Sinclair ZX Spectrum 128 Capacitor Replacement Guide

Sinclair ZX Spectrum 128 "Toastrack".

The Sinclair ZX Spectrum 128 has a full component-level parts list in Sinclair's Service Manual, so 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).

Safety and board handling

There is no mains voltage inside the computer: it runs from an external power pack, and the highest rail on the board is +12 V.

The Service Manual asks for recognised desoldering and heatsinking techniques and a suitably earthed, low-power iron, and for the board to be checked for solder splatter after any component is renewed. Add fresh flux and solder to old joints before removing parts, and keep the time on each joint short.

When to replace capacitors

There are 13 electrolytics on the main board, so a targeted recap is quick when there is a reason for it:

• Visible failure: bulging, leakage, residue or a split sleeve.
• An unstable supply, or rails out of tolerance or showing ripple. In the power circuit described in section 1.10, D15 and C44 rectify and smooth the inverter output to +12 V, and C111 and C112 with D28 and D29 form the charge pump that makes −12 V.
• A dead RS232 port or keypad: the RS232 driver IC33 runs from +12 V and −12 V, and the keypad from +12 V, so check C44, C111 and C112.
• Wavy lines across the display. Sinclair's 48K service manual gives the test: press down on each electrolytic in turn, and if the lines disappear, change that capacitor.

Vertical banding ("jailbars") on the RGB output is not cured by a routine recap; it has its own modification, described below.

Main board capacitors

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

Electrolytic capacitors

ZX Spectrum 128 main PCB: electrolytic capacitors (Table 5.4)

Ref	Value	Rating / tolerance	Notes
C25	100 µF	10 V, −10% +80%
C27	1 µF	50 V, 10%
C28	22 µF	10 V, −10% +80%	Changed to 47 µF in the jailbars modification
C34	22 µF	10 V, −10% +80%
C44, C45	100 µF	10 V, −10% +80% (as printed; see below)	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 and D29
C123	1 µF	10 V, −10% +80%
C127	1 µF	10 V, −10% +80%

The parts list gives C44 and C45 a 10 V rating, but section 1.10 places C44 on the +12 V rail, above that rating. Sinclair's 48K service manual lists C44 and C45, in the same inverter arrangement, as 100 µF 16 V. Fit parts rated at 16 V or more at C44 and C45.

Ceramic capacitors

ZX Spectrum 128 main PCB: ceramic capacitors (Table 5.4)

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%
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%

Sinclair's printed table lists "C29, C31" as 22 nF and then lists C31 again as 100 nF, while C30 does not appear at all, so the first entry is probably C29 and C30. Check the board.

Note 6 to Table 5.4 says the 20 pF capacitor at C124 may be split into two 10 pF ±2% capacitors in parallel at C124 and C130, so whether C130 is fitted depends on how the board was built. Note 3 records that the PCB has provision for a 2–22 pF film-dielectric trimmer should it be needed. C115, C124 and C130 are 2% parts; match the tolerance as well as the value.

Keypad PCB capacitors

The numeric keypad has its own small board with 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

Some Toastracks show faint vertical banding on plain, light colours on the RGB output that does not appear over RF or composite video. Spectrum for Everyone suspects the PCB ground plane and gives this fix:

• Replace C28 (22 µF, to the right of the 5-way keyboard connector) with a 47 µF axial electrolytic.
• Replace C7 and C8 (next to the IC12 and IC13 RAM chips, bottom left of the board) with 1 µF ceramic capacitors.

This reduces the banding significantly without removing it entirely. The equivalent change on the 48K is C5 to C8 to 150–220 nF, and on a grey +2 (Z70500 Issue 3) it is C52.

Choosing replacements

• Use aluminium electrolytics of the same capacitance and the same or a higher voltage rating, rated for 105 °C, and at least 16 V at C44 and C45.
• Most of the originals are axial. If you fit radial parts, form the leads neatly and sleeve any long lead.
• Match the tolerance of the 2% parts (C115, C124, C130).
• Leave the ceramics alone except as part of the jailbars modification.

Replacement procedure

1. Unplug the power, open the case and disconnect both keyboard ribbon cables.
2. Lift the board out so it can be supported flat.
3. Record the value, voltage and polarity of each electrolytic before removing it, and check it against the table above.
4. Add fresh flux and solder to both joints.
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 it and trim the leads.
7. Check for bridges and solder splatter before reassembly.

Post-recap checks

1. Power up, preferably on a current-limited bench supply, and check that the 128 menu appears.
2. Check the supplies against the basic-checks table: +5 V ± 0.25 V at the regulator output, +12 V ± 0.5 V at IC33 pin 14, −12 V at IC34 pin 1, and −5 V at the anode of D19.
3. Check the clocks: 17.73447 MHz at IC37 pin 6, 3.54689 MHz at IC2 pin 6 and 8.8 MHz at IC1 pins 46 and 47, all with no jitter.
4. Check the display for wavy lines or new artefacts.
5. Test the AY sound and the beeper, a tape load and the whole keyboard.

If a function that worked before the recap now fails, 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
• Sinclair ZX Spectrum +2 Capacitor Replacement Guide
• Capacitor Failure Symptoms