Editing Sinclair ZX Spectrum Keyboard Membrane Repair

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[[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.]]

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.

'''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>

== How the keyboard works ==

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" />

=== Sinclair's diagnostic rule ===

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" />

{| 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
|}

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 ==

* 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 ==

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

== Inspection ==

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 ==

=== All models ===

# 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.

=== Spectrum+ and 128 ===

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" />

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" />

* '''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 ===

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.

== The STOP key fix ==

'''If Symbol Shift + A (STOP) does not work after fitting a Spectrum+ keyboard, connect a 22 k resistor across R68.'''<ref name="sm" />

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.<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:Sinclair Computers]]
[[Category:Maintenance Guides]]