Sinclair ZX Spectrum Capacitor Replacement Guide

Sinclair's Issue 3 circuit diagram. The regulator, the TR4/TR5 inverter and the capacitors around them are at the bottom left.

This guide covers the capacitors on the original rubber-key Sinclair ZX Spectrum (16K and 48K), board Issues 1 to 6A. Every designator, value, voltage rating and tolerance below is taken from the parts lists in Sinclair's own ZX Spectrum Service Manual, which is hosted on this wiki.

For the later machines, which use different boards, see the Spectrum+, Spectrum 128 and Spectrum +2 guides.

Is the Spectrum a recap candidate?

No — not as a matter of course. The Spectrum uses ordinary leaded electrolytics and has no systematic capacitor-failure problem. There is no reason to shotgun every capacitor on a board that works.

Capacitors do nevertheless cause real, documented faults here, and Sinclair published a neat test for it: if the picture shows wavy lines, press down on each electrolytic in turn with the machine running. If the lines disappear while you are pressing one, that is the faulty capacitor.

Worth attention on any board:

• C50 and C34 — the regulator input and output. These are Sinclair's own supply test points, so a rail that is out of tolerance or visibly rippling points straight at them.
• C44 and C45 — the rectifier smoothing for the +12 V and +12 VA rails produced by the TR4/TR5 inverter. C44 is one of the parts to check when the inverter has failed short.
• C46 — Sinclair specified this be replaced with a high-temperature component as a service modification, on all units.
• C65 — implicated by Sinclair in a dim picture, where it has gone high-impedance.
• Anything bulging, leaking, or that changes the symptom when you press it.

Which board have you got?

This matters more on the Spectrum than on most machines: the capacitor complement genuinely differs between board issues. C1–C8 are 47 nF on Issue 2 and Issue 3 but 22 nF from Issue 4A onwards; C44 and C45 are not fitted at all on Issue 5; C47 disappears after Issue 4B; C78, C79 and C80 exist only on the later boards. A capacitor list that does not say which issue it applies to is wrong even where the numbers happen to be right for one of them.

Sinclair shipped Issue 1, 2, 3, 3B, 4A, 4B, 5 and 6A boards. There was no Issue 4 and no Issue 6 — neither was ever issued.

The issue number is printed on the board. Work from it.

Board issues and what changed

Issue	Notes
1	The 16K RAM is on the main board; the 32K extension is a subsidiary board on special DIL connectors. Approximately 26,000 units were made. Sinclair published no separate Issue 1 capacitor parts list, and none is reconstructed here
2	All 48K of RAM can be fitted directly to the board. Has the colour presets VR1/VR2 and trimmers TC1/TC2. Some early units used disc capacitors where axials were later fitted
3	Colour tuning made automatic, so the presets and trimmers are gone; allows OKI parts in the extension memory; the heatsink is redesigned and moved to the back of the case
3B	Functionally like Issue 3 but a different layout, with an improved DC converter: C77 (100 nF) added, C49 changed from 47 nF to 560 pF, R60 from 270 R to 68 R
4A	R32 deleted; two spare gates of IC24 used to improve ULA timing, which requires ULA 6C001-7 or later
4B	Issue 4A plus the DC-DC converter modification. C61 was omitted for want of board space and reinstated on later issues
5	Only 1,000 boards were supplied, UK market only. A single ULA (IC27, Mullard ZX 84/01) replaces six 74LS packages, and IC28 is added
6A	Like Issue 5, but IC1 may be a Ferranti or a SAGA part

Capacitors, by board issue

Sinclair's two parts lists, combined. Unless otherwise stated all are axial types.

ZX Spectrum 16K/48K main board capacitors

Ref	Issue 2	Issue 3	Issue 4A	Issue 4B	Issue 5	Issue 6A	Rating / tol.	Type
C1–C8	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C9–C24	Not used
C25	22 µF	22 µF	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C26	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C27	1 µF	1 µF	1 µF	1 µF	1 µF	1 µF	50 V	Electrolytic
C28	22 µF	22 µF	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C29	47 nF	47 nF	47 nF	47 nF	47 nF (printed "44nf" — see below)	22 nF	25 V, 10%	Ceramic
C30	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C31	100 nF	100 nF	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C32	100 nF	100 nF	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C33	47 nF	47 nF	22 nF	22 nF	22 nF	47 nF	25 V, 10%	Ceramic
C34	22 µF	22 µF	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C35	10 nF	10 nF	10 nF	10 nF	10 nF	10 nF (100 nF with the Ferranti ULA)	25 V, 10%	Ceramic
C36	47 nF	Not fitted	Not used	Not used	Not used	Not used	25 V, 10%	Ceramic
C37, C38	33 pF	33 pF	33 pF	33 pF	33 pF	33 pF	25 V, 10%	Ceramic
C39	10 nF	10 nF	10 nF	10 nF	10 nF	10 nF	25 V, 10%	Ceramic
C40, C42	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C43	100 nF	100 nF	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C44, C45	100 µF	100 µF	100 µF	100 µF	Not used	100 µF	16 V, −10% +80%	Electrolytic
C46	1 µF	1 µF	1 µF	1 µF	1 µF	1 µF	50 V, −10% +80%	Electrolytic
C47	22 µF	22 µF	22 µF	22 µF	Not used	Not used	10 V, −10% +80%	Electrolytic
C48	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C49	47 nF	47 nF (560 pF from Issue 3B)	560 pF	560 pF	560 pF	560 pF	25 V, 10%	Ceramic
C50	22 µF	22 µF	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C51	Not used
C52, C53	150 pF	150 pF	150 pF	150 pF	150 pF	150 pF	25 V, 10%	Ceramic
C54	470 pF	Not fitted	Not used	Not used	Not used	Not used	25 V, 10%	Ceramic
C55–C60	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C61	47 nF	47 nF	22 nF	Not used	22 nF	22 nF	25 V, 10%	Ceramic
C62	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C63	47 pF	47 pF	47 pF	47 pF	Not used	Not used	25 V, 10%	Ceramic
C64	100 pF	100 pF	180 pF	180 pF	Not used	Not used	25 V, 10%	Ceramic
C65	100 µF (22 µF after the colour modification)	22 µF	22 µF	22 µF	22 µF	22 µF	20 V, −10% +80%	Electrolytic
C66	47 nF	47 nF	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C67	Not fitted (100 pF once modified)	100 pF	100 pF	100 pF	100 pF	100 pF	25 V, 10%	Ceramic
C68–C71	Not fitted	100 nF	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C72, C73	Not fitted	16 pF	16 pF	16 pF	16 pF	16 pF	25 V, 5%	Ceramic
C74	Not fitted (47 µF once modified)	4.7 µF	4.7 µF	4.7 µF	4.7 µF	4.7 µF	5 V minimum	Electrolytic
C75	Not fitted	100 nF	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C76	Not fitted	47 nF	47 nF	47 nF	47 nF	22 nF	25 V, 10%	Ceramic
C77	Not fitted	Added on Issue 3B (100 nF)	100 nF	100 nF	Not used	Not used	25 V, 10%	Ceramic
C78	Not fitted	Not fitted	Not used	Not used	22 µF	Not used	25 V	Electrolytic
C79	Not fitted	Not fitted	Not used	1 µF	1 µF	1 µF	16 V, 10%	Electrolytic
C80	Not fitted	Not fitted	Not used	22 µF	Not used	22 µF	25 V	Electrolytic
C100	Not fitted	Not fitted	Not used	Not used	Not used	10 nF (VHF models only)	25 V, 10%	Ceramic
C101	Not fitted	Not fitted	Not used	Not used	Not used	22 nF (VHF models only)	25 V, 10%	Ceramic
C104	Not fitted	Not fitted	Not used	Not used	100 nF	100 nF	25 V, 10%	Ceramic
C105	Not fitted	Not fitted	Not used	Not used	180 pF	180 pF	25 V, 10%	Ceramic

C41 appears in Sinclair's modification notes as a 47 nF part that should be an axial rather than a disc type, but it is not listed in either parts-list table; neither is a value for it given anywhere else in the manual. It is therefore not tabulated above.

Where the two parts lists disagree with themselves

Sinclair's tables contain several obvious misprints. They are recorded here so that you can check this page against the manual and see why it differs:

• C44/C45 are given as type "Ceramic" in section 9.13 but "Electrolytic" in section 8.5. A 100 µF part with a −10% +80% tolerance is an electrolytic; the 9.13 type cell is wrong.
• C47 is given as "22nf" in section 9.13, with an electrolytic type and a 10 V, −10% +80% rating. Section 8.5 and the DC-DC converter modification in section 9.2 both call it a 22 µF electrolytic. It is 22 µF.
• C29 on Issue 5 is printed as "44nf", which is not a preferred value. The table above shows 47 nF, matching every other issue except 6A, but that is an inference and not what the manual prints — check your own board.
• C78 is 1 µF 25 V in section 9.2 but 22 µF 25 V in the section 9.13 parts list. Section 9.2 carries the editor's warning that its instructions are incorrect, so the parts list is the better record.

Every one of these is flagged rather than quietly corrected, so that this page can be checked line by line against the manual.

The electrolytics — the actual shopping list

These are the service items. Everything else on the board is ceramic and is not normally replaced.

Electrolytics by board issue

Board	Electrolytics fitted
Issue 2 / Issue 3	C25, C27, C28, C34, C44, C45, C46, C47, C50, C65, and C74 on Issue 3 (or on an Issue 2 board that has had the modification)
Issue 4A	C25, C27, C28, C34, C44, C45, C46, C47, C50, C65, C74
Issue 4B	As Issue 4A, plus C79 and C80
Issue 5	C25, C27, C28, C34, C46, C50, C65, C74, C78, C79 — no C44, C45 or C47
Issue 6A	C25, C27, C28, C34, C44, C45, C46, C50, C65, C74, C79, C80 — no C47

Choosing replacements

• Use 105 °C aluminium electrolytics, equal capacitance, equal or higher voltage. The original 10 V, 16 V and 20 V parts are comfortably replaced by 16 V, 25 V or 35 V modern equivalents.
• Note how wide the original tolerances are — several parts are specified −10% +80%. A modern ±20% capacitor is well inside Sinclair's specification.
• C46 should be a high-temperature type. Sinclair made this a service change to be applied to all units, so it is worth doing on any board still carrying the original.
• C74 is essential on Issue 2 boards and should be fitted retrospectively if missing, and R60 must be changed to the larger value at the same time.
• Sinclair's originals are axial unless the parts list says otherwise. Axials are still made; if you fit radial parts instead, form and sleeve the leads so nothing can touch an adjacent track.
• No lead-pitch or body-height figures are given here. Sinclair published none, and the case clearance depends on which board issue and which heatsink you have. Measure your own board.

Replacement procedure

1. Unplug the supply. There are no mains voltages inside the computer: it runs from an external 9 V pack and the highest rail on the board is +12 V.
2. Open the case. Five self-tapping screws in the base, and one more holding the board.
3. Withdraw the keyboard tails from KB1 (5-way) and KB2 (8-way). Pull them straight. Sinclair's own instruction is that the ribbon connectors must be fully inserted and not kinked during insertion; they crack where they bend.
4. Record value, voltage and polarity of every electrolytic before you remove it.
5. Add fresh flux and solder to the old joints and let it mix before wicking. Use an earthed, low-power iron and recognised desoldering technique — Sinclair's own instruction, aimed at not lifting pads.
6. Fit the replacements the right way round, solder and trim.
7. Inspect for solder splatter under magnification before reassembly. The manual is explicit that hairline splatter can measure several ohms and produce very misleading fault symptoms.
8. Check the heatsink. If you disturbed the regulator, make sure it makes good contact with its heatsink, apply thermal grease to the back of its body, and on an Issue 3 board make sure the heatsink is positioned away from the edge connector — the fixing hole allows enough play to foul it.

Checks after the work

Sinclair's own setting-up voltage check, with the machine out of the case. IC6 is one of the lower-RAM 4116 devices, which is where all three rails meet.

Internal supply voltages

Test point	Nominal	Tolerance	Source
IC6 pin 9	+5 V	±0.25 V
IC6 pin 1	−5 V	−5.5 V to −4 V
IC6 pin 8	+12 V	±1.2 V
LT side of C52	+12 VA	±1.2 V
Positive side of C50 (regulator input)	+9 V	±2.0 V; below +7 V the regulator will not function correctly
Positive side of C34 (regulator output)	+5 V	±0.25 V, with no discernible ripple

Sinclair published no ripple figure in millivolts, for any rail. The manual's criterion is "no discernible ripple" at the regulator output, and none is invented here.

Then confirm the machine initialises to a clear screen with the copyright message in the lower left, that the picture is stable with no wavy lines, and that sound, tape and the whole keyboard work.

The rails, and why the Spectrum has three

Unlike the ZX81 and ZX80, which have a single +5 V rail, the Spectrum generates three supplies on board from the single external 9 V input:

• +5 V from a 7805 regulator, for the logic, the ULA and the modulator.
• −5 V and +12 V for the 4116 lower RAM, produced by the TR4/TR5 inverter, which raises the 9 V input above 12 V. The square wave at the junction of the TR4 collector and the inverter coil is rectified and smoothed by D15/C44.
• +12 VA for the colour circuits, taken from the same rectifier through R62/C45 so that it is free of the switching noise the RAM generates. It is also available on the expansion port.

This is the reason the +12 V and −5 V rails matter on a 16K/48K Spectrum and not on a 128 — the 4116 DRAM needs all three supplies, and the later machines' RAM does not.

Failure of TR4 is a significant cause of problems in Sinclair's own words, and the recommended DC-DC converter modification is specifically aimed at units in which either TR4 or the 16K DRAM has blown. If a recap has not cured a rail fault, that circuit is where to look next; the symptom-by-symptom treatment is in the Sinclair ZX Spectrum Troubleshooting Guide.

Tools and materials

See Recommended Tools. For this job specifically:

• Earthed, temperature-controlled iron with a fine tip, and normal anti-static precautions
• Desoldering braid and/or a pump
• 63/37 leaded solder and fresh flux
• 105 °C aluminium electrolytics to the list above for your board issue
• Thermal grease, if the regulator comes off
• Isopropyl alcohol, a soft antistatic brush and magnification

Notes

• Check the power supply before you fit new capacitors. A supply that is out of specification will simply damage the new ones. The external pack is unregulated and the polarity is unusual — see Sinclair ZX Spectrum General Maintenance.
• If a previously working function fails after a recap, check the polarity of every capacitor you replaced before looking anywhere else.
• See Capacitor Failure Symptoms for general failure modes, and Battery Explosion, Capacitor or Corrosion Damage if there is corrosion to clean up.

Related pages

• Sinclair ZX Spectrum
• Sinclair ZX Spectrum General Maintenance
• Sinclair ZX Spectrum Troubleshooting Guide
• Sinclair ZX Spectrum Service Manual
• Sinclair ZX Spectrum+ Capacitor Replacement Guide
• Sinclair ZX Spectrum 128 Capacitor Replacement Guide
• Capacitor Failure Symptoms