Sinclair ZX Spectrum Keyboard Membrane Repair

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.

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.

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:

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

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

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

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.
3. 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.

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

• 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. 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.

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.
• 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.

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.

• 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

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.
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.

Further reading

• Sinclair ZX Spectrum Service Manual — sections 1.8, 5.5, 6.7 and 10
• Membrane schematic

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