Sinclair ZX Spectrum Keyboard Membrane Repair

A 48K Spectrum membrane with a cracked tail (inset).

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

How the keyboard works

The keyboard is an 8 × 5 matrix that the CPU scans every 20 ms, once per maskable interrupt. Two ribbon tails 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 the columns through diodes to the high address lines A8–A15. Pull-up resistors hold the row outputs high when no key is pressed. Section 1.8 of the manual numbers the pull-ups R64–R68, but the Issue 3 circuit diagram and the parts lists give them as R65–R69 (10 kΩ); R64 is a 15 Ω resistor elsewhere on the board.

Period drawing of the matrix

A hand-drawn rear view of the matrix in Ferranti's Spectrum hardware file, now in the Ferranti Archives 2026, labels the 8-way tail KB2 "ADDS A8–A15" and the 5-way tail KB1 "D0–D4". Each of the eight lines on the 8-way tail serves one half-row of five keys and goes to one of the address lines A8 to A15; each of the five lines on the 5-way tail serves one key from every half-row. (Half-row here means five keys side by side on the keyboard; section 1.8 of Sinclair's manual names the electrical lines the other way round, calling the five ULA lines rows.) The same sheet maps both tails to the pins of a 16-pin DIL plug. On the board the 8-way lines pass through diodes D1 to D8 (1N4148), and the 5-way lines go to the ULA's keyboard inputs, KB0 to KB4 on pins 19, 20, 23, 24 and 26, which the ULA passes to data lines D0 to D4 when the Z80 reads the keyboard.

A key press is a connection between one line of each tail, so a key can be simulated at the board's connectors without the membrane. Ferranti's 1983 ULA test box did this electronically, with 4066 analogue switches wired across the connector pins and closed for about 30 ms per key press.

Reading the pattern of dead keys

Count the dead keys before opening the case. The keys are wired in eight blocks of five and five blocks of eight.

Sinclair's keyboard fault rules

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
KB1 fault	Check for a short between two or more of the 5 tracks; change the membrane
KB2 fault	Check for a short between two or more of the 8 tracks; change the membrane
Whole keyboard dead	Change IC1 (the ULA)
Wrong keyboard response	Change IC1
Keyboard response too fast	Change IC1

T.I.S.'s field-service notes add two checks before changing the ULA for a dead keyboard: that +5 V is reaching the keyboard, and the 10 kΩ row resistors (R65 to R69), a failed one of which kills one row. Ian Worsley's repair guide says that keys which fail only when the machine is warm, with a good ULA, mean a membrane change.

Tools and materials

• small cross-head screwdrivers
• an anti-static mat or strap
• isopropyl alcohol and lint-free swabs
• the correct replacement membrane: the rubber-key 16K/48K and the Spectrum+ membranes are different parts, and the Spectrum+ one has five layers
• for the rubber-key model: 12 mm and 6 mm double-sided tape for the keyboard template
• for the Spectrum+ and 128: double-sided tape for the tails, and Paxolin or similar about 0.75 mm thick for clamp packing

Rubber-key Spectrum (16K and 48K)

Sinclair's procedure:

1. Unplug all leads. Turn the computer upside down and remove the five self-tapping screws.
2. Hold the case halves together, turn the computer keys-up and lift the top half. It stays connected by the two keyboard tails; note how they lie in the case, and take care not to damage them.
3. To reach the membrane or the rubber key mat, remove the escutcheon plate (the metal template), which is held by double-sided tape around its edges. Lever it away from one end with a screwdriver. Sinclair notes that it is not normally possible to remove the template without damaging it.
4. On reassembly the membrane keys into projections on the case. Place the key mat over it and make sure it is properly seated, or depressed keys can be trapped under the template.
5. Fit the template with fresh double-sided tape around all four edges: 12 mm tape on the sides and bottom, 6 mm at the top. Press firmly round the edges. Early Issue 2 machines had no tape on the top edge, and Sinclair added the strip to stop the template lifting.
6. When closing the case, make sure neither tail is trapped between components. A tail bent double will go open circuit sooner or later.
7. Push each tail fully into its connector without kinking it.

Spectrum+ and Spectrum 128

The Spectrum+ keyboard has a five-layer membrane, and the key springing comes from a bubble mat. On the 128, Sinclair's procedure is to remove the two tail clamps and then the ten screws holding the reaction plate, and lift out the plate, the membrane and the bubble mat. Individual keys come off for cleaning by pressing the key and prising the retaining sleeve off its underside with a small screwdriver. Worsley describes the Spectrum+ backing plate as held by nine screws.

Sinclair's assembly instructions for the Spectrum+ keyboard:

1. Lay the tails in an S shape between the keyboard and the clamps, so that there is no tension in the membrane. To get this, leave the ends of the tracks on the middle layer protruding 1 mm past the clamps. This is easy to see on the wide tail and harder on the narrow one.
2. Fit a packing piece under the tails at each clamp, about 0.75 mm thick and as wide as the tail. Paxolin is suitable. Current production had the packing fitted, and Sinclair says the instruction does not apply to a Spectrum+ whose serial number starts with S.
3. Stick the two surfaces of each tail together near the connector end, with double-sided tape or a pair of tabs around the edges, to prevent short circuits on the board.
4. Tighten the screws in order: the centre line of three first, the screws next to the tail clamps last. Tighten each to a firm fit only, about a quarter turn back from fully tight.
5. The reaction plate may be metal or plastic. A metal plate needs its plastic insulator and 6.5 mm screws; a plastic plate takes 8 mm screws.
6. Check every key on a newly assembled keyboard.

The 128 manual gives the same tail and screw instructions, and adds that the bubble mat, membrane and reaction plate locate on a peg at each end, and that the upper case should be supported face down with the keys clear of the bench while the keyboard is assembled.

Worsley's fixes for Spectrum+ keyboards:

• Keys on the bottom row that do not work can often be cured without a new membrane. Remove the tail clamps, clean between the layers of each tail around the clamped area with WD-40 on a cotton bud, and add extra packing between tail and clamp when refitting.
• A key that prints repeatedly means the backing plate is clamped too tightly. Slacken the screw nearest that key by a quarter turn.

Electrical load of the Spectrum+ membrane

In November 1984 Timex and Ferranti investigated keyboard faults on Spectrum+ machines and measured the new keyboard. Ferranti describes it as a 6-layer membrane, against 2 layers on the rubber-key model, and measured about 100 pF of stray capacitance on each ULA keyboard input, about three times the old keyboard; Sinclair's manual calls it a five-layer membrane. Timex's oscilloscope photographs show the rise time on a keyboard line going from about 330 ns with the old keyboard to about 1500 ns with the new one, and more crosstalk between columns.

A good ULA still read the keyboard with more than 220 pF added to each input. The failures were ULAs with out-of-specification input thresholds, which the slower Spectrum+ lines exposed. The ULA's keyboard inputs switch low below 1.4 V and high above 2.8 V, and on the 6C001 each has an internal 18K pull-up in addition to R65–R69. So a Spectrum+ keyboard that misbehaves in one machine and works in another may be exposing a marginal ULA; see the Spectrum+ troubleshooting guide.

Protecting the tails from chafing

When converting a Spectrum to a Spectrum+, Sinclair fits foam pads to stop the tails chafing: one on top of the modulator, and on Issue 3 and later boards two on top of the heatsink.

Keys that make two contacts

The Spectrum+ keyboard is two switching membranes in parallel. Some keys close a second contact on the top membrane as well as their own position:

• TRUE VIDEO, INV VIDEO, BREAK, DELETE, GRAPH, EXTEND MODE, EDIT, CAPS LOCK and the four cursor keys also connect row 1 / column 6 on the top membrane. TRUE VIDEO, for example, connects row 1 / column 6 on the top membrane and row 3 / column 1 on the lower one.
• Full stop, semicolon, inverted commas and comma also connect row 2 / column 8 on the top membrane.

Row 1 / column 6 is the Caps Shift position and row 2 / column 8 the Symbol Shift position in Sinclair's matrix description, so each of these keys produces a shifted code with one press. If a whole group fails together while the rest of the keyboard works, suspect the shared contact on the top membrane.

Damaged tails

Most keyboard faults are the membrane: the pads under the keys wear out, or a tail cracks with age or handling. A crinkled, creased or cracked tail end can be cut back by up to an inch to reach good material. Strip back the insulating layer by about another quarter inch, or the keyboard will stop working altogether. Worsley says this applies to both keyboard types.

Light tarnish on the tail contacts cleans off with isopropyl alcohol on a swab.

The STOP key fix

Step 5 of Sinclair's Spectrum-to-Spectrum+ conversion procedure is: test the keyboard, especially STOP, and if STOP does not work, connect a 22 kΩ resistor across R68. On an Issue 1 board, connect the 22 kΩ between pins 1 and 5 of RP1, on the reverse side of the board.

R68 is one of the 10 kΩ row pull-ups on Issues 2 to 5. On Issue 6A it is 6.8 kΩ. Timex's Issue 6A drawing of September 1984 also marks R68 as 6.8K; on that drawing it is the pull-up on KB1, ULA pin 20. The fix belongs to fitting a Spectrum+ keyboard to a rubber-key board; on an unmodified rubber-key Spectrum with its own membrane, check the tails first.

Reassembly and test

1. Refit the keyboard with no folds or creases in any membrane layer.
2. Fit the top cover without creasing the tails.
3. Replace the case screws evenly. Sinclair warns against overtightening the self-tapping screws.
4. Power up and test every key, including the shifted and extended combinations.

Further reading

• Sinclair ZX Spectrum Service Manual, sections 1.8, 2, 5.5, 6.7 and 10
• Sinclair ZX Spectrum 128 Service Manual, section 2

Track layout of a 2011 replacement keyboard PCB for the rubber-key Spectrum ("ZX Spectrum keyboard PCB ver. 0.1, MCBX 06.2011"), with the matrix links drawn in red. Not a Sinclair drawing.

Related pages

• Sinclair ZX Spectrum
• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum 128
• Sinclair ZX Spectrum General Maintenance
• Sinclair ZX Spectrum Troubleshooting Guide
• Sinclair ZX Spectrum+ General Maintenance
• Sinclair ZX Spectrum+ Troubleshooting Guide
• Ferranti Archives 2026