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Sinclair ZX Spectrum Z80 CPU Replacement

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NEC D780C-1 in a ZX Spectrum
An NEC D780C-1, the Z80-compatible CPU Sinclair fitted at IC2 on many boards, on a 48K Issue 3B board

The CPU of the Sinclair ZX Spectrum (16K and 48K) is IC2, a Zilog Z80A or NEC µPD780, clocked at 3.5 MHz from the ULA through TR3.[1][2] The Sinclair ZX Spectrum+ uses the same boards.[3] The CPU is soldered to the board, not socketed.[4] Two kinds of CPU fault turn up: a CPU that has failed outright, so the machine never starts, and one that works on its own but drives a weak M1 signal, so that interfaces which use M1 do not work.[5][6][4]

Symptoms

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  • The machine does not start and the border is not white, because no code is running. Retroleum's suspects are the CPU, the ROM, or a RAM chip holding the data bus.[5]
  • Alternate thick black and white vertical bars. Worsley checks that the CPU and ROM are seated and that no leg cut during earlier tests has been left unsoldered, and then tests TR6 and TR3.[7]
  • Crashes or odd effects only when an expansion is plugged in. Retroleum cleans the edge connector, then replaces the Z80, then the ROM.[5]
  • Interface 1 not recognised, or a modern interface such as a DivMMC not working, while the Spectrum works on its own. Sinclair's servicing addendum says to inspect the edge connector for damaged pins and check the CPU's M1 signal: only Interface 1 uses M1, so a failure is not evident when the Spectrum is used alone.[8] Adam's Vintage Computer Restorations traced a DivMMC Future that would not work to a partly failed Z80 without a valid M1 signal, a fault he had found on several 48K machines.[4]

The T.I.S. notes rate IC2 and IC5 as the most likely culprits after the ULA, and neither as "particularly reliable".[9] Why some Spectrum CPUs have a weak M1 is not settled: Adam's write-up records two theories, part-working Z80s bought cheaply at the factory and damage from an expansion shorting M1 to 12 V, without supporting either.[4]

Before you start

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Sinclair ZX Spectrum Issue 3 CPU IC2 pinout
IC2 on Sinclair's Issue 3 circuit diagram, showing M1 on pin 27, the clock on pin 6, Vcc on pin 11 and ground on pin 29

Check the things the CPU depends on before removing it:[10][2]

  1. +5 V at IC2 pin 11 and ground at pin 29.
  2. The clock at IC2 pin 6: a square wave of about 5 V amplitude. If it is low in amplitude, check R25; then check TR3, IC1 pin 32, IC1 pin 39 (14 MHz) and crystal X1.
  3. The control lines, comparing them with a known-good board if possible: pin 16 INT, pin 17 NMI, pin 19 MREQ, pin 20 IOREQ, pin 21 RD, pin 22 WR, pin 24 WAIT, pin 25 BUSRQ and pin 26 RESET.
  4. The address and data lines of IC1, IC2 and IC5 for activity.

For the M1 fault, look at pin 27, which the Issue 3 diagram labels M1, and at the edge connector contact that carries it (24B in Sinclair's lettering).[11][8]

Tools and parts

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  • A replacement Z80. Sinclair's parts lists give Z80A or µPD780 (NEC).[1][12] Retroleum sells modern CMOS Z80s from Zilog's last production as a direct replacement for the NMOS originals, and says this also cures the out-of-specification Z80s with a weak M1.[6]
  • A 40-pin DIL socket, so the CPU can be changed again without desoldering.[4]
  • A desoldering pump or station, braid, and a hot-air tool if available.[13]

Procedure

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  1. Open the case (five screws in the base) and lift the board out (one screw next to the IC22 socket).[14]
  2. On a Sinclair ZX Spectrum+ the base has eight screws, seven 8 mm pan-head screws round the overhang and one 13 mm countersunk screw at the back next to the expansion slot, and the board is held by two 6.5 mm pan-head screws at its front corners.[15]
  3. Desolder the CPU carefully. The board's tracks, pads and plated-through holes are easily damaged.[16] Take your time and do not force the chip out even when every pin looks free; a little hot air once the pins are clear of solder helps.[13]
  4. Clear the holes and check every pad and track around IC2 for damage.[16]
  5. Fit a 40-pin socket, with its notch the same way round as the original chip.[4][7]
  6. Fit the new CPU in the socket, notch to notch, without bending any pin under.[7]
  7. Look over the board for solder splatter, which Sinclair warns can measure several ohms and cause misleading faults.[17]
  8. Refit the board and close the case without trapping the keyboard ribbons or overtightening the screws.[14]

Testing

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  1. Power on and check for the copyright message.[18]
  2. Connect the interface that failed before (Interface 1, DivMMC or similar) with the power off, and check that it now works. Never plug or unplug an interface with the Spectrum switched on; Worsley puts most Spectrum faults down to that.[7][4]
  3. Run a diagnostic program if one is available. Adam's restoration used ZX Spectrum Diagnostics, which checks the ROM contents and all the RAM.[4]
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References

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  1. ↑ 1.0 1.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 8.10 "Integrated Circuits" (p. 44): IC2 Z80A or µ780. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  2. ↑ 2.0 2.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 6.3.2 "No 3MHz at IC2 pin 6" (p. 32). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  3. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.1 (p. 57). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Adam, "Sinclair ZX Spectrum 48k 'Rubber Key' Restoration", Adam's Vintage Computer Restorations, 6 July 2021: a DivMMC Future failing through a weak M1, socketing and replacing the Z80, the two theories, and the diagnostic run.
  5. ↑ 5.0 5.1 5.2 "Repairing A ZX Spectrum", Retroleum blog (retrieved 2026-10-25): no-start with a non-white border, and crashes with expansions.
  6. ↑ 6.0 6.1 "Spectrum Chips", Retroleum shop (retrieved 2026-10-25): CMOS Z80s as direct replacements, and out-of-specification Z80s with a weak M1.
  7. ↑ 7.0 7.1 7.2 7.3 Ian Worsley, "Repairing Spectrum & Spectrum Plus", from the Lil Old Sinclair Technical Information Repository, republished by World of Spectrum (retrieved 2026-10-25): interfaces plugged in with the power on, and the vertical-bar screen.
  8. ↑ 8.0 8.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 9.10 "Fault Finding – Servicing Addenda" (p. 51): the Interface 1 fault and the M1 signal. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  9. ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data" appended, symptom 11 (PDF p. 65). Third-party field-service material. Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  10. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.3.1 "Basic Checks" and 5.3.1.1 (p. 26): IC2 pins 11 and 29, the clock at IC2 pin 6, the control lines, and the address and data lines. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  11. ↑ ZX Spectrum - Issue 3 circuit diagram, Sinclair Research Ltd: IC2 pin numbers and the edge-connector table. Hosted as File:Sinclair ZX Spectrum Issue 3 circuit diagram.png.
  12. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 9.19 "Integrated Circuits" (p. 55): IC2 Z80A, NEC or Zilog. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  13. ↑ 13.0 13.1 "Fitting an EPROM to a 48K Spectrum", Spectrum for Everyone (retrieved 2026-10-25): removing a soldered chip without forcing it, with hot air.
  14. ↑ 14.0 14.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 2 "Disassembly/Assembly" (p. 16). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  15. ↑ Sinclair Research Ltd, ZX Spectrum + Upgrade Instructions, undated leaflet, Step 6 and final assembly: the Spectrum+ case screws and the two 6.5 mm board screws. Hosted as Sinclair ZX Spectrum+ Upgrade Instructions (PDF).
  16. ↑ 16.0 16.1 1024MAK, post of 9 November 2019 in "Two faulty 4A Spectrums", Spectrum Computing forum: the risk to tracks, pads and plated-through holes when desoldering.
  17. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 7.2 "Repair" (p. 37). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  18. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.3 "Initialization" (p. 26). Hosted as Sinclair ZX Spectrum Service Manual (PDF).


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