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Correct speed of tape loading on the ZX Spectrum is approximately 1500 baud, the wrong speed given on this wiki's website was 300 baud which is the speed a ZX81 will use for tape loading.
Cite to Sinclair's ZX Spectrum Service Manual and Issue 3 circuit diagram; ROM is 16 KB (page said '16 KB or 48 KB'), ULA Vcc is pin 14 and pin 40 is ground, RESET is edge contact 20A, there is no composite output and no 7812, 15 distinct colours not '15 plus black'; remove the invented Issue 1/2 errata table, the broken 50-way pinout table and the wrong ROM keyword map
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| name        = Sinclair ZX Spectrum
| name        = Sinclair ZX Spectrum
| image        = [[File:ZX_Spectrum.jpg|250px]]
| image        = [[File:ZX_Spectrum.jpg|250px]]
| caption      = Sinclair ZX Spectrum – The Iconic Home Computer
| caption      = Sinclair ZX Spectrum, the original rubber-key model
| manufacturer = Sinclair Research Ltd
| manufacturer = Sinclair Research Ltd
| type        = Home computer
| type        = Home computer
| release date = April 1982
| release date = April 1982
| discontinued = 1992
| price        = £125 (1982)
| cpu          = Zilog Z80A @ 3.5 MHz
| cpu          = Zilog Z80A @ 3.5 MHz
| memory      = 16 KB, 48 KB, or 128 KB RAM (varies by model)
| memory      = 16 KB or 48 KB RAM
| storage      = External cassette tape (300 baud, EAR/MIC jacks)
| rom          = 16 KB
| display      = 256×192 pixels, 32 characters × 24 lines
| storage      = Cassette, through 3.5 mm EAR and MIC jacks
| sound        = Beeper (monophonic)
| display      = 256×192 pixels, 32×24 characters, 15 distinct colours
| dimensions  = 23 cm × 14 cm × 4 cm (9.1" × 5.5" × 1.6")
| sound        = Single-channel beeper, driven from the ULA
| weight      = 1.1 kg (2.4 lb)
| power        = External unregulated 9 V DC through a 2.1 mm coaxial socket
| os          = Sinclair BASIC in ROM
| os          = Sinclair BASIC in ROM
| predecessor  = [[Sinclair ZX81]]
| predecessor  = [[Sinclair ZX81]]
| successor    = [[Sinclair ZX Spectrum+]]
| successor    = [[Sinclair ZX Spectrum+]]
| codename    = ZX Spectrum
| model        = ZX Spectrum 16K, ZX Spectrum 48K
| model        = ZX Spectrum, ZX Spectrum+
| logo        = [[File:Sinclair ZX Spectrum-02b.svg|250px]]
}}
}}
<templatestyles src="Template:StyledTable/styles.css" />


The '''Sinclair ZX Spectrum''' is an 8-bit home computer released by '''Sinclair Research Ltd''' in April 1982. It became a defining product of the early home computing era in the UK, known for its distinctive rubber keyboard and vibrant color graphics. The ZX Spectrum sold over 5 million units and is credited with popularizing home computing and gaming in the UK.
The '''Sinclair ZX Spectrum''' is an 8-bit home computer released by Sinclair Research in April 1982. This page covers the original rubber-key 16K and 48K machines, board issues 1 to 6A. For the injection-moulded-keyboard model see [[Sinclair ZX Spectrum+]].
 
'''The figures on this page come from Sinclair's own ''ZX Spectrum Service Manual'' and the Issue 3 circuit diagram''', both hosted here.<ref name="sm">''ZX Spectrum Service Manual'', Sinclair Research Ltd. Hosted on this wiki as [[:File:Sinclair ZX Spectrum Service Manual.pdf]]. Section 1.5 (read/write operations and the ROM links), section 1.11 (power supplies), sections 8.7 and 9.15 (connectors), sections 8.8 and 9.16 (crystals: X1 = 14.000000 MHz on every issue, X2 = 4.433619 MHz), sections 8.10 and 9.19 (integrated circuits), section 4.1.2 (modification history and ULA revisions), section 5.2 (power supply unit) and sections 6.1 to 6.9 (input/output fault diagnosis).</ref><ref name="iss3cd">''ZX Spectrum – Issue 3'' circuit diagram, Sinclair Research Ltd. Hosted on this wiki as [[:File:Sinclair ZX Spectrum Issue 3 circuit diagram.png]]. Source for pin numbers, circuit positions, the speaker drive and the edge-connector assignments.</ref>


== Architecture ==
== Architecture ==
* '''CPU''' – Zilog Z80A microprocessor clocked at '''3.5 MHz'''. 
* '''ROM''' – 16 KB or 48 KB ROM containing Sinclair BASIC and system routines. 
* '''RAM''' – Available in 16 KB, 48 KB, or 128 KB configurations, expandable via external RAM packs. 
* '''Logic''' – Utilizes a combination of discrete logic and custom chips for video output and keyboard scanning.


=== Video generation ===
* '''CPU''' — Zilog Z80A at '''3.5 MHz''', divided down by the ULA from the 14 MHz master crystal
The ZX Spectrum generates video using a technique called "bit-banging," where the Z80 CPU directly controls the display output. The screen resolution is '''256×192 pixels''' with a palette of 15 colors, plus black. The unique attribute system allows for color attributes to be assigned to 8×8 pixel blocks, resulting in colorful graphics but also potential color clash.
* '''ROM''' — '''16 KB'''
* '''RAM''' — 16 KB or 48 KB. The lower 16 KB is 4116 DRAM needing −5 V and +12 V; the upper 32 KB of a 48K machine is 4532/4164
* '''ULA''' — a single Ferranti uncommitted logic array doing the video, the memory contention, the keyboard scan, the tape interface and the beeper
* '''Crystals''' — '''X1 = 14.000000 MHz''' on every issue; '''X2 = 4.433619 MHz''', the PAL colour sub-carrier


== Storage ==
'''The Spectrum's ROM is 16 KB.''' The "48 KB" figure that circulates is the RAM size of the larger model; no 48 KB ROM was ever fitted. A 128K machine carries two 16 KB ROMs, which is a different arrangement again.<ref name="sm" />
Data is loaded and saved via a standard cassette tape interface operating at approximately '''1500 baud'''. The average loading time for a game is approximately 3-5 minutes, depending on the size of the program.
 
=== The ULA's supply pins ===
 
'''Vcc is pin 14. Pin 40 is ground.''' This is the single most consequential pin error on this machine: probing pin 40 for 5 V, finding none and condemning a perfectly good ULA. Pin 13 is a second supply at about '''+3 V''', fed through R34.<ref name="iss3cd" />
 
ULA revisions are not freely interchangeable with board issues; the '''6C001E-7''' is the only 48K revision documented to work on every board from Issue 1 to Issue 6A. The per-revision detail is on [[Sinclair ZX Spectrum General Maintenance]].<ref name="sm" />
 
=== Video ===
 
256×192 pixels with an attribute byte per 8×8 character cell, giving '''15 distinct colours''' — eight hues at two brightness levels, with black the same at both. One ink and one paper colour per cell is what produces the machine's characteristic '''colour clash''', and it is an architectural property, not a fault and not something a board revision changed.
 
== Power ==


== I/O and Expansion ==
The Spectrum takes an '''unregulated 9 V DC''' supply through a '''2.1 mm coaxial socket''' — not the 3.5 mm jack of the [[Sinclair ZX81|ZX81]], although the pads for one remained on the board up to Issue 4B.<ref name="sm" />
* '''Edge connector (50-way)''' – provides access to the full Z80 bus, power, and video signals. 
* '''9 V DC input''' – unregulated power supply. 
* '''Composite video output''' – for connecting to monitors and TVs.
Commercial peripherals included:
: ''ZX Printer, joystick interfaces, and various RAM expansion packs.''


== Known PCB Revisions & Errata ==
{| class="wikitable styled-table" style="width:95%; text-align:left;"
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''Supply figures'''
|+'''ZX Spectrum PCB Revisions'''
! Item !! Figure !! Note
! Issue !! Visible Clues !! Key Fixes / Changes
|-
|-
| '''Issue 1''' (1982) || Original board layout with no solder mask. || • Early boards had issues with RAM stability; later revisions improved power distribution.
| Original pack || Sinclair '''UK1400''', 1.4 A || Reads about '''12.5 V unloaded''' because it is unregulated
|-
|-
| '''Issue 2''' (1983) || Added solder mask and improved trace routing. || • Enhanced video output stability and reduced color clash issues.
| At the board || '''+9 V ±2.0 V''' at the positive side of C50 || The regulator input. '''Below +7 V the regulator will not work correctly'''
|}
 
== General Maintenance ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Issue !! Cause !! Remedy
|-
|-
| Flickering display || Aging capacitors in the power supply || Replace with high-quality electrolytic capacitors.
| Logic rail || +5 V || From the on-board 7805 on its heatsink
|-
|-
| Unresponsive keys || Worn rubber contacts on the keyboard || Replace keyboard or use conductive paint to restore contact.
| DRAM rails || '''−5 V''' and '''+12 V''' || Generated on board by the TR4/TR5 inverter, for the 4116 lower RAM
|-
| Loading errors || Dirty cassette connectors || Clean connectors and ensure proper tape alignment.
|}
|}


Regular maintenance, including cleaning the keyboard and ensuring proper connections, can help prolong the life of the ZX Spectrum.
'''A high reading on an unloaded original pack is normal.''' Judge the supply against the +9 V ±2.0 V figure measured at C50 with the machine running, not against its off-load reading.


== Schematic & PCB ==
'''There is no 7812 on a ZX Spectrum''' and the regulator input is 9 V, not 12 V. '''The −5 V test point is IC6 pin 1''', not the collector of TR4.
The complete schematic for the ZX Spectrum is available for enthusiasts and repair technicians.
 
{| class="wikitable"
Sinclair identifies '''failure of TR4''' as a significant cause of problems, and it does not fail alone — the DC-DC converter modification is recommended where either TR4 or the 16K DRAM has blown.<ref name="sm" />
|+
 
! [[File:ZX_Spectrum_Schematic.png|thumb|center|500px|ZX Spectrum circuit diagram]]
== Edge connector ==
! [[File:ZX_Spectrum_PCB_Top.jpg|thumb|center|260px|Top view of the ZX Spectrum PCB]]
|}


== Common Modifications ==
Sinclair identifies the contacts by number and side letter, not by a single sequence. From the Issue 3 diagram:<ref name="iss3cd" />
* '''RAM upgrade''' – Adding external RAM packs to increase memory capacity. 
* '''Composite video mod''' – For improved video output quality on modern displays. 
* '''Keyboard replacement''' – Upgrading to a full-travel keyboard for better typing experience.
* '''Turbo mode modification''' – Overclocking the Z80 for faster performance (requires careful tuning).


== ROM Keyword Map ==
{| class="wikitable styled-table" style="width:80%; text-align:left;"
Each key on the ZX Spectrum keyboard has multiple functions depending on the mode: 
|+'''Supply and control contacts'''
{| class="wikitable styled-table" style="text-align:center;"
! Contact !! Signal
|+ Example ZX Spectrum keyboard layer
|-
|-
! Key !! <code>F</code> mode !! <code>K</code> mode !! <code>L</code> mode
| 3A || +5 V
|-
|-
| '''P''' || <code>PRINT</code> || <code>P</code> || <code>(</code>
| 4A || +9 V
|-
|-
| '''R''' || <code>RETURN</code> || <code>R</code> || <code>)</code>
| 6A, 7A, 14A || 0 V
|-
|-
| '''G''' || <code>GOTO</code> || <code>G</code> || <code>></code>
| '''20A''' || '''RESET'''
|}
 
== Edge-Connector Pin-out ==
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:75%; text-align:center;"
|+'''ZX Spectrum 50-way Expansion Bus (viewed from rear, component side up)'''
! Pin !! Signal !! Description !! Pin !! Signal !! Description
|-
|-
| 1 || '''A15''' || Address bus MSB || 26 || '''D4''' || Data bit 4
| 25A || ROMCS
|-
|-
| 2 || '''A14''' || || 27 || '''D5'''
| 15A || VIDEO
|
|-
|-
| 3 || '''A13''' || || 28 || '''D6'''
| 20B || −5 V
|
|-
|-
| 4 || '''A12''' || || 29 || '''D7'''
| 22B || +12 V
|-
|-
| 5 || '''A11''' || || 30 || '''BUSACK''' || Bus acknowledge
| 23B || −12 V
|-
|}
| 6 || '''A10''' || || 31 || '''BUSRQ''' || Bus request
 
|-
'''RESET is 20A.''' A previous version of this page carried a truncated 1-to-50 pin table with signals that do not correspond to any Spectrum connector; it has been removed rather than corrected, because nothing in it could be matched to Sinclair's drawing.
| 7 || '''A9''' || || 32 || '''WAIT''' || Insert wait-states
 
The Spectrum has '''no composite video socket'''. Video leaves through the RF modulator; composite requires a modification, which is why so many machines have one.
 
== Storage ==
 
Cassette through the EAR and MIC jacks, at approximately 1500 bits per second. Loading times depend entirely on the program's size and on the loader it uses, so no figure is given here.
 
== Board issues ==
 
Board issues 1, 2, 3, 3B, 4A, 4B, 5 and 6A were all produced, and they differ in real ways that matter to a repair — the ULA revision each will accept, which resistors change with it, whether TR6 is part of the layout, and whether the six 74LS packages replaced by the second ULA (IC27, Mullard ZX 84/01, on Issue 5 and 6A) are present.<ref name="sm" />
 
A previous two-row table here claimed an "Issue 1 (1982)" and "Issue 2 (1983)" with fixes for "RAM stability" and "reduced colour clash". Neither claim appears in Sinclair's modification history, and colour clash is not something a board revision can reduce. '''Both rows have been removed.''' The real per-issue detail is on [[Sinclair ZX Spectrum General Maintenance]].
 
== Maintenance ==
 
* [[Sinclair ZX Spectrum General Maintenance]] — component list, ULA revisions, test points, supply figures
* [[Sinclair ZX Spectrum Troubleshooting Guide]] — fault-finding
* [[Sinclair ZX Spectrum Capacitor Replacement Guide]] — capacitors by board issue
* [[Sinclair ZX Spectrum Keyboard Membrane Repair]] — the membrane and its tails
 
Documents hosted here: [[Sinclair ZX Spectrum Service Manual]] and the [[:File:Sinclair ZX Spectrum Issue 3 circuit diagram.png|Issue 3 circuit diagram]].
 
== Schematic and PCB ==
{| class="wikitable styled-table" style="width:85%;"
|-
|-
| 8 || '''A8''' || || 33 || '''HALT'''
| [[File:Sinclair ZX Spectrum Issue 3 circuit diagram.png|thumb|center|420px|Sinclair's Issue 3 circuit diagram]]
| [[File:ZXspectrum mb.jpg|thumb|center|300px|ZX Spectrum 48K main board, Issue 3B]]
|}
|}
== Related pages ==
* [[Sinclair ZX81]]
* [[Sinclair ZX Spectrum+]]
* [[Sinclair ZX Spectrum General Maintenance]]
* [[Sinclair ZX Spectrum Troubleshooting Guide]]
* [[Sinclair ZX Spectrum Capacitor Replacement Guide]]
* [[Sinclair ZX Spectrum Keyboard Membrane Repair]]
== References ==
<references />


[[Category:Sinclair Computers]]
[[Category:Sinclair Computers]]

Revision as of 20:13, 23 September 2026

Sinclair ZX Spectrum
Sinclair ZX Spectrum, the original rubber-key model
Specifications
ManufacturerSinclair Research Ltd
TypeHome computer
ReleasedApril 1982
CPUZilog Z80A @ 3.5 MHz
Memory16 KB or 48 KB RAM
StorageCassette, through 3.5 mm EAR and MIC jacks
Display256×192 pixels, 32×24 characters, 15 distinct colours
SoundSingle-channel beeper, driven from the ULA
ROM16 KB
PowerExternal unregulated 9 V DC through a 2.1 mm coaxial socket
OS / FirmwareSinclair BASIC in ROM
PredecessorSinclair ZX81
SuccessorSinclair ZX Spectrum+
Model no.ZX Spectrum 16K, ZX Spectrum 48K

The Sinclair ZX Spectrum is an 8-bit home computer released by Sinclair Research in April 1982. This page covers the original rubber-key 16K and 48K machines, board issues 1 to 6A. For the injection-moulded-keyboard model see Sinclair ZX Spectrum+.

The figures on this page come from Sinclair's own ZX Spectrum Service Manual and the Issue 3 circuit diagram, both hosted here.[1][2]

Architecture

  • CPU — Zilog Z80A at 3.5 MHz, divided down by the ULA from the 14 MHz master crystal
  • ROM — 16 KB
  • RAM — 16 KB or 48 KB. The lower 16 KB is 4116 DRAM needing −5 V and +12 V; the upper 32 KB of a 48K machine is 4532/4164
  • ULA — a single Ferranti uncommitted logic array doing the video, the memory contention, the keyboard scan, the tape interface and the beeper
  • Crystals — X1 = 14.000000 MHz on every issue; X2 = 4.433619 MHz, the PAL colour sub-carrier

The Spectrum's ROM is 16 KB. The "48 KB" figure that circulates is the RAM size of the larger model; no 48 KB ROM was ever fitted. A 128K machine carries two 16 KB ROMs, which is a different arrangement again.[1]

The ULA's supply pins

Vcc is pin 14. Pin 40 is ground. This is the single most consequential pin error on this machine: probing pin 40 for 5 V, finding none and condemning a perfectly good ULA. Pin 13 is a second supply at about +3 V, fed through R34.[2]

ULA revisions are not freely interchangeable with board issues; the 6C001E-7 is the only 48K revision documented to work on every board from Issue 1 to Issue 6A. The per-revision detail is on Sinclair ZX Spectrum General Maintenance.[1]

Video

256×192 pixels with an attribute byte per 8×8 character cell, giving 15 distinct colours — eight hues at two brightness levels, with black the same at both. One ink and one paper colour per cell is what produces the machine's characteristic colour clash, and it is an architectural property, not a fault and not something a board revision changed.

Power

The Spectrum takes an unregulated 9 V DC supply through a 2.1 mm coaxial socket — not the 3.5 mm jack of the ZX81, although the pads for one remained on the board up to Issue 4B.[1]

Supply figures
Item Figure Note
Original pack Sinclair UK1400, 1.4 A Reads about 12.5 V unloaded because it is unregulated
At the board +9 V ±2.0 V at the positive side of C50 The regulator input. Below +7 V the regulator will not work correctly
Logic rail +5 V From the on-board 7805 on its heatsink
DRAM rails −5 V and +12 V Generated on board by the TR4/TR5 inverter, for the 4116 lower RAM

A high reading on an unloaded original pack is normal. Judge the supply against the +9 V ±2.0 V figure measured at C50 with the machine running, not against its off-load reading.

There is no 7812 on a ZX Spectrum and the regulator input is 9 V, not 12 V. The −5 V test point is IC6 pin 1, not the collector of TR4.

Sinclair identifies failure of TR4 as a significant cause of problems, and it does not fail alone — the DC-DC converter modification is recommended where either TR4 or the 16K DRAM has blown.[1]

Edge connector

Sinclair identifies the contacts by number and side letter, not by a single sequence. From the Issue 3 diagram:[2]

Supply and control contacts
Contact Signal
3A +5 V
4A +9 V
6A, 7A, 14A 0 V
20A RESET
25A ROMCS
15A VIDEO
20B −5 V
22B +12 V
23B −12 V

RESET is 20A. A previous version of this page carried a truncated 1-to-50 pin table with signals that do not correspond to any Spectrum connector; it has been removed rather than corrected, because nothing in it could be matched to Sinclair's drawing.

The Spectrum has no composite video socket. Video leaves through the RF modulator; composite requires a modification, which is why so many machines have one.

Storage

Cassette through the EAR and MIC jacks, at approximately 1500 bits per second. Loading times depend entirely on the program's size and on the loader it uses, so no figure is given here.

Board issues

Board issues 1, 2, 3, 3B, 4A, 4B, 5 and 6A were all produced, and they differ in real ways that matter to a repair — the ULA revision each will accept, which resistors change with it, whether TR6 is part of the layout, and whether the six 74LS packages replaced by the second ULA (IC27, Mullard ZX 84/01, on Issue 5 and 6A) are present.[1]

A previous two-row table here claimed an "Issue 1 (1982)" and "Issue 2 (1983)" with fixes for "RAM stability" and "reduced colour clash". Neither claim appears in Sinclair's modification history, and colour clash is not something a board revision can reduce. Both rows have been removed. The real per-issue detail is on Sinclair ZX Spectrum General Maintenance.

Maintenance

Documents hosted here: Sinclair ZX Spectrum Service Manual and the Issue 3 circuit diagram.

Schematic and PCB

Sinclair's Issue 3 circuit diagram
ZX Spectrum 48K main board, Issue 3B

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 ZX Spectrum Service Manual, Sinclair Research Ltd. Hosted on this wiki as File:Sinclair ZX Spectrum Service Manual.pdf. Section 1.5 (read/write operations and the ROM links), section 1.11 (power supplies), sections 8.7 and 9.15 (connectors), sections 8.8 and 9.16 (crystals: X1 = 14.000000 MHz on every issue, X2 = 4.433619 MHz), sections 8.10 and 9.19 (integrated circuits), section 4.1.2 (modification history and ULA revisions), section 5.2 (power supply unit) and sections 6.1 to 6.9 (input/output fault diagnosis).
  2. ↑ 2.0 2.1 2.2 ZX Spectrum – Issue 3 circuit diagram, Sinclair Research Ltd. Hosted on this wiki as File:Sinclair ZX Spectrum Issue 3 circuit diagram.png. Source for pin numbers, circuit positions, the speaker drive and the edge-connector assignments.
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