Sinclair ZX Spectrum 128 Capacitor Replacement Guide

ZX Spectrum 128 circuit diagram, sheet 1 of 3. Every designator in the tables below can be located on the three sheets hosted on this wiki.

This guide covers the capacitors on the Sinclair ZX Spectrum 128. Sinclair published a full component-level parts list for the 128, so unlike the Amstrad-built +2 every capacitor's designator, value, rating and type is documented — the tables below are transcribed from Table 5.4 (main PCB) and Table 5.5 (keypad PCB) of the Service Manual.

Safety and board handling

There are no mains voltages inside the computer — it runs from an external supply and the highest rail on the board is +12 V, so capacitor work here is low voltage.

The board still needs care. Use recognised desoldering technique and an earthed, low-power iron; add fresh flux and solder to old joints before removing anything, keep dwell times short, and inspect for solder splatter afterwards — hairline bridges can measure several ohms and cause very misleading faults.

Pay particular attention to the area around the heatsink fixings, where hairline cracks in the board are a documented cause of intermittent faults on Spectrums.

Is the 128 a recap candidate?

The 128 uses ordinary leaded electrolytics and is not a machine with a systematic capacitor plague — it is not a "recap on sight" case. There are, however, only 13 electrolytics on the whole main board, so a targeted recap is quick and cheap when it is justified.

Recap when there is evidence:

• Visibly failed parts — bulging, leaking, crusty residue or a cracked sleeve.
• Supply instability — random crashes, or rails out of tolerance or showing ripple. The parts to look at are C25, C44/C45 (supply smoothing) and C111/C112 (the −12 V charge pump).
• Dead RS-232 or keypad — check C111/C112 in the −12 V charge pump, since both ports depend on that rail.
• Wavy lines across the display — press each electrolytic in turn; if the artefact clears while pressing one, that is the culprit.

Do not recap to cure jailbars. Vertical banding on the RGB output is a noise artefact that survives a routine recap; it has its own specific modification, described below.

Main board capacitors

From Table 5.4 of the Service Manual. Unless otherwise stated all capacitors are axial types. Designators not listed here are marked "not used" in the parts list — the 128 board has many unpopulated positions.

Electrolytics — the recap targets

ZX Spectrum 128 main PCB — electrolytic capacitors

Ref	Value	Rating / tolerance	Notes
C25	100 µF	10 V, −10% +80%	Supply smoothing
C27	1 µF	50 V, 10%
C28	22 µF	10 V, −10% +80%	Right of the 5-way keyboard connector. Changed to 47 µF in the jailbars modification
C34	22 µF	10 V, −10% +80%
C44, C45	100 µF	10 V, −10% +80%	C44 smooths the +12 V rail with D15
C50	22 µF	10 V, −10% +80%
C74	4.7 µF	5 V minimum
C80	22 µF	10 V, −10% +80%
C111, C112	47 µF	16 V, −10% +80%	The −12 V charge pump, with D28/D29. Feeds the MC1488 RS-232 driver and the keypad
C123	1 µF	10 V, −10% +80%
C127	1 µF	10 V, −10% +80%

Ceramics — normally left alone

ZX Spectrum 128 main PCB — ceramic capacitors

Ref	Value	Rating / tolerance
C1–C8	22 nF	25 V, 10%
C26	22 nF	25 V, 10%
C29, C30	22 nF	25 V, 10% (see note)
C31, C32	100 nF	25 V, 10%
C35	10 nF	25 V, 10%
C41	22 nF	25 V, 10%
C43	100 nF	25 V, 10%
C49	560 pF	25 V, 10%
C55–C62	22 nF	25 V, 10%
C66	22 nF	25 V, 10%
C67	100 pF	25 V, 10%
C75	100 nF	25 V, 10%
C100	10 nF	25 V, 10%
C101	22 nF	25 V, 10%
C104	100 nF	25 V, 10%
C105	180 pF	25 V, 10%
C106–C110	22 nF	25 V, 10%
C113, C114	47 nF	25 V, 10%
C115	330 pF	25 V, 2%
C116	10 nF	25 V, 10%
C118, C119	1 nF	25 V, 10%
C120	100 pF	25 V, 10%
C121	47 pF	25 V, 10%
C122	1 nF	25 V, 10%
C124	20 pF	25 V, 2% — see note below
C125	100 nF	25 V, 10%
C126	22 pF	25 V, 10%
C128	47 nF	25 V, 10%
C129	100 pF	25 V, 10%
C130	See note	25 V, 2%

Note on C29 and C30. Sinclair's printed table lists "C29, C31" as 22 nF and then lists C31 again as 100 nF. C30 is otherwise absent from the table entirely, so the first entry is evidently C29 and C30. Check your own board rather than trusting either reading.

Note on C124 and C130. The Service Manual states that if preferred, the 20 pF capacitor used for C124 may be split into two parallel capacitors of 10 pF ±2% fitted at positions C124 and C130. That is why C130's value is given by reference rather than as a number — whether it is populated depends on which option the board was built with. Check your own board.

The manual also notes that provision was made on the PCB for a 2–22 pF film dielectric trimmer should the need arise.

The tight-tolerance parts (C115, C124, C130 at 2%) sit in timing and colour circuitry — if you replace them, match the tolerance, not just the value.

Keypad PCB capacitors

The numeric keypad has its own small board carrying a General Instrument PIC1652 microcontroller:

Keypad PCB (Table 5.5) — axial types

Ref	Value	Rating / tolerance	Type
C1	1 µF	10 V, −10% +80%	Electrolytic
C2, C3	47 pF	25 V, 10%	Ceramic

The jailbars modification

Faint vertical banding on plain, light-coloured areas, visible on the RGB output but not over RF or composite, is a well-known Toastrack characteristic. It is a noise and ground-plane artefact, so an ordinary recap does not cure it. The established modification is:

• Replace C28 with a 47 µF axial electrolytic in place of the original 22 µF. C28 sits to the right of the 5-way keyboard connector.
• Replace C7 and C8 with 1 µF ceramic capacitors in place of the original 22 nF parts. These sit next to the IC12 and IC13 RAM devices at the bottom left of the board.

This significantly reduces the banding without eliminating it entirely. On the 48K machines the equivalent change is C5–C8 to 150–220 nF; on a grey +2 it is C52.

Choosing replacements

• Use 105 °C aluminium electrolytics of equal capacitance and equal-or-higher voltage. The originals are 10 V and 16 V parts; modern 16 V or 25 V equivalents are fine.
• Note the wide original tolerances (−10% +80% on most electrolytics) — modern ±20% parts are comfortably within specification.
• Most originals are axial. Axial parts remain available; if fitting radial replacements, form and insulate the leads neatly so nothing shorts to adjacent tracks.
• Match the tolerance on the 2% parts (C115, C124, C130).
• The ceramics are stable and are not normally replaced — except deliberately, as part of the jailbars modification.

Replacement procedure

1. Unplug the power, open the case and disconnect both keyboard tails carefully.
2. Lift the board out so it is supported flat.
3. Record the value, voltage and polarity of each electrolytic before removing it, cross-checking against the table above and the circuit diagrams.
4. Add fresh flux and solder to both joints and let it mix with the old solder.
5. Lift one lead at a time with the joint fully molten, then clear the hole with wick.
6. Fit the replacement the correct way round; solder and trim.
7. Inspect for bridges and splatter before reassembly.

Post-recap verification

1. Power up — ideally on a current-limited supply — and confirm the 128 menu appears.
2. Check the rails: +5 V, +12 V, −12 V and −5 V. A missing −12 V rail will show up as a dead RS-232 port and dead keypad.
3. Verify the clocks are clean and jitter-free: 17.734475 MHz at IC37 pin 6, 3.54689 MHz at IC2 pin 6, and 8.8 MHz at IC1 pins 46/47.
4. Check the display for wavy lines or new artefacts.
5. Test AY sound (via PLAY) and the beeper (via BEEP), a tape load, and the full keyboard.

If a previously-working function fails after the recap, check the polarity of every replaced electrolytic first.

Related pages

• Sinclair ZX Spectrum 128
• Sinclair ZX Spectrum 128 General Maintenance
• Sinclair ZX Spectrum 128 Troubleshooting Guide
• Sinclair ZX Spectrum+ Capacitor Replacement Guide — the 48K machine
• Sinclair ZX Spectrum +2 Capacitor Replacement Guide — the Amstrad repackaging, which has no published parts list
• Capacitor Failure Symptoms