when touched || Loose 7805 regulator tab shorts to case || Insulate tab / fit TO-220 spacer
| 9V Input || C4 (+) || 9.0V ±0.5V
|-
|-
| RAM pack “wobble” resets || Leverage on edge bus ⇒ poor contact || Fit retention bracket or right-angle header
| CPU Clock || IC1 (Z80) Pin 6 || 3.25MHz ±0.05MHz
|-
|-
| Overheating || No ventilation slots (black “stripes” are cosmetic) || Drill discreet vents or run uncased board
| Video Sync || IC5 Pin 3 || 5Vpp square wave
|}
|}


Full recapping, regulator heatsinking, and replacement of the brittle membrane keyboard are popular reliability upgrades.
== Essential Tools ==
 
* Antistatic wrist strap & mat
== 📜 Schematic & PCB ==
* Multimeter (with continuity and voltage)
The complete single-layer PCB schematic is freely available 
* Oscilloscope (≥10MHz, for clock/video)
{| class="wikitable"
* Soldering iron (fine tip, ESD safe)
|+
* Solder sucker or wick
! [[File:ZX80_Schematic.png|thumb|center|500px|Sinclair ZX80 circuit diagram]]
* Small flat and Phillips screwdrivers
! [[File:ZX80_PCB_Top.jpg|thumb|center|260px|Original Issue 1 board – note hand-drawn track work]]
* IC extractor
|}
* Fine tweezers
* Isopropyl alcohol & brush (for cleaning)


== 🔧 Common Modifications ==
== Preventive Maintenance Checklist ==
* '''8 KB ZX81 ROM upgrade''' – drops-in to give floating-point BASIC, SAVE/LOAD VERIFY and better editing (still no SLOW mode).
# Power off and unplug the unit.
* '''Composite video mod''' – bypass RF can for crisp LCD/monitor output.
# Discharge static and open the case.
* '''Full-travel keyboard''' – external matrix wired to KB pins 8-15.
# Inspect PCB for corrosion, broken traces, or burnt components.
* '''Internal 16 KB SRAM''' – replaces external pack, eliminating “wobble.” 
# Check all capacitors for bulging or leakage.
* '''Turbo crystal (3.58 MHz)''' – simple Xtal swap for ≈10% speed gain (requires retune TV).
# 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.


== 🔢 ROM Keyword Map ==
== Common Faults & Quick Checks ==
Each key performs multiple functions depending on mode: 
{| class="wikitable styledtable"
{| class="wikitable styled-table" text-align:center;"
! Symptom !! Likely Cause !! Quick Check
|+ Example ZX80 keyboard layer
|-
|-
! Key !! <code>F</code> mode !! <code>K</code> mode !! <code>L</code> mode
| No power || PSU or 7805 failure || Measure 9V in, 5V out at IC3
|-
|-
| '''P''' || <code>PRINT</code> || <code>P</code> || <code>(</code>
| No video || C2, IC5, or ULA fault || Scope video out, check C2
|-
|-
| '''R''' || <code>RETURN</code> || <code>R</code> || <code>)</code>
| Keyboard dead || Membrane or connector || Inspect for cracks, reseat
|-
|-
| '''G''' || <code>GOTO</code> || <code>G</code> || <code>></code>
| Random resets || C1, C4, or RAM || Replace caps, test RAM
|-
| Overheating || 7805 or shorted cap || Check C4/C5, IC3 temp
|}
|}


== 🔗 Edge-Connector Pin-out ==
== Troubleshooting Flowcharts ==
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# '''No Power'''
{| class="wikitable styled-table" style="width:75%; text-align:center;"
## → Check PSU output (9V) 
|+'''ZX80 44-way Expansion Bus (viewed from rear, component side up)'''
## → Check 7805 output (5V) 
! Pin !! Signal !! Description !! Pin !! Signal !! Description
## → If 5V missing, replace 7805 and C4/C5 
|-
## → If still dead, inspect PCB for shorts
| 1 || '''A15''' || Address bus MSB || 23 || '''D4''' || Data bit 4
 
|-
# '''No Video Output'''
| 2 || '''A14''' || || 24 || '''D5'''
## → Confirm 5V present 
|-
## → Test C2 and IC5 (video circuit)
| 3 || '''A13''' || || 25 || '''D6'''
## → Swap ULA if available 
|-
## → Inspect video connector and cable
| 4 || '''A12''' || || 26 || '''D7'''
 
|-
# '''Keyboard Not Responding'''
| 5 || '''A11''' || || 27 || '''BUSACK''' || Bus acknowledge
## → Inspect membrane for cracks 
|-
## → Clean and reseat connector 
| 6 || '''A10''' || || 28 || '''BUSRQ''' || Bus request (NC on ZX80)
## → Test continuity across matrix
|-
 
| 7 || '''A9''' || || 29 || '''WAIT''' || Insert wait-states
# '''Random Freezing/Resets'''
|-
## → Replace C1, C4, C5 
| 8 || '''A8''' || || 30 || '''HALT'''
## → Test RAM ICs 
|-
## → Check for dry joints on IC sockets
| 9 || '''A7''' || || 31 || '''INT'''
 
|-
[[Category:Sinclair Computers]]
|10 || '''A6''' || || 32 || '''NMI'''
|-
|11 || '''A5''' || || 33 || '''RESET'''
|-
|12 || '''A4''' || || 34 || '''M1''' || Machine-cycle 1
|-
|13 || '''A3''' || || 35 || '''RFSH'''
|-
|14 || '''A2''' || || 36 || '''CLK''' || CPU clock (3.25 MHz)
|-
|15 || '''A1''' || || 37 || '''GND'''
|-
|16 || '''A0''' || || 38 || '''+5 V'''
|-
|17 || '''D0''' || Data bit 0 || 39 || '''+9 V DC'''
|-
|18 || '''D1''' || || 40 || '''/MREQ'''
|-
|19 || '''D2''' || ||
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