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Sinclair ZX Spectrum Voltage Regulator Replacement

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Sinclair ZX Spectrum Issue 4B 7805 regulator and power socket
The 7805 regulator (here a National LM340T5) bolted to the heatsink of an Issue 4B board, next to the 2.1 mm power socket

The Sinclair ZX Spectrum (16K and 48K) and the Sinclair ZX Spectrum+ take an unregulated 9 V supply through a 2.1 mm coaxial socket and make their +5 V rail with a single 7805 regulator bolted to a heatsink. That rail feeds the logic, the ULA and the UHF modulator.[1][2][3] The regulator and the socket fail in a few well-known ways: a supply plugged in with the wrong polarity, a poor contact between the regulator and its heatsink, and a socket whose joints have cracked or corroded.[4][5][6] The −5 V and +12 V rails come from a separate inverter, covered in Sinclair ZX Spectrum DC-DC Converter Repair.

Symptoms

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  • Nothing happens at all: no key clicks and no picture, as if there were no power. Retroleum checks the supply, the power socket and the 7805's output.[4]
  • Faults that appear only when the machine is warm. The T.I.S. notes check the heatsink first.[7]
  • Crashes or resets when the machine or its lead is moved. Worsley blames the power plug or socket.[6]
  • The +5 V rail above 5.2 V. Worsley changes the regulator at once, and hopes not too many chips have been damaged.[6]

Before you start

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  • The Spectrum's power socket is centre negative, outer positive, the opposite of most modern adaptors. A supply of the wrong polarity is likely to have destroyed the 7805.[4][8]
  • The original pack is unregulated: Retroleum expects about 13.5 V off load from a Sinclair UK1400, and 1024MAK says the nominal 9 V can be anywhere between 10 V and 13 V.[4][9]
  • Unplugging or plugging interfaces with the power on damages the power resistors and the regulator, according to the T.I.S. service data.[10]

Measure

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Sinclair ZX Spectrum Issue 3 regulator and power input circuit
The power input and 7805 on Sinclair's Issue 3 circuit diagram: the PWR socket, C50 and C66 on the input, C34 on the output, and the +9 V, 0 V and +5 V edge-connector contacts
  1. Measure the regulator's input at the positive side of C50: +9 V ±2.0 V. Below +7 V the regulator will not work correctly.[11]
  2. Measure its output at the positive side of C34: +5 V ±0.25 V, with no discernible ripple.[11] Worsley measures the right-hand leg of the 7805, with the tab away from you, and expects 4.85 V to 5.2 V.[6]
  3. If the output is high, change the regulator. If it is low, but not below 4.3 V, something on the board is shorting the rail; feel the chips for one running hot, remembering that the ULA always runs warm. If +5 V is all but missing, replace the regulator but do not run the machine yet; Worsley's next step is the resistance check on the RAM supply pins described in the DC-DC converter guide.[6]
  4. With no +5 V, Sinclair also checks that the power socket is not shorted and that the power pack works, and checks the voltage drop across R64.[12]
  5. If the voltages are right but the machine misbehaves when warm, check the regulator's contact with the heatsink.[7][5]

Tools and parts

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Parts
Item Original Notes
Regulator 7805[13] Switch-mode drop-in replacements exist; Adam's restoration fitted a Traco Power TSR-2450 and removed the heatsink[14]
Fixings 4BA cheese-head screw (15/16 in), 4BA shakeproof washer and 4BA nut[13]
Heat transfer Thermal grease on the back of the regulator[5]
Power socket 2.1 mm coaxial socket (Hoseiden)[15] New sockets are sold by Retroleum[14]

Procedure

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Open the machine

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  1. Unplug every lead, remove the five self-tapping screws from the base and lift the top half, minding the keyboard ribbons.[16]
  2. Remove the board screw next to the IC22 socket and lift the board out.[16]
  3. 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.[17]

Inspect

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  1. Check that the heatsink is not loose and that its screws are not overtightened.[18]
  2. Look for cracks in the board, especially radiating from the heatsink screws, and for damage to the print round the screws.[19][20]
  3. Look at the power socket's solder joints for cracks, and at the socket for corrosion.[4]

Replace the regulator

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  1. Undo the regulator's bolt and lift the heatsink away.[14][13]
  2. Desolder the regulator using proper desoldering and heatsinking technique, so that the board is not damaged.[5]
  3. For a 7805: apply thermal grease to the back of the new regulator's body, fit it, and bolt it to the heatsink with the 4BA screw, shakeproof washer and nut so that the body makes good contact with the heatsink.[5][13]
  4. On Issue 3 boards the heatsink's hole allows enough play for the heatsink to foul the edge connector. Fit it away from the edge connector.[5]
  5. For a switch-mode replacement, solder it into the 7805's three holes; Adam's restoration left the heatsink off.[14]

Replace the power socket

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  1. Do not try to tighten a loose socket; replace it.[6]
  2. Desolder the old socket and fit a new 2.1 mm socket in its place.[6][14]

Close the machine

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  1. Refit the board and its screw, check that the keyboard ribbons are not trapped, and refit the five case screws without overtightening them.[16]

Testing

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  1. Confirm the polarity of the supply before connecting it.[8]
  2. Measure +5 V ±0.25 V at the positive side of C34, and +5 V at IC1 pin 14, IC2 pin 11 and IC5 pin 28.[11]
  3. Check the other rails: −5 V and +12 V at IC6 pins 1 and 8.[21]
  4. Run the machine for some minutes and check that any faults that used to appear when warm have gone.[7]

Spectrum+ boards

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When Sinclair moved Issue 1 and Issue 2 boards into Spectrum+ cases, it changed the heatsink, keeping the triangular insulator if one was fitted.[22] Later boards keep their own heatsink.

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References

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  1. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 1.11 "Power Supplies" (p. 15). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  2. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 8.7 "Connectors" (p. 44): PWR, 2.1 mm co-axial socket. 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 "Repairing A ZX Spectrum", Retroleum blog (retrieved 2026-10-25): the dead-machine checks, socket polarity, the UK1400's off-load voltage, the power socket and a 7805 destroyed by reversed polarity.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 ZX Spectrum Service Manual, Sinclair Research Ltd, section 7.2 "Repair" (p. 37): regulator contact with the heatsink, the Issue 3 heatsink fouling the edge connector, and thermal grease. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Ian Worsley, "Repairing Spectrum & Spectrum Plus", from the Lil Old Sinclair Technical Information Repository, republished by World of Spectrum (retrieved 2026-10-25): measuring the 7805, what high and low readings mean, and the power socket.
  7. ↑ 7.0 7.1 7.2 ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data", symptom 8 "Various faults which appear only when warm" (PDF p. 65). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  8. ↑ 8.0 8.1 "Sinclair ZX Spectrum Power Supplies – What Types Are There (Part 1)?", PopeyMon Games, 3 February 2021 (retrieved 2026-10-25): centre-negative polarity.
  9. ↑ 1024MAK, post of 15 May 2024 in "Issue 4B, 12V power missing", Spectrum Computing forum: the unregulated input between 10 V and 13 V.
  10. ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Service Data" (PDF p. 59). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  11. ↑ 11.0 11.1 11.2 ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.3.1 "Basic Checks" (p. 26). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  12. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 6.2.3 "No +5V input" (p. 31). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  13. ↑ 13.0 13.1 13.2 13.3 ZX Spectrum Service Manual, Sinclair Research Ltd, section 8.13 "Miscellaneous" (p. 46): 7805 regulator, special heatsink, and the 4BA screw, washer and nut. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  14. ↑ 14.0 14.1 14.2 14.3 14.4 Adam, "Sinclair ZX Spectrum 48k 'Rubber Key' Restoration", Adam's Vintage Computer Restorations, 6 July 2021: the Traco TSR-2450 replacement, removing the heatsink, and new jacks from Retroleum.
  15. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 9.15 "Connectors" (p. 53): PWR, 2.1 mm co-axial socket, Hoseiden. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  16. ↑ 16.0 16.1 16.2 ZX Spectrum Service Manual, Sinclair Research Ltd, section 2 "Disassembly/Assembly" (p. 16). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  17. ↑ 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).
  18. ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data" appended, note 1 "General repair procedure" (PDF p. 60). Third-party field-service material. Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  19. ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data", symptom 9 "Intermittent faults" (PDF p. 65). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  20. ↑ ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data", symptom 5 "No coil buzz from inverter, REG7805 open circuit" (PDF p. 64). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  21. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 3.1.2 "Voltage Check" (p. 17). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  22. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.2 "Conversion Procedure – Spectrum to Spectrum+" (p. 57). Hosted as Sinclair ZX Spectrum Service Manual (PDF).


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