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Sinclair ZX Spectrum DC-DC Converter Repair

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Sinclair ZX Spectrum Issue 4B DC-DC converter area
The DC-DC converter area of an Issue 4B board: the coil (marked COIL) beside the 7805 regulator, C34 and C50 below it, C46 and C74 lower down, and TR4 and TR5 at the bottom edge of the picture

The Sinclair ZX Spectrum (16K and 48K) makes the −5 V and +12 V rails that its 4116 lower RAM needs with a small inverter on the main board, built round TR4, TR5 and a two-winding coil. The same circuit is on the Sinclair ZX Spectrum+, which uses the 48K boards.[1][2] Sinclair's service manual calls failure of TR4 "a significant cause of problems", and 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.[3]

The Spectrum repairer 1024MAK gives the usual causes as edge-connector abuse (an interface plugged in or pulled out with the power on, or another short), one or more faulty lower DRAM chips (IC6 to IC13), or a failed electrolytic capacitor in the converter.[4] The faults feed each other. A dead 4116 overloads the +12 V and −5 V rails and kills a new TR4, which can take TR5 with it; and a 4116 left running with +5 V and +12 V but no −5 V can be damaged internally.[5][4] Repair the converter and then find the RAM chip that caused or suffered the failure; replacing the RAM itself is covered in Sinclair ZX Spectrum RAM Replacement.

Symptoms

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  • Dead machine, with smoke on switch-on. Sinclair's first check is TR4 shorted base to collector.[6]
  • A screen of random glitchy horizontal lines with a black border, or a bright, shimmering, fine herring-bone pattern. Retroleum and Ian Worsley both trace these to missing lower-RAM supplies and a failed TR4.[7][8]
  • No faint buzz from the coil. The T.I.S. field-service notes treat a silent inverter with −5 V absent as a sure sign that TR4 must be replaced.[9]
  • A high-pitched whine from the coil is something else: the winding vibrating at the switching frequency, which 1024MAK says is common on older boards.[10] Sinclair changes the coil only if it is excessively noisy.[6]

A machine that starts but shows a dim "Sinclair" logo also points at the +12 V rail; Sinclair's first check for that symptom is +12 V.[6] For other no-picture faults see the Sinclair ZX Spectrum Troubleshooting Guide.

Before you start

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  • Keep each power-up short. 1024MAK's advice is to leave the board on for no more than 20 to 30 seconds until you know the −5 V rail is good, because the 4116 chips can be damaged by +5 V and +12 V without −5 V.[11][4]
  • A failing TR4 gets hot quickly. 1024MAK's method is to hold a finger on TR4 while the board is powered, with the other hand ready to pull the power plug.[12] Retroleum warns that new transistors can be cooked again while you test, so do not leave the power on.[7]
  • The original Sinclair power pack is unregulated, and its nominal 9 V output can rise to between 10 V and 13 V off load.[4] The coil is fed from this input, not from the 7805.[13]
  • The board runs from the 9 V power pack, and the mains transformer is inside that external pack.[1]

Tools and parts

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Parts for the converter
Position Original part Replacements in the sources
TR4 ZTX650 or TIPP31[14][3][15] ZTX651 or ZTX653.[4][12] Sinclair's Issue 4A to 6A list adds the KTC2236A-Y, whose base and collector pins are reversed relative to every other type[3]
TR5 ZTX213 (PNP)[14] BC213, BX214 or FRB925 in Sinclair's list.[3] 1024MAK: any reasonable small-signal PNP transistor, checking the lead-out, because few share the ZTX213's lead-out and outline[12]
Coil "Spectrum TFR", two windings on a toroidal former (N. Devon), part 1703[14][3] No modern equivalent is named in the sources; see the coil
D15 BA157 rectifier[14]
C44, C45 100 µF 16 V electrolytic[14]
C46 1 µF 50 V electrolytic[14] 1 µF 50 V 105 °C, on Issue 1 and Issue 2 boards[16]
C74 4.7 µF electrolytic, 5 V minimum[14] 4.7 µF 16 V radial[16]

Also needed: a multimeter, a desoldering pump or braid, and ideally a bench supply with adjustable current limiting.[12] The parts for bringing an older board up to Sinclair's final arrangement are listed under that step.

How the converter works

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Sinclair ZX Spectrum Issue 3 DC-DC converter circuit
The DC-DC converter on Sinclair's Issue 3 circuit diagram, with the 7805's output capacitor C34 at the left. The box lists the Issue 3B changes.

Sinclair's description: the inverter raises the 9 V input to more than 12 V; the square wave at the junction of TR4's collector and the coil is rectified and smoothed to give +12 V for the RAM, and further smoothing through R62 and C45 gives a separate +12 V for the video circuits and the expansion port.[1] On the Issue 3 diagram the rectifier is D15 (BA157) with C44 on its output; the −5 V rail is taken from TR4's collector through C46, D11, R55 and the 5.1 V zener D16, with D12, C47 and R54 on the output; and the −12 V on edge-connector contact 23B is taken straight from TR4's collector.[13]

1024MAK explains the regulation. TR4 runs as an oscillator, with feedback from the coil's collector winding to its base winding. R59 (1.8 kΩ) and R58 (1 kΩ) divide the +12 V rail down to TR5's base, and TR5 compares that with the +5 V rail on its emitter. If +12 V falls, TR5 turns on harder and drives more current through R60 and R61 into TR4's base, so TR4 transfers more energy to C44; if +12 V rises, TR5 backs off.[17] With +12 V at exactly 12 V, the junction of R58, R59 and TR5's base sits at 4.285 V.[17]

Procedure

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Open the machine

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  1. Unplug every lead. Turn the computer upside down and remove the five self-tapping screws.[18]
  2. Hold the two halves together, turn the computer keys-up and lift the top half. It stays connected by the two keyboard ribbons; note how they lie and do not strain them.[18]
  3. To take the board out, remove the single self-tapping screw next to the IC22 socket and lift the board out. Sinclair suggests making two short pluggable extensions for the keyboard tails, so the keyboard can stay connected with the board out.[18]
  4. 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.[19]

Measure the rails

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  1. Power up briefly and measure at IC6 (the first 4116), negative probe on 0 V: pin 9 should be +5 V ±0.25 V, pin 1 between −4 V and −5.5 V, and pin 8 +12 V ±1.2 V.[20]
  2. Measure the +12 VA rail at the LT side of C52 (+12 V ±1.2 V).[20] 1024MAK also checks it on IC14 (LM1889) pins 14, 15 and 16, which are linked together.[10]
  3. Measure the input at the positive side of C50 (+9 V ±2 V) and the 7805 output at the positive side of C34 (+5 V ±0.25 V). The converter's regulation depends on +5 V being present.[21][17]
  4. If −5 V is missing or wrong, or +12 V is missing or low, the converter has failed. If all the rails are correct, the fault is more likely in one or more of the RAM chips.[11]

Test TR4 and TR5

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Sinclair ZX Spectrum Issue 4B TR4 and TR5
TR4 and TR5 on an Issue 4B board, with D15 below TR4 and the H and N ROM links at the lower right
  1. With the power on, measure TR4's base. 1024MAK expects just over 0.5 V, and never much above 1 V; a base at the same voltage as the collector means TR4 has failed with its base-collector junction shorted and its emitter open.[17]
  2. With the power off, check TR4 for a base-collector short, Sinclair's first check for this fault.[6] Retroleum's test is that the centre pin of a failed TR4 or TR5 normally reads as a dead short to either outer pin.[7]
  3. Remove TR4 if it is shorted. If the short is still there with TR4 out, check TR5 and C44 and look for a damaged track.[6]
  4. Check TR5 out of circuit with a transistor tester, or simply replace it. The T.I.S. notes warn that TR4, TR5 and the coil sometimes fail together, and that a new TR4 can fail again immediately if the others are faulty.[9][12]

Find the RAM that killed TR4

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A new TR4 will not survive a shorted 4116 on the +12 V rail, so clear the RAM before powering the new transistor for any length of time.[5][12]

Worsley's resistance method, with the power off and the meter on its 20 kΩ range, negative lead on the modulator case:[8]

  1. Measure IC6 pin 1 (−5 V). It should read above 12 kΩ. If it reads lower, cut the track to the right of C4, above IC6, and measure pin 1 again.
  2. If pin 1 is still low, measure IC6 pin 8 (+12 V). If it is below 700 Ω, cut the track to the left of C44, at the bottom right corner of IC6.
  3. Power up with the tracks cut and measure on the 20 V DC range: the right-hand side of the cut by C4 should read −4.5 V to −5.2 V, and the right-hand side of the cut by C44 +11.5 V to +12.3 V. If both are in range, go on to test the RAM chips.
  4. To find the faulty chips, cut pin 8 of each 4116 (IC6 to IC13), leaving a stub long enough to solder back. Any chip whose pin 8 stub reads below 20 kΩ is faulty. If IC6 pin 1 still reads below 11 kΩ, cut pin 1 of the remaining chips in the same way; a stub below 12 kΩ marks a faulty chip.
  5. Solder the good chips' legs back, remove the faulty chips and repair the cut tracks without bridging their neighbours.

1024MAK gives the same approach in other forms: isolate the +12 V rail from all the 4116 chips and test the converter alone; cut pin 8 of each 4116 and test one chip at a time with a finger on TR4; or remove all eight, fit sockets and try chips one at a time. He warns that desoldering the RAM can easily damage the tracks, pads and plated-through holes.[12] Retroleum clips out each 4116 until the voltages come back, and notes that all eight may have been damaged.[7] A RAM chip that runs hot is shorted; T.I.S. replaces any very hot RAM.[9]

Where a bench supply with current-limited +12 V and −5 V outputs is available, 1024MAK's alternative is to leave TR4 out and feed the RAM rails from the bench supply while testing the rest of the board. He says this assumes C78 or C80 is fitted; Sinclair's parts lists show one of them on Issues 4B, 5 and 6A.[12][3]

Replace the transistors

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  1. Fit a new TR4 (ZTX650, ZTX651 or ZTX653).[4][12] A KTC2236A-Y has pin 1 as the emitter, pin 2 as the collector and pin 3 as the base, so its base and collector are reversed relative to every other replacement type.[3]
  2. Fit a new TR5, matching the lead-out to the board.[12]
  3. If a replacement TR4 blows again, change TR4 and TR5 together even if TR5 tests good, and check that the components round TR5 are to the latest specification.[6]
  4. Check the other parts of the converter. Retroleum lists the BA157 and 1N4148 diodes, the 5.1 V zener and the 15 Ω resistors on the underside of the board.[7] Worsley checks that R55 (56 Ω, Issue 4 and earlier) or R79 and R62 (Issue 6) are not discoloured, and changes D15 if R55 is badly discoloured.[8]
  5. 1024MAK renews C44 (100 µF), C45 (100 µF), C46 (1 µF), C74 (4.7 µF), C79 (1 µF) and C80 (22 µF) along with TR4 on boards that have the final modification, and calls C43 critical to TR4's operation.[17][22]

The coil

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Worsley puts the likelihood that the coil has shorted turns at 20%.[8] 1024MAK explains why: if TR4 fails short and the machine is left on, the coil overheats and its thin varnish insulation can break down, which is hard to find with a meter because the winding resistance is low anyway.[23] One owner whose replacement TR4s kept failing found discoloured wire on the coil although its resistance and isolation measured correctly; he rewound it with 0.32 mm (28 AWG) wire, using 39 turns (74 cm) on one winding and 15 turns (21 cm) on the other, and the converter then worked.[24] Sinclair's Issue 3 diagram marks the windings 13 and 39 turns, so check the original before rewinding.[13]

Bringing an early board up to the final modification

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Sinclair ZX Spectrum Issue 6A DC-DC converter circuit
The converter on Sinclair's Issue 6A circuit diagram, with the final arrangement built in: C80 and D17 at D15's anode, D18, C79, R79 and the BZY88 zener D19 on the −5 V rail. The larger "560pF" label on C49 is an annotation added to this copy of the drawing.

Sinclair's DC-DC converter modification adds and changes protection components without changing what the circuit does.[3] Do not follow the step-by-step instructions printed in section 9.2 of the hosted service manual: its editor notes that they are incorrect and are kept only for fidelity to the original.[3] 1024MAK has published a corrected version. He says it is needed only on Issue 4A and earlier boards that do not already have it, and that it keeps the −5 V rail from failing if the supply input rises above 11.9 V.[16]

New parts: D15 BA157 (the old D15 may be reusable), D17 BA157, D18 1N4148, D19 BZY88C 5.1 V 500 mW zener, C47 1 µF 16 V, C49 560 pF ceramic if not already that value, C74 4.7 µF 16 V radial if not already fitted, C78 or C80 22 µF 25 V axial, C79 1 µF 16 V, R60 220 Ω, R79 2.2 kΩ, and on Issue 1 and Issue 2 boards only, C46 1 µF 50 V 105 °C.[16]

  1. Remove D12 (1N4148), D16 (5.1 V zener), C47 (22 µF), R54 (100 kΩ), R55 (56 Ω) and R60. Remove C49 if it is not 560 pF, and remove C77.[16]
  2. Lift the anode of D15 out of the board.[16]
  3. Fit C78/C80 between D15's former anode pad (the track that goes to C49) and D15's free anode lead, positive end to the pad.[16]
  4. Fit D17 with its anode to 0 V and its cathode to the new junction of C78/C80 and D15's anode.[16]
  5. Fit R79 (2.2 kΩ) in place of D12, C79 (1 µF, positive to 0 V) in place of D16, the new C47 (1 µF), D19 (cathode to 0 V) in place of R54, D18 (cathode to C46) in place of R55, and R60 (220 Ω). Leave C77's position empty.[16]
  6. If C74 is missing, fit it between the R58 lead at the junction of R58, R59 and TR5's base and the positive lead of C34 (+5 V), positive end to C34.[16]

The Issue 6A circuit diagram shows the same arrangement, with C80 (22 µF) between TR4's collector and D15, D17 (BA157) from that junction to 0 V, R60 220 Ω, and D18, C79, R79 and D19 (BZY88) on the −5 V rail.[25] 1024MAK's own schematic draws the old and new parts together, so read it with his notes.[22]

Close the machine

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  1. Refit the board with its screw, and check that neither keyboard ribbon is trapped between components when the case halves go together.[18]
  2. Refit the five case screws without overtightening them.[18]

Testing

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  1. Power up with a finger on TR4 and the meter on the +12 V rail. If TR4 gets hot, switch off at once and note the voltage that was showing.[17]
  2. Check the rails at IC6 against Sinclair's limits: pin 1 −4 V to −5.5 V, pin 8 +12 V ±1.2 V, pin 9 +5 V ±0.25 V, and +12 VA at C52 ±1.2 V.[20]
  3. Listen for the coil. Worsley expects a buzz when the converter is running.[8] A loud whine can be calmed with nail varnish or conformal coating on the windings, applied sparingly in case the coil has to come out again.[10]
  4. Once the machine starts, run Sinclair's memory check (PRINT PEEK 23732+PEEK 23733*256, and the POKE 85 and POKE 170 tests) to find any RAM chip still faulty; the bit-to-IC table is in the troubleshooting guide.[26]
  5. Worsley's final check is to feel each lower RAM chip after a few minutes' running: none should even be warm.[8]

Revision notes

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Converter changes by board issue
Board Changes in the sources
Issue 1 and 2 Thorn EMI's 1984 manual lists, for Issue 1 boards and again for Issue 2, a change of R60 from 100 Ω to 270 Ω and a 4.7 µF electrolytic from TR5's emitter (C34 positive) to TR5's base (at R58).[27] Sinclair's parts-list note makes C74 "essential on all Issue 2 boards", fitted retrospectively with the larger R60[14]
Issue 3 R60 270 Ω, C49 47 nF, TR4 ZTX650[13]
Issue 3B R60 changed to 68 Ω, C49 to 560 pF, C77 (100 nF) added, TIPP31 allowed for TR4[13][15]
Issue 4A R60 220 Ω; D16 still fitted and D17 to D19 not listed. Sinclair says some 4A boards have the modification built in[3]
Issue 4B Modification built in: D17 (BA157), D18 (1N4148), D19 (5.1 V zener), C79 (1 µF) and C80 (22 µF) fitted; D12 and D16 not used. The parts list shows R79 as not used on 4B[3]
Issue 5 As 4B, with C78 (22 µF) in place of C80, D19 a BZY88C and R79 2.2 kΩ[3]
Issue 6A As 4B, with D19 a BZY88C and R79 2.2 kΩ[3][25]

1024MAK warns that the various board issues' schematics contain errors, and compares one with another and with the service manual.[22]

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References

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  1. ↑ 1.0 1.1 1.2 ZX Spectrum Service Manual, Sinclair Research Ltd, section 1.11 "Power Supplies" (p. 15): the 7805, the TR4/TR5 inverter, the +12 V for the RAM and the separately smoothed +12 V through R62 and C45, and the external pack. Hosted as Sinclair ZX Spectrum Service Manual (PDF). The text names the rectifier "D5"; the diagram and parts list call it D15.
  2. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.1 (p. 57): the Spectrum+ is "basically the same as the 48k ZX Spectrum with a different keyboard and case, and a reset button". Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 1: sections 9.1 and 9.2 (p. 48), with the editor's warning that the printed modification instructions are incorrect; 9.2.1 (p. 50); 9.5 (p. 50); 9.12 notes (p. 51), including note 7 on the KTC2236A pin connections and the TR5 alternatives BC213, BX214 and FRB925; and the Issue 4A to 6A parts lists in 9.13 to 9.20 (pp. 52–56). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 1024MAK, post of 15 May 2024 in "Issue 4B, 12V power missing", Spectrum Computing forum: TR4 replacements, the usual causes of TR4 failure, the unregulated supply rising to 13 V, and RAM damage without −5 V.
  5. ↑ 5.0 5.1 1024MAK, post of 3 February 2020 in "48k power supply inverter circuit", Spectrum Computing forum: a dead 4116 overloading the rails and killing replacement transistors.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 ZX Spectrum Service Manual, Sinclair Research Ltd, sections 6.1 "TV screen dead, smoke appears on switch-on" (p. 30), 6.4.2 "Dim 'Sinclair' logo" (p. 32) and 6.8 "Regulator Coil" (p. 34). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  7. ↑ 7.0 7.1 7.2 7.3 7.4 "Repairing A ZX Spectrum", Retroleum blog (retrieved 2026-10-25): the glitchy-lines symptom, the TR4/TR5 short test, the converter's diodes and 15 Ω resistors, and clipping out 4116 chips.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Ian Worsley, "Repairing Spectrum & Spectrum Plus", from the Lil Old Sinclair Technical Information Repository, republished by World of Spectrum (retrieved 2026-10-25): the resistance checks on IC6, the converter-side voltages after cutting the tracks, the coil, and the screen symptoms.
  9. ↑ 9.0 9.1 9.2 ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data" appended, symptom 3 "No coil buzz from inverter, −5V absent" (PDF p. 64). Third-party field-service material. Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  10. ↑ 10.0 10.1 10.2 1024MAK, post of 4 November 2019 in "Two faulty 4A Spectrums", Spectrum Computing forum: test points for −5 V, +12 V and +12 VA, and the coil whine.
  11. ↑ 11.0 11.1 1024MAK, post of 13 August 2018 in "Have I killed my ZX Spectrum?", Spectrum Computing forum: short power-ups until −5 V is confirmed, and renewing TR4 and TR5 when −5 V or +12 V is wrong.
  12. ↑ 12.00 12.01 12.02 12.03 12.04 12.05 12.06 12.07 12.08 12.09 1024MAK, post of 9 November 2019 in "Two faulty 4A Spectrums", Spectrum Computing forum: why replacement TR4s fail, TR5 replacement, and four ways to isolate the faulty RAM.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 ZX Spectrum - Issue 3 circuit diagram, Sinclair Research Ltd. Hosted as File:Sinclair ZX Spectrum Issue 3 circuit diagram.png. Source for the converter's parts and connections, the 13- and 39-turn windings and the Issue 3B change box.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 14.6 14.7 ZX Spectrum Service Manual, Sinclair Research Ltd, section 8 parts lists for Issues 2 and 3 (pp. 41–46): note 3 (C74 essential on Issue 2, with the larger R60), note 7 (Issue 3B converter changes), C44/C45 100 µF 16 V, C46 1 µF 50 V, C74 4.7 µF, D15 BA157, the coil (Spectrum, type 1703), TR4 ZTX650/TIPP31 and TR5 ZTX213. Hosted as Sinclair ZX Spectrum Service Manual (PDF). The re-typeset Issue 3 transistor column prints TR4 as ZTX313, which does not match the Issue 3 diagram's ZTX650.
  15. ↑ 15.0 15.1 ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), section 4, paragraph 1.5 "Modifications - Issue 3B Board" (PDF p. 31). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  16. ↑ 16.00 16.01 16.02 16.03 16.04 16.05 16.06 16.07 16.08 16.09 1024MAK, "ZX Spectrum 16K, 48K and Plus models – DC-DC Converter Modification", 8 & 16 bit computer repairs and mods, 23 August 2019, version 7E3823_02, posted by markkinsey: the corrected parts list and procedure.
  17. ↑ 17.0 17.1 17.2 17.3 17.4 17.5 1024MAK, post of 23 May 2024 in "Issue 4B, 12V power missing", Spectrum Computing forum: how the converter regulates, TR4 base voltages, the capacitors to renew and the finger test.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 ZX Spectrum Service Manual, Sinclair Research Ltd, section 2 "Disassembly/Assembly" (p. 16). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  19. ↑ 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).
  20. ↑ 20.0 20.1 20.2 ZX Spectrum Service Manual, Sinclair Research Ltd, section 3.1.2 "Voltage Check" (p. 17): IC6 pin 9 +5 V ±0.25 V, pin 1 −5 V (−5.5 V to −4 V), pin 8 +12 V ±1.2 V, LT side of C52 +12 VA ±1.2 V. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  21. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.3.1 "Basic Checks" (p. 26): regulator input at the positive side of C50, +9 V ±2.0 V; regulator output at the positive side of C34, +5 V ±0.25 V. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  22. ↑ 22.0 22.1 22.2 1024MAK, posts of 27 May 2024, 28 May 2024 and 28 May 2024 in "Issue 4B, 12V power missing", Spectrum Computing forum: errors on the schematics, C43 critical to TR4, C49 560 pF and C77 removed after the final modification, and how to read his schematic.
  23. ↑ 1024MAK, post of 19 February 2020 in "48k power supply inverter circuit", Spectrum Computing forum: coil overheating and insulation failure.
  24. ↑ gmc, post of 19 February 2020 in "48k power supply inverter circuit", Spectrum Computing forum: the repaired Issue 3 board and the coil rewind. One owner's repair.
  25. ↑ 25.0 25.1 ZX Spectrum (Issue 6A) Circuit Diagram, Sinclair Research Ltd. Hosted as File:Sinclair ZX Spectrum Issue 6A circuit diagram.png.
  26. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.4 "Memory Check" (pp. 27–28). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  27. ↑ ZX Spectrum Servicing Manual, prepared by Thorn (EMI) Datatech Ltd for Sinclair Research Ltd, March 1984, section 4, paragraph 1.3(h) (PDF p. 30). Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).


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