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C105 corrected 150 pF -> 180 pF per Service Manual Table 5.4; table numbers corrected 5.2/5.3 -> 5.4/5.5; manual now hosted and cited; note the C29/C30 printing error
Flag C44/C45 10 V parts-list rating below the +12 V rail (48K manual: 16 V); remove invented role notes, the 48K heatsink-crack claim and the wrong tolerance claim; replace mislabelled 48K schematic image with the 128 photo; copyedit
 
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[[File:Sinclair ZX Spectrum 128 circuit diagram (sheet 1).png|thumb|420px|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.]]
[[File:Sinclair ZX Spectrum 128.jpg|thumb|380px|Sinclair ZX Spectrum 128 "Toastrack".]]


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 [[Sinclair ZX Spectrum +2|+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.<ref name="sm128">''ZX Spectrum 128 Service Manual'', Sinclair Research Ltd, section 5.4 "Main PCB Components" (Table 5.4), section 5.5 "Keypad PCB Components" (Table 5.5) and section 5.2, the notes to Table 5.4; also section 1.10, the power supply description, and section 4.3, the repair notes. Hosted on this wiki as [[Sinclair ZX Spectrum 128 Service Manual]] ([[:File:ZX Spectrum 128 Service Manual.pdf|PDF]]).</ref>
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).<ref name="sm128">''[[Sinclair ZX Spectrum 128 Service Manual]]'' ([[:File:ZX Spectrum 128 Service Manual.pdf]]), Sinclair Research Ltd; re-typeset Revision 1.1 by Brendan Alford. Table 5.4 (main PCB components), Table 5.5 (keypad PCB components), section 5.2 (notes to Table 5.4), section 1.10 (power supplies), section 4.2.3 (basic checks) and section 4.3 (repair notes).</ref>


== Safety and board handling ==
== 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.<ref name="sm128" />
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.<ref name="sm128" />


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.<ref name="sm128" />
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.<ref name="sm128" /> Add fresh flux and solder to old joints before removing parts, and keep the time on each joint short.


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.<ref name="tis">''ZX Spectrum Servicing Manual'', prepared by Thorn (EMI) Datatech Ltd for Sinclair Research Ltd, March 1984, Repair Data symptom 9. Hosted on this wiki as [[ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984)]] ([[:File:ZX Spectrum Servicing Manual Thorn EMI Datatech 1984.pdf|PDF]]). Third-party T.I.S. field-service material, not Sinclair's own text.</ref><ref name="ian">Worsley, Ian. ''Repairing Spectrum &amp; Spectrum Plus'', Lil Old Sinclair Technical Information Repository. Source for the wavy-lines capacitor test and general Spectrum repair practice.</ref>
== When to replace capacitors ==


== Is the 128 a recap candidate? ==
There are 13 electrolytics on the main board, so a targeted recap is quick when there is a reason for it:<ref name="sm128" />


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.
* 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.<ref name="sm128" />
* 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.<ref name="sm128" />
* 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.<ref name="sm48">''[[Sinclair ZX Spectrum Service Manual]]'' ([[:File:Sinclair ZX Spectrum Service Manual.pdf]]), Sinclair Research Ltd, section 6.4.3 "Wavy lines across screen"; parts list section 8.5 (C44/C45 100 µF 16 V).</ref>


Recap when there is evidence:
Vertical banding ("jailbars") on the RGB output is not cured by a routine recap; it has its own modification, described below.<ref name="jail">[https://spectrumforeveryone.com/technical/fixing-vertical-jailbars-48k-128k-spectrum/ "Fixing Vertical 'Jailbars' on your 48K, 128K and Grey Spectrum +2"], Spectrum for Everyone.</ref>
 
* '''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.<ref name="sm128" />
* '''Wavy lines across the display''' — press each electrolytic in turn; if the artefact clears while pressing one, that is the culprit.<ref name="ian" />
 
'''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 ==
== 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.<ref name="sm128" />
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.<ref name="sm128" />


=== Electrolytics — the recap targets ===
=== Electrolytic capacitors ===


{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''ZX Spectrum 128 main PCB — electrolytic capacitors'''
|+'''ZX Spectrum 128 main PCB: electrolytic capacitors (Table 5.4)'''
! Ref !! Value !! Rating / tolerance !! Notes
! Ref !! Value !! Rating / tolerance !! Notes
|-
|-
| '''C25''' || 100 µF || 10 V, −10% +80% || Supply smoothing
| C25 || 100 µF || 10 V, −10% +80% ||
|-
|-
| '''C27''' || 1 µF || 50 V, 10% ||
| 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'''
| C28 || 22 µF || 10 V, −10% +80% || Changed to 47 µF in the jailbars modification
|-
|-
| '''C34''' || 22 µF || 10 V, −10% +80% ||
| C34 || 22 µF || 10 V, −10% +80% ||
|-
|-
| '''C44, C45''' || 100 µF || 10 V, −10% +80% || C44 smooths the +12 V rail with D15
| 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% ||
| C50 || 22 µF || 10 V, −10% +80% ||
|-
|-
| '''C74''' || 4.7 µF || 5 V minimum ||
| C74 || 4.7 µF || 5 V minimum ||
|-
|-
| '''C80''' || 22 µF || 10 V, −10% +80% ||
| 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
| C111, C112 || 47 µF || 16 V, −10% +80% || The −12 V charge pump, with D28 and D29
|-
|-
| '''C123''' || 1 µF || 10 V, −10% +80% ||
| C123 || 1 µF || 10 V, −10% +80% ||
|-
|-
| '''C127''' || 1 µF || 10 V, −10% +80% ||
| C127 || 1 µF || 10 V, −10% +80% ||
|}
|}


=== Ceramics — normally left alone ===
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.<ref name="sm128" /><ref name="sm48" /> Fit parts rated at 16 V or more at C44 and C45.
 
=== Ceramic capacitors ===


{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''ZX Spectrum 128 main PCB — ceramic capacitors'''
|+'''ZX Spectrum 128 main PCB: ceramic capacitors (Table 5.4)'''
! Ref !! Value !! Rating / tolerance
! Ref !! Value !! Rating / tolerance
|-
|-
Line 100: Line 98:
| C113, C114 || 47 nF || 25 V, 10%
| C113, C114 || 47 nF || 25 V, 10%
|-
|-
| C115 || 330 pF || 25 V, '''2%'''
| C115 || 330 pF || 25 V, 2%
|-
|-
| C116 || 10 nF || 25 V, 10%
| C116 || 10 nF || 25 V, 10%
Line 112: Line 110:
| C122 || 1 nF || 25 V, 10%
| C122 || 1 nF || 25 V, 10%
|-
|-
| C124 || 20 pF || 25 V, '''2%''' — see note below
| C124 || 20 pF || 25 V, 2%
|-
|-
| C125 || 100 nF || 25 V, 10%
| C125 || 100 nF || 25 V, 10%
Line 122: Line 120:
| C129 || 100 pF || 25 V, 10%
| C129 || 100 pF || 25 V, 10%
|-
|-
| C130 || See note || 25 V, '''2%'''
| 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.<ref name="sm128" />
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.<ref name="sm128" />
 
'''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.<ref name="sm128" />
 
The manual also notes that provision was made on the PCB for a '''2–22 pF film dielectric trimmer''' should the need arise.<ref name="sm128" />


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.
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.<ref name="sm128" /> C115, C124 and C130 are 2% parts; match the tolerance as well as the value.


== Keypad PCB capacitors ==
== Keypad PCB capacitors ==


The numeric keypad has its own small board carrying a General Instrument PIC1652 microcontroller:<ref name="sm128" />
The numeric keypad has its own small board with a General Instrument PIC1652 microcontroller:<ref name="sm128" />


{| class="wikitable styled-table" style="width:80%; text-align:left;"
{| class="wikitable styled-table" style="width:80%; text-align:left;"
|+'''Keypad PCB (Table 5.5) — axial types'''
|+'''Keypad PCB (Table 5.5), axial types'''
! Ref !! Value !! Rating / tolerance !! Type
! Ref !! Value !! Rating / tolerance !! Type
|-
|-
Line 148: Line 142:
== The jailbars modification ==
== 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:<ref name="jail">[https://spectrumforeveryone.com/technical/fixing-vertical-jailbars-48k-128k-spectrum/ "Fixing Vertical 'Jailbars' on your 48K, 128K and Grey Spectrum +2"], Spectrum for Everyone.</ref>
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:<ref name="jail" />


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


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 [[Sinclair ZX Spectrum +2|+2]] it is C52.<ref name="jail" />
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.<ref name="jail" />


== Choosing replacements ==
== 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.
* 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.
* Note the wide original tolerances (−10% +80% on most electrolytics) — modern ±20% parts are comfortably within specification.
* Most of the originals are axial. If you fit radial parts, form the leads neatly and sleeve any long lead.
* 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 of the 2% parts (C115, C124, C130).
* '''Match the tolerance on the 2% parts''' (C115, C124, C130).
* Leave the ceramics alone except as part of the jailbars modification.
* The ceramics are stable and are not normally replaced — except deliberately, as part of the jailbars modification.


== Replacement procedure ==
== Replacement procedure ==


# Unplug the power, open the case and disconnect both keyboard tails carefully.
# Unplug the power, open the case and disconnect both keyboard ribbon cables.
# Lift the board out so it is supported flat.
# Lift the board out so it can be supported flat.
# '''Record the value, voltage and polarity of each electrolytic before removing it''', cross-checking against the table above and the circuit diagrams.
# Record the value, voltage and polarity of each electrolytic before removing it, and check it against the table above.
# Add fresh flux and solder to both joints and let it mix with the old solder.
# Add fresh flux and solder to both joints.
# Lift one lead at a time with the joint fully molten, then clear the hole with wick.
# Lift one lead at a time with the joint fully molten, then clear the hole with wick.
# Fit the replacement the correct way round; solder and trim.
# Fit the replacement the correct way round, solder it and trim the leads.
# Inspect for bridges and splatter before reassembly.
# Check for bridges and solder splatter before reassembly.<ref name="sm128" />


== Post-recap verification ==
== Post-recap checks ==


# Power up — ideally on a '''current-limited supply''' — and confirm the '''128 menu''' appears.
# Power up, preferably on a current-limited bench supply, and check that the 128 menu appears.
# 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.
# 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.<ref name="sm128" />
# 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.<ref name="sm128" />
# 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.<ref name="sm128" />
# Check the display for wavy lines or new artefacts.
# Check the display for wavy lines or new artefacts.
# Test AY sound (via <code>PLAY</code>) and the beeper (via <code>BEEP</code>), a tape load, and the full keyboard.
# Test the AY sound and the beeper, a tape load and the whole keyboard.


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


== Related pages ==
== Related pages ==
Line 188: Line 181:
* [[Sinclair ZX Spectrum 128 General Maintenance]]
* [[Sinclair ZX Spectrum 128 General Maintenance]]
* [[Sinclair ZX Spectrum 128 Troubleshooting Guide]]
* [[Sinclair ZX Spectrum 128 Troubleshooting Guide]]
* [[Sinclair ZX Spectrum+ Capacitor Replacement Guide]] — the 48K machine
* [[Sinclair ZX Spectrum+ Capacitor Replacement Guide]]
* [[Sinclair ZX Spectrum +2 Capacitor Replacement Guide]] — the Amstrad repackaging, which has no published parts list
* [[Sinclair ZX Spectrum +2 Capacitor Replacement Guide]]
* [[Capacitor Failure Symptoms]]
* [[Capacitor Failure Symptoms]]


Latest revision as of 12:12, 1 October 2026

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).[1]

Safety and board handling

[edit | edit source]

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.[1]

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.[1] Add fresh flux and solder to old joints before removing parts, and keep the time on each joint short.

When to replace capacitors

[edit | edit source]

There are 13 electrolytics on the main board, so a targeted recap is quick when there is a reason for it:[1]

  • 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.[1]
  • 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.[1]
  • 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.[2]

Vertical banding ("jailbars") on the RGB output is not cured by a routine recap; it has its own modification, described below.[3]

Main board capacitors

[edit | edit source]

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.[1]

Electrolytic capacitors

[edit | edit source]
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.[1][2] Fit parts rated at 16 V or more at C44 and C45.

Ceramic capacitors

[edit | edit source]
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.[1]

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.[1] C115, C124 and C130 are 2% parts; match the tolerance as well as the value.

Keypad PCB capacitors

[edit | edit source]

The numeric keypad has its own small board with a General Instrument PIC1652 microcontroller:[1]

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

[edit | edit source]

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:[3]

  • 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.[3]

Choosing replacements

[edit | edit source]
  • 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

[edit | edit source]
  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.[1]

Post-recap checks

[edit | edit source]
  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.[1]
  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.[1]
  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.

[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Sinclair ZX Spectrum 128 Service Manual (File:ZX Spectrum 128 Service Manual.pdf), Sinclair Research Ltd; re-typeset Revision 1.1 by Brendan Alford. Table 5.4 (main PCB components), Table 5.5 (keypad PCB components), section 5.2 (notes to Table 5.4), section 1.10 (power supplies), section 4.2.3 (basic checks) and section 4.3 (repair notes).
  2. ↑ 2.0 2.1 Sinclair ZX Spectrum Service Manual (File:Sinclair ZX Spectrum Service Manual.pdf), Sinclair Research Ltd, section 6.4.3 "Wavy lines across screen"; parts list section 8.5 (C44/C45 100 µF 16 V).
  3. ↑ 3.0 3.1 3.2 "Fixing Vertical 'Jailbars' on your 48K, 128K and Grey Spectrum +2", Spectrum for Everyone.
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