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Remove the live ChatGPT citation placeholder; the 22k/R68 STOP fix is real but is step 5 of Sinclair's Spectrum-to-Spectrum+ conversion (and is RP1 pins 1-5 on an Issue 1 board), now cited to the service manual; add Sinclair's block-of-five/block-of-eight fault rule, the section 10.3 assembly figures, the foam pads and the dual-contact keys; fix markdown numbering, the H1 title, the red links and the non-existent categories
 
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= ZX Spectrum Keyboard Membrane Repair =
<templatestyles src="Template:StyledTable/styles.css" />
[[File:Sinclair ZX Spectrum Broken Membrane.jpg|alt=A cracked Sinclair ZX Spectrum keyboard membrane|thumb|340px|A failed Spectrum membrane. The tails are where these almost always go.]]


This guide provides a thorough, step-by-step process for diagnosing, removing, cleaning, and replacing the ZX Spectrum keyboard membrane (both "rubber-key" 16/48K and 48K+/128K / Spectrum+ variants), including advanced troubleshooting and upgrade options.
Diagnosing, removing and replacing the keyboard membrane on the rubber-key '''[[Sinclair ZX Spectrum]]''' (16K and 48K) and on the '''[[Sinclair ZX Spectrum+|Spectrum+]]''' and 128.


== Background ==
'''The assembly figures and the fault rules on this page are Sinclair's own''', from the ''ZX Spectrum Service Manual'' hosted on this wiki.<ref name="sm">''ZX Spectrum Service Manual'', Sinclair Research Ltd, hosted at [[Sinclair ZX Spectrum Service Manual]]. Section 1.8 (keyboard scanning: an 8 × 5 matrix scanned every 20 ms, ribbon cables KB1 and KB2, pull-ups R64–R68 holding KB0–KB4 high); section 5.5 (keyboard structure and the block-of-five / block-of-eight fault rule); section 6.7 (keyboard faults 6.7.1 to 6.7.5); section 10.1 (the Spectrum+ five-layer membrane and bubble mat); section 10.2 (Spectrum-to-Spectrum+ conversion procedure, including step 5, the 22 k resistor across R68, and step 9, the foam pads); section 10.3 (assembly of the Spectrum+ keyboard: the S-shape flextails, the 1 mm protrusion, the Paxolin packing piece, the screw order and the reaction plates); section 10.4 (keys that make contact in two places at once); section 8.11 (resistor values, R68).</ref>
[[File:Sinclair ZX Spectrum Broken Membrane.jpg|alt=Broken Sinclair ZX Spectrum Keyboard Membrane|thumb|Broken Sinclair ZX Spectrum Keyboard Membrane]]
The Spectrum keyboard relies on a multi-layer printed plastic membrane; pressing a key bridges conductive traces to signal the row/column. Over time, these membranes become brittle or delaminate, and the ribbon "tails" frequently crack, resulting in unresponsive keys.


== Required Tools and Materials ==
== How the keyboard works ==
* Small Phillips or Pozidriv screwdrivers 
* Anti-static wrist strap or mat 
* Low-pressure air blower or canned air 
* 90%+ isopropyl alcohol and lint-free swabs 
* Clean cloths, trays to organize screws 
* Correct replacement membrane for your model (16/48K, Plus/128K).
* Insulating tape, optional resistor (22 KΩ for STOP/Symbol-Shift fix on 16/48K), optional spare ULA for compatibility troubleshooting.


== Detailed Disassembly ==
The keyboard is an '''8 × 5 matrix''' scanned by the CPU '''every 20 ms''', once per maskable interrupt. Two ribbon cables run from the membrane to the board: '''KB1''', the 5-way tail, carries the row outputs KB0–KB4 to the ULA; '''KB2''', the 8-way tail, connects to the high address lines A15–A8. Pull-up resistors '''R64 to R68''' hold the row outputs high when nothing is pressed.<ref name="sm" />
1. Power off and unplug your Spectrum; discharge static.


2. Remove the five or eight screws on the underside, depending on model.
=== Sinclair's diagnostic rule ===


3. Gently lift the top case half. Early machines use clips; later ones may use adhesive—apply heat or thin tools carefully to avoid bending.
This is the most useful thing on the page, and it turns a vague "some keys don't work" into a one-minute diagnosis:<ref name="sm" />


4. Lift out the painted faceplate and keyboard assembly, noting membrane orientation and ribbon tail routing. Photograph for reference.
{| class="wikitable styled-table" style="width:90%; text-align:left;"
|+'''What the pattern of dead keys tells you'''
! Symptom !! Where the fault is
|-
| '''A block of five keys dead''' || KB2 circuitry or the '''8-way''' membrane tail
|-
| '''A block of eight keys dead''' || KB1 circuitry or the '''5-way''' membrane tail
|-
| Two or more of the '''5''' KB1 tracks shorted || Change the membrane
|-
| Two or more of the '''8''' KB2 tracks shorted || Change the membrane
|-
| '''Whole keyboard dead''', with good tails || Sinclair's remedy is to change '''IC1, the ULA'''
|-
| '''Wrong''' keyboard response || Change IC1
|-
| Keyboard response '''too fast''' (repeat rate) || Change IC1
|}


== Membrane Removal and Inspection ==
So: count the dead keys before opening anything. Five, or a multiple of five in one column, points at one tail; eight points at the other.
1. Locate and gently release the ribbon tail clamp(s) holding the membrane tails.


2. Carefully extract the ribbon tails—these are fragile and susceptible to damage.
== Tools and materials ==


3. Check membrane integrity:  
* Small Phillips or Pozidriv drivers
* Anti-static mat or strap
* 90 %+ isopropyl alcohol and lint-free swabs
* Air blower
* The correct replacement membrane — '''rubber-key 16/48K and Spectrum+ membranes are different parts'''
* For a Spectrum+ rebuild: double-sided tape, and Paxolin or similar about 0.75 mm thick for the clamp packing pieces


# Brittle, cracked, or delaminated membranes need replacing.
== Disassembly ==
# Inspect tail ribbons for cracks or broken conductors—the usual failure point.


== Cleaning (Optional Reuse) ==
# Power off and unplug.
* Blow away dust with a blower; avoid stirring debris onto the contacts.
# Remove the case screws from the underside and lift the top half. Note that the faceplate and the keyboard assembly come as a unit.
* Clean keycaps and faceplate with a damp cloth, using IPA carefully on stubborn grime—avoid wetting membrane layers.
# Photograph the membrane orientation and the tail routing before disturbing anything.
* Clean contact pads gently with IPA on lint-free swabs if lightly corroded; avoid abrasion.
# Release the tail clamps and withdraw the tails '''straight out'''. They are brittle and this is where most of them break.


== Installing the Replacement Membrane ==
== Inspection ==
1. Insert the membrane into the support plate in the same orientation as the original. Ensure conductive pads lie flat and align over PCB contact areas. 


2. Route ribbon tails in a gentle “S-shape” to avoid strain; this is critical for Plus membranes.
The membrane itself rarely fails in the middle. '''The tails are the failure point''', and a crack in a tail is often invisible until the tail is flexed against the light. Check:


3. Seat tail ends fully and clamp; ensure clamps are neither loose nor overtightened. 
* Cracks or breaks in either tail, particularly where it leaves the membrane and where it enters the clamp
* Delamination of the printed layers
* Corrosion or dullness on the contact pads
* Continuity along each track, tail end to pad


* For Spectrum+, confirm clamp assembly is correct—misalignment leads to extended key malfunctions.
Light corrosion on the pads will clean off with isopropyl alcohol on a swab. Do not abrade them.


4. Apply insulating tape if the ribbon tail contacts the RF modulator—this can cause failures even without electrical contact.
== Fitting a replacement ==


== Reassembly ==
=== All models ===
1. Replace the keyboard assembly into the case, ensuring no folds or folds in the membrane inner layers.


2. Replace screws securely and evenly.  
# Seat the membrane in the support plate in the original orientation, with the pads flat.
# Route the tails in a gentle '''S-shape'''. Sinclair specifies this explicitly so that there is no tension in the membrane.<ref name="sm" />
# Seat the tail ends fully in their clamps and tighten to a firm fit, no more.


3. Power the unit and proceed to testing.
=== Spectrum+ and 128 ===


== Testing Procedure ==
The Spectrum+ keyboard needs a '''five-layer''' membrane because of the more complex legend layout, and the key spring action comes from a '''bubble mat''' rather than from rubber keys.<ref name="sm" />
1. Boot a simple keyboard test (e.g. BASIC or keyboard-testing ROM).  


2. Press each key individually. 
Sinclair's assembly instructions for it are specific, and they are the difference between a keyboard that works and one that half works:<ref name="sm" />


3. Test combination/extended keys (Symbol Shift, Caps Shift, cursor keys).
* '''The tails must lie in an S shape.''' The way to achieve it is to make '''the end of the tracks on the middle layer protrude 1 mm past the clamps'''. This is easy to see on the wide tail and hard on the narrow one.
* '''Fit a packing piece under the tails at each clamp''', about '''0.75 mm thick''' and as wide as the tail. Paxolin is suitable. Sinclair notes this does not apply to a Spectrum+ whose serial number begins with '''S''' — those were built with the packing already fitted.
* '''Stick the two surfaces of each tail together''' near the connector end, with double-sided tape or a pair of tabs around the edges. This prevents short circuits against the board.
* '''Tighten the screws in order''': the centre line of three first, the screws next to the tail clamps last. Tighten to a firm fit only — Sinclair says about '''a quarter turn back from fully tight'''.
* '''Reaction plates come in metal and plastic.''' The metal plate must have its plastic insulator and takes '''6.5 mm''' screws; the plastic plate takes '''8 mm'''. Using the longer screws with the metal plate is a way to damage the assembly.


* If Symbol-Shift/A (“STOP”) fails on 16/48K models, try adding a 22 KΩ resistor atop R68 on the mainboard.:contentReference[oaicite:10]{index=10}
=== Protecting the tails from chafing ===


== Advanced Troubleshooting ==
Sinclair's own answer to tails rubbing on the hardware underneath is '''foam pads''': one on top of the modulator, and on Issue 3 and later boards, two on top of the heatsink.<ref name="sm" /> That is a better fix than insulating tape, because the problem is abrasion rather than electrical contact.
* [[File:Sinclair ZX Spectrum Membrane Schematic.png|alt=Sinclair ZX Spectrum Membrane Schematic|thumb|383x383px|Sinclair ZX Spectrum Membrane Schematic]]'''Intermittent or non-registering keys''': Reinspect tail seating, clamp condition, and tail length. 
* '''Extended keys not working''': Especially on Spectrum+, re-verify clamp integrity and tail length.
* '''Random unresponsive keys''': Possibly caused by ULA incompatibility; attempting ULA replacement has resolved such issues in some cases.


== Sources and Further Reading ==
== The STOP key fix ==
* '''Step-by-step installation''': RetroComputing.SE Q&A covers core steps.
* '''Component fix and troubleshooting''': ByteDelight offers detailed guidance including STOP key hinge fix.
* '''Restoration narrative''': Retro Repairs & Refurbs provides real-world rebuild context.
* '''Official service manual''': For advanced membrane layer schematics and fault diagnosis, see the Spectrum Service Manual.


== See Also ==
'''If Symbol Shift + A (STOP) does not work after fitting a Spectrum+ keyboard, connect a 22 k resistor across R68.'''<ref name="sm" />
* [[Sinclair ZX Spectrum]]
 
* [[Keyboard membrane#Repair and replacement|Membrane keyboard general guide]]
Two things about this instruction matter, and both were missing from earlier versions of this page:
* [[ZX Spectrum maintenance]]
 
* '''It is step 5 of Sinclair's Spectrum-to-Spectrum+ conversion procedure''', not a general membrane-repair tip. It addresses the rubber-key board driving the heavier Spectrum+ membrane. R68 is one of the '''R64–R68 pull-ups''' on the row outputs, 10 k on most issues.<ref name="sm" />
* '''On an Issue 1 board there is no discrete R68 to work across.''' Sinclair's instruction there is to connect the 22 k '''between pins 1 and 5 of RP1, on the reverse side of the board'''.<ref name="sm" />
 
If STOP fails on an unmodified rubber-key Spectrum with its original membrane, this is not the fix — check the tails first.
 
== Keys that contact in two places ==
 
On the Spectrum+ some keys close a second contact as well as their own, which produces fault patterns that look impossible until you know about them.<ref name="sm" />
 
* '''TRUE VIDEO''' connects row 1 / column 6 on the top membrane '''and''' row 3 / column 1 on the lower.
* '''. ; " and ,''' each additionally bridge '''row 2 / column 8''' on the top membrane.
 
So if those four punctuation keys fail together while everything else works, the fault is at one shared contact, not four separate ones.
 
== Reassembly and test ==
 
# Refit the keyboard assembly with no folds or creases in any membrane layer.
# Fit the top cover '''taking care not to crease the tails''' — Sinclair calls this out as its own step.<ref name="sm" />
# Replace the screws evenly.
# Power up and test '''every''' key, including the shifted and extended combinations. Sinclair's instruction is to check the operation of all keys on a newly assembled keyboard.<ref name="sm" />
 
== Further reading ==
 
* [[Sinclair ZX Spectrum Service Manual]] — sections 1.8, 5.5, 6.7 and 10
* [[:File:Sinclair ZX Spectrum Membrane Schematic.png|Membrane schematic]]
 
[[File:Sinclair ZX Spectrum Membrane Schematic.png|alt=Sinclair ZX Spectrum membrane schematic|thumb|383x383px|The membrane layout]]
 
== Related pages ==
* [[Sinclair ZX Spectrum]]
* [[Sinclair ZX Spectrum+]]
* [[Sinclair ZX Spectrum 128]]
* [[Sinclair ZX Spectrum General Maintenance]]
* [[Sinclair ZX Spectrum Troubleshooting Guide]]
 
== References ==
<references />


[[Category:ZX Spectrum]]
[[Category:Sinclair Maintenance Guides]]
[[Category:Sinclair Computers]]
[[Category:Sinclair Computers]]
[[Category:Maintenance Guides]]

Latest revision as of 20:15, 23 September 2026

A cracked Sinclair ZX Spectrum keyboard membrane
A failed Spectrum membrane. The tails are where these almost always go.

Diagnosing, removing and replacing the keyboard membrane on the rubber-key Sinclair ZX Spectrum (16K and 48K) and on the Spectrum+ and 128.

The assembly figures and the fault rules on this page are Sinclair's own, from the ZX Spectrum Service Manual hosted on this wiki.[1]

How the keyboard works

[edit | edit source]

The keyboard is an 8 × 5 matrix scanned by the CPU every 20 ms, once per maskable interrupt. Two ribbon cables run from the membrane to the board: KB1, the 5-way tail, carries the row outputs KB0–KB4 to the ULA; KB2, the 8-way tail, connects to the high address lines A15–A8. Pull-up resistors R64 to R68 hold the row outputs high when nothing is pressed.[1]

Sinclair's diagnostic rule

[edit | edit source]

This is the most useful thing on the page, and it turns a vague "some keys don't work" into a one-minute diagnosis:[1]

What the pattern of dead keys tells you
Symptom Where the fault is
A block of five keys dead KB2 circuitry or the 8-way membrane tail
A block of eight keys dead KB1 circuitry or the 5-way membrane tail
Two or more of the 5 KB1 tracks shorted Change the membrane
Two or more of the 8 KB2 tracks shorted Change the membrane
Whole keyboard dead, with good tails Sinclair's remedy is to change IC1, the ULA
Wrong keyboard response Change IC1
Keyboard response too fast (repeat rate) Change IC1

So: count the dead keys before opening anything. Five, or a multiple of five in one column, points at one tail; eight points at the other.

Tools and materials

[edit | edit source]
  • Small Phillips or Pozidriv drivers
  • Anti-static mat or strap
  • 90 %+ isopropyl alcohol and lint-free swabs
  • Air blower
  • The correct replacement membrane — rubber-key 16/48K and Spectrum+ membranes are different parts
  • For a Spectrum+ rebuild: double-sided tape, and Paxolin or similar about 0.75 mm thick for the clamp packing pieces

Disassembly

[edit | edit source]
  1. Power off and unplug.
  2. Remove the case screws from the underside and lift the top half. Note that the faceplate and the keyboard assembly come as a unit.
  3. Photograph the membrane orientation and the tail routing before disturbing anything.
  4. Release the tail clamps and withdraw the tails straight out. They are brittle and this is where most of them break.

Inspection

[edit | edit source]

The membrane itself rarely fails in the middle. The tails are the failure point, and a crack in a tail is often invisible until the tail is flexed against the light. Check:

  • Cracks or breaks in either tail, particularly where it leaves the membrane and where it enters the clamp
  • Delamination of the printed layers
  • Corrosion or dullness on the contact pads
  • Continuity along each track, tail end to pad

Light corrosion on the pads will clean off with isopropyl alcohol on a swab. Do not abrade them.

Fitting a replacement

[edit | edit source]

All models

[edit | edit source]
  1. Seat the membrane in the support plate in the original orientation, with the pads flat.
  2. Route the tails in a gentle S-shape. Sinclair specifies this explicitly so that there is no tension in the membrane.[1]
  3. Seat the tail ends fully in their clamps and tighten to a firm fit, no more.

Spectrum+ and 128

[edit | edit source]

The Spectrum+ keyboard needs a five-layer membrane because of the more complex legend layout, and the key spring action comes from a bubble mat rather than from rubber keys.[1]

Sinclair's assembly instructions for it are specific, and they are the difference between a keyboard that works and one that half works:[1]

  • The tails must lie in an S shape. The way to achieve it is to make the end of the tracks on the middle layer protrude 1 mm past the clamps. This is easy to see on the wide tail and hard on the narrow one.
  • Fit a packing piece under the tails at each clamp, about 0.75 mm thick and as wide as the tail. Paxolin is suitable. Sinclair notes this does not apply to a Spectrum+ whose serial number begins with S — those were built with the packing already fitted.
  • Stick the two surfaces of each tail together near the connector end, with double-sided tape or a pair of tabs around the edges. This prevents short circuits against the board.
  • Tighten the screws in order: the centre line of three first, the screws next to the tail clamps last. Tighten to a firm fit only — Sinclair says about a quarter turn back from fully tight.
  • Reaction plates come in metal and plastic. The metal plate must have its plastic insulator and takes 6.5 mm screws; the plastic plate takes 8 mm. Using the longer screws with the metal plate is a way to damage the assembly.

Protecting the tails from chafing

[edit | edit source]

Sinclair's own answer to tails rubbing on the hardware underneath is foam pads: one on top of the modulator, and on Issue 3 and later boards, two on top of the heatsink.[1] That is a better fix than insulating tape, because the problem is abrasion rather than electrical contact.

The STOP key fix

[edit | edit source]

If Symbol Shift + A (STOP) does not work after fitting a Spectrum+ keyboard, connect a 22 k resistor across R68.[1]

Two things about this instruction matter, and both were missing from earlier versions of this page:

  • It is step 5 of Sinclair's Spectrum-to-Spectrum+ conversion procedure, not a general membrane-repair tip. It addresses the rubber-key board driving the heavier Spectrum+ membrane. R68 is one of the R64–R68 pull-ups on the row outputs, 10 k on most issues.[1]
  • On an Issue 1 board there is no discrete R68 to work across. Sinclair's instruction there is to connect the 22 k between pins 1 and 5 of RP1, on the reverse side of the board.[1]

If STOP fails on an unmodified rubber-key Spectrum with its original membrane, this is not the fix — check the tails first.

Keys that contact in two places

[edit | edit source]

On the Spectrum+ some keys close a second contact as well as their own, which produces fault patterns that look impossible until you know about them.[1]

  • TRUE VIDEO connects row 1 / column 6 on the top membrane and row 3 / column 1 on the lower.
  • . ; " and , each additionally bridge row 2 / column 8 on the top membrane.

So if those four punctuation keys fail together while everything else works, the fault is at one shared contact, not four separate ones.

Reassembly and test

[edit | edit source]
  1. Refit the keyboard assembly with no folds or creases in any membrane layer.
  2. Fit the top cover taking care not to crease the tails — Sinclair calls this out as its own step.[1]
  3. Replace the screws evenly.
  4. Power up and test every key, including the shifted and extended combinations. Sinclair's instruction is to check the operation of all keys on a newly assembled keyboard.[1]

Further reading

[edit | edit source]
Sinclair ZX Spectrum membrane schematic
The membrane layout
[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 ZX Spectrum Service Manual, Sinclair Research Ltd, hosted at Sinclair ZX Spectrum Service Manual. Section 1.8 (keyboard scanning: an 8 × 5 matrix scanned every 20 ms, ribbon cables KB1 and KB2, pull-ups R64–R68 holding KB0–KB4 high); section 5.5 (keyboard structure and the block-of-five / block-of-eight fault rule); section 6.7 (keyboard faults 6.7.1 to 6.7.5); section 10.1 (the Spectrum+ five-layer membrane and bubble mat); section 10.2 (Spectrum-to-Spectrum+ conversion procedure, including step 5, the 22 k resistor across R68, and step 9, the foam pads); section 10.3 (assembly of the Spectrum+ keyboard: the S-shape flextails, the 1 mm protrusion, the Paxolin packing piece, the screw order and the reaction plates); section 10.4 (keys that make contact in two places at once); section 8.11 (resistor values, R68).
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