Sinclair ZX Spectrum
 | |
| Sinclair ZX Spectrum 48K, the original rubber-key model (photo: Bill Bertram, CC BY-SA 2.5) | |
| Specifications | |
|---|---|
| Manufacturer | Sinclair Research Ltd |
| Type | Home computer |
| Released | 23 April 1982[1] |
| Intro price | £125 (16K), £175 (48K)[1] |
| CPU | Zilog Z80A at 3.5 MHz |
| Memory | 16 KB or 48 KB RAM |
| Storage | Cassette, through 3.5 mm EAR and MIC jacks |
| Display | 256×192 pixels, 32×24 characters, 15 colours |
| Sound | Single-channel beeper, driven from the ULA |
| ROM | 16 KB |
| Power | External unregulated 9 V DC through a 2.1 mm coaxial socket |
| OS / Firmware | Sinclair BASIC in ROM |
| Predecessor | Sinclair ZX81 |
| Successor | Sinclair ZX Spectrum+ |
| Model no. | ZX Spectrum 16K, ZX Spectrum 48K |
The Sinclair ZX Spectrum is an 8-bit home computer launched by Sinclair Research on 23 April 1982, in 16K and 48K versions priced at £125 and £175.[1] The rubber-key 16K and 48K machines were built on board issues 1 to 6A; the later model with an injection-moulded keyboard is the Sinclair ZX Spectrum+.
The technical figures below are from Sinclair's ZX Spectrum Service Manual and the Issue 3 circuit diagram.[2][3]
Architecture
[edit | edit source]| Part | Detail |
|---|---|
| CPU (IC2) | Zilog Z80A (or NEC µPD780) at 3.5 MHz, clocked from the ULA, which divides down the 14 MHz master crystal |
| ULA (IC1) | Ferranti uncommitted logic array: video, memory contention, keyboard scan, tape interface and beeper |
| ROM (IC5) | 16 KB, addresses 0000–3FFF |
| Lower RAM (IC6–IC13) | 16 KB in eight 16 kbit (4116) DRAMs, addresses 4000–7FFF, needing +5 V, −5 V and +12 V |
| Upper RAM (48K only) | 32 KB in eight 32 kbit DRAMs, addresses 8000–FFFF, with multiplexer and control ICs IC23–IC26 |
| Crystals | X1 14.000000 MHz and X2 4.43361875 MHz (PAL colour sub-carrier), the same on every issue |
| Video encoder | IC14 (LM1889) and the UHF modulator, channel 36 |
The ROM is 16 KB on every rubber-key Spectrum; "48K" refers to the RAM.[2]
The ULA's supply pins
[edit | edit source]The Issue 3 circuit diagram labels ULA pin 14 as Vcc1 (+5 V) and pin 40 as ground. Pin 13 (Vcc2) is fed through R34 (15 Ω, 1 W).[3] The Service Manual's fault-finding steps check IC1 pin 14 for +5 V with no discernible ripple, pin 40 for ground, and pin 13 for +3 V.[2] Probing pin 40 for +5 V will find nothing on a working board.
The ULA type changed with the board issues (5C102, 5C112 and 6C001 versions), and several resistor values depend on which ULA is fitted. The per-issue detail is on Sinclair ZX Spectrum General Maintenance.[2]
The ULA in Ferranti's records
[edit | edit source]Ferranti made the ULA, and its working files on the chip from 1981 to 1984 are in the Ferranti Archives 2026. The specification of the first production part, the 5C102, was first issued on 14 December 1981; the 5C112 logic diagram, "an updated version of the ULA 5C102", is dated 30 June 1982; and the 6C001 specification was issued on 26 August 1982.[4][5][6] Ferranti's lineage chart runs from the 5C102 through the 5C112 to the 6C001 marks 3, 5, 6 and 7, with a separate NTSC line (5C114 and 6C011). The mark 7, agreed with Sinclair in September 1983, added delay to the ULA's RAM timing; seven weeks earlier Ferranti had photographed the timing of fast mark 6 parts, one of which failed when cold.[7][8] The same files hold correspondence with Timex in Dundee, which built Spectrums for Sinclair, about memory faults on early boards in 1983 and about ULAs that misread the Spectrum+ keyboard in 1984.[9][10] The chip is described in Sinclair ZX Spectrum ULA and the memory-timing history in Sinclair ZX Spectrum Memory Timing.
Video
[edit | edit source]The display is 256×192 pixels, or 24 lines of 32 characters, with one attribute byte for each 8×8 character cell. The palette has 15 colours: seven colours at two brightness levels, plus black.[1] All the pixels in a cell share one ink and one paper colour, which produces the Spectrum's colour clash; it is a property of the design, not a fault.[1]
The ULA outputs luminance with sync (Y) and two colour-difference signals (U and V) on pins 15, 16 and 17. IC14 (LM1889) encodes the colour onto a 4.43 MHz sub-carrier, and the composite signal goes to the UHF modulator on channel 36.[2]
Power
[edit | edit source]The Spectrum takes an unregulated 9 V DC supply through a 2.1 mm coaxial socket.[2] The external pack contains a mains transformer, a full-wave rectifier and capacitive smoothing, with a thermal fuse at the transformer input.[2]
| Item | Figure |
|---|---|
| Power pack design limits | At 215 V AC mains and 1.4 A load, the voltage trough must not fall below 7.0 V; at 265 V AC and 600 mA, the peak must stay below 13 V (14 V in the re-typeset edition) |
| Regulator input (+ side of C50) | +9 V DC ± 2.0 V; below +7 V the regulator will not work correctly |
| Regulator output (+ side of C34) | +5 V DC ± 0.25 V, no discernible ripple |
| +5 V at the RAM | IC6 pin 9 |
| −5 V | IC6 pin 1 |
| +12 V | IC6 pin 8 |
| +12 VA (to IC14 only) | LT end of C52 |
The 13 V peak limit is from the 1984 Thorn EMI edition.[11]
The +5 V rail comes from a 7805 regulator on the board. The −5 V and +12 V rails for the 4116 lower RAM come from an inverter built around TR4 and TR5, which raises the 9 V input to more than +12 V; the square wave at TR4's collector is rectified and smoothed by D15 and C44, and R62 with C45 gives a separately smoothed +12 V for the video circuits and the expansion port.[2] The Issue 3 diagram also shows a −12 V output on edge-connector contact 23B.[3]
Sinclair recommends its DC-DC converter modification for any machine on which TR4 or the 16K DRAM has blown; some Issue 3B and 4A boards, and all later issues, have it built in.[2]
Edge connector
[edit | edit source]Sinclair's drawing numbers the contacts 1 to 28 on each side, with A on the underside and B on the component side.[3]
| Contact | Signal |
|---|---|
| 3A | +5 V |
| 4A | +9 V |
| 6A, 7A, 14A | 0 V |
| 15A | VIDEO |
| 20A | RESET |
| 25A | ROMCS |
| 20B | −5 V |
| 22B | +12 V |
| 23B | −12 V |
The full contact list is printed on the Issue 3 diagram.[3] The Spectrum has no composite video socket; video leaves through the UHF modulator.[2]
Storage
[edit | edit source]Programs are saved and loaded on cassette through the EAR and MIC jacks at about 1500 baud.[1] A single ULA pin (pin 28) handles both directions.[2]
Board issues
[edit | edit source]Sinclair's service documentation covers board issues 1, 2, 3, 3B, 4A, 4B, 5 and 6A; there was no Issue 4 or Issue 6.[2] Samsung also made an Issue 4S board, almost identical to the Issue 3B, which Sinclair's documentation does not cover; it is found in rubber-key as well as Spectrum+ cases.[12] The issues differ in ways that matter for repair: the ULA each takes, resistor values that change with the ULA, the DC-DC converter modification, and from Issue 5 a second ULA, IC27 (Mullard ZX 84/01), replacing six 74LS chips. Sinclair supplied 1000 Issue 5 boards, for the UK market only; IC27 is also listed for Issue 6A.[2] The per-issue detail is on Sinclair ZX Spectrum General Maintenance.
Ferranti's archive also holds period drawings of these boards: a 1982 Sinclair print with the early factory modifications pencilled in; Sinclair's Issue Two diagram of 16 August 1982, which draws the upper 32K on the main board as "optional 32K RAM"; a 1982 plug-in 32K board for Issue 1 machines, "RAM 32 Issue 1"; and Timex's Issue 3 (June 1983) and Issue 6A (September 1984) drawings, the first calling for a 5C112E ULA and the second for a 6C001E-7.[13][14][15][16][17] They are listed under Documentation.
Maintenance and repair
[edit | edit source]- Sinclair ZX Spectrum General Maintenance: ULA revisions, test points and 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
Documentation
[edit | edit source]- Sinclair ZX Spectrum Service Manual: Sinclair's service manual for the ZX Spectrum and Spectrum+, with the supplement for Issues 4A to 6A (re-typeset edition)
- ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984): the March 1984 servicing manual, with the T.I.S. Repair Data section
- Issue 3 circuit diagram
- Ferranti Archives 2026: Ferranti's files on the Spectrum ULA, 1981 to 1984, including these board drawings:
- Sinclair circuit diagram with modifications 1 to 3 (© 1982, modifications pencilled to 15 October 1982)
- Sinclair Issue Two circuit diagram (SRC 102, 16 August 1982)
- 32K memory board, RAM 32 Issue 1 (1982)
- Timex Issue 3 circuit diagram (drawing 356-201891-02, June 1983)
- Timex Issue 6A circuit diagram, Ferranti version (drawing 356-280106-01, 25 September 1984)
Schematic and PCB
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Related pages
[edit | edit source]References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "ZX Spectrum", Wikipedia. Source for the launch date, launch prices, the 15-colour palette and the 1500 baud tape speed.
- ↑ 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 Sinclair ZX Spectrum Service Manual (File:Sinclair ZX Spectrum Service Manual.pdf), Sinclair Research Ltd; re-typeset Revision 1.3 edited by Brendan Alford, Spectrum For Everyone. Sections 1.4 (memory organisation), 1.7 (TV picture generation), 1.11 (power supplies), 5.2 (power supply unit), 5.3.1 (basic checks), 8.7 and 9.15 (connectors), 8.8 and 9.16 (crystals), 9.1 (supplement introduction) and 9.7 (Issue 5 board).
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 ZX Spectrum – Issue 3 circuit diagram, Sinclair Research Ltd (File:Sinclair ZX Spectrum Issue 3 circuit diagram.png). Source for the ULA supply pins and the edge-connector assignments.
- ↑ Ferranti ULA 5C102 specification and pinning, issue 2, 4 February 1982, Ferranti Archives 2026 (SPEC-FER-DAT-093). Source for the supply pins (Vcc1 5 V ±5% on pin 14, Vcc2 2 V nominal on pin 13, ground on pin 40), typical currents of 60 mA and 120 mA, and the input and output levels.
- ↑ Ferranti logic diagram LD6761/ULA5C112/1, issue 1, 30 June 1982, Ferranti Archives 2026 (SPEC-FER-SCH-112). Source for the date of the 5C112, described on the drawing as "an updated version of the ULA 5C102".
- ↑ Ferranti ULA 6C001 specification, issue 1, 26 August 1982, Ferranti Archives 2026 (SPEC-FER-DAT-097). Source for Vcc2 "2 V (nominal)", typical currents of 40 mA and 80 mA, keyboard input thresholds of 1.4 V and 2.8 V, TAPE/LS thresholds of 0.4 V and 1.0 V, other inputs 0.8 V and 2.0 V, and the output levels.
- ↑ Ferranti lineage chart of the Spectrum ULA types, Ferranti and Sinclair telexes of 23 and 29 September 1983, and Ferranti memo of 22 May 1984, Ferranti Archives 2026 (SPEC-FER-FAX-MEM-048). Source for the ULA types and marks (5C102 to 6C001 Mk7; 5C114 and 6C011 Mk2 to Mk4), the Mk7 delay change (4 inverters on N.CAS, about 22 ns, and 2 on ADSEL, about 11 ns, typical at 25 °C, worst case 1.7 times) and the order to bring the 6C011 up to Mk7 timing.
- ↑ RAS, A0 and CAS oscilloscope Polaroids of 6C001-6 "low R" ULAs on Issue 2 and Issue 3 boards, cold, hot and with 150 pF on CAS, 10 August 1983, Ferranti Archives 2026 (SPEC-FER-REP-050). Source for a 6C001-6 sample that "fails cold".
- ↑ Ferranti report on a visit to Timex, with ITT's memo of 27 January 1983 on Spectrum RAS and refresh memory faults, 2 February 1983, Ferranti Archives 2026 (SPEC-FER-REP-059). Source for ITT's description of the fault (spurious characters, then a crash), its six candidate fixes and ULAs that failed in one board working in another.
- ↑ Timex letter to Ferranti, "Spectrum ULA-7 Failure Analysis, Keyboard Faults", with oscilloscope Polaroids, 7 November 1984, Ferranti Archives 2026 (SPEC-TMX-LET-041). Source for the two fault categories (columns of keys dead; wrong keys in a column pair), the rise time on the keyboard lines (about 330 ns with the old keyboard, about 1500 ns with the Spectrum+ keyboard) and the increased crosstalk.
- ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (File:ZX Spectrum Servicing Manual Thorn EMI Datatech 1984.pdf), prepared by Thorn (EMI) Datatech Ltd for Sinclair Research, March 1984; OCR by Andy Dansby. Section 4, paragraph 2.2 (p. 4.7) gives the power pack peak limit as 13 V.
- ↑ "ZX Spectrum Models", Spectrum for Everyone (accessed 2026-10-10). Community source for the Samsung-built Issue 4S board.
- ↑ Sinclair Research ZX Spectrum circuit diagram, © 1982, unnumbered, with pencil modifications mod 1 to mod 3 (mod 3 dated 15 October 1982), Ferranti Archives 2026 (SPEC-SIN-SCH-068). Source for mod 1 (the 74LS00 patch, "5C102 only"), mod 2 (a ZTX312 from Z80 A0 to the ULA's IORQ input), mod 3 (the TR3 clock buffer) and the NTSC encoder note.
- ↑ Sinclair Research "ZX Spectrum Circuit Diagram - Issue Two", SRC 102, 16 August 1982, Ferranti Archives 2026 (SPEC-SIN-SCH-071). Source for the Issue 2 edge-connector table and lettering, the optional 32K RAM section with its RAM TYPE SELECT link, and the positions of R47, R49, R56 and R63 in the video circuit.
- ↑ Sinclair "RAM 32 Issue 1" 32K memory board, © 1982: layout sketches on Ferranti forms, socket connection list, photocopies of a board and pencil circuit diagram, Ferranti Archives 2026 (SPEC-FER-DRW-072). Source for the parts and socket connections of the plug-in 32K board.
- ↑ Timex Corporation (Dundee) "ZX Spectrum Circuit Diag. Issue 3", drawing 356-201891-02, May 1983, change 02 of 10 June 1983, Ferranti Archives 2026 (SPEC-TMX-SCH-074). Source for the 5C112E ULA types, the RAM and ROM options and the expansion RAM select links on Issue 3, Timex's edge-connector lettering and the Issue 3 capacitor values.
- ↑ Timex Corporation (Dundee) "ZX Spectrum Circuit Diag. Issue 6A", Ferranti version, drawing 356-280106-01, sheet 1 of 2, 25 September 1984, Ferranti Archives 2026 (SPEC-TMX-SCH-069). Source for the 6C001E-7 ULA, the ZX8401 (IC27), the IC28 link points, C105 180 pF, R68 6.8K and the Issue 6A capacitor values.