Quick answer: on a ZX Spectrum, look at which keys are dead before you buy anything. The 40 keys are wired as a matrix: eight groups of five keys and five groups of eight keys. If a group of five neighbouring keys has died together (say 1 to 5, or A to G), one track on the 8-way membrane tail has failed. If eight keys spread across the keyboard have died (for example 1, Q, A, Caps Shift, 0, P, Enter and Space), one track on the 5-way tail has failed. Odd single keys are usually worn contact pads. All three faults normally mean a new membrane. Only a completely dead keyboard on a machine that otherwise works points to the board or the ULA.
The membrane is the most common keyboard fault on every Spectrum. The plastic sheets go brittle with age, and the two ribbon tails crack where they bend into the motherboard connectors. This guide covers the rubber-key 16K/48K and the Spectrum+ and 128, which share the same electrical layout. Our general ZX Spectrum repair guide covers power supplies, capacitors and other faults.
How the Spectrum keyboard is wired
The keyboard has no electronics of its own. It is two layers of printed plastic (more on the Spectrum+), and pressing a key connects one track on the 8-way tail to one track on the 5-way tail. According to Sinclair’s service manual, as summarised by RetroTechCollection:
- The 8-way tail carries eight lines, one per half-row of five keys. On the board, each line passes through a diode (D1 to D8, 1N4148) to one of the Z80’s high address lines, A8 to A15.
- The 5-way tail carries five lines, KB0 to KB4. Each line connects one key in every half-row and goes to the ULA’s keyboard inputs. Pull-up resistors hold these lines high until a key is pressed.
The computer scans the keyboard by pulling each address line low in turn and reading which of the five ULA inputs go low. That’s why failures come in groups of five or eight rather than at random.
Step 1: map exactly which keys are dead
Press every key and write down which ones do nothing. In 48K BASIC most keys produce a keyword or a letter on screen, so this is quick. Then compare your list with these two tables.
The eight groups of five (8-way tail)
This mapping is from the Sinclair Wiki keyboard page and the original Spectrum manual, chapter 23.
| Address line | Port (decimal) | The five keys |
|---|---|---|
| A8 | 65278 | Caps Shift, Z, X, C, V |
| A9 | 65022 | A, S, D, F, G |
| A10 | 64510 | Q, W, E, R, T |
| A11 | 63486 | 1, 2, 3, 4, 5 |
| A12 | 61438 | 0, 9, 8, 7, 6 |
| A13 | 57342 | P, O, I, U, Y |
| A14 | 49150 | Enter, L, K, J, H |
| A15 | 32766 | Space, Symbol Shift, M, N, B |
The five groups of eight (5-way tail)
Each 5-way line takes the key in the same position from every half-row. KB0 is the outermost key, and KB4 is nearest the middle of the keyboard.
| 5-way line | The eight keys |
|---|---|
| KB0 (D0) | Caps Shift, A, Q, 1, 0, P, Enter, Space |
| KB1 (D1) | Z, S, W, 2, 9, O, L, Symbol Shift |
| KB2 (D2) | X, D, E, 3, 8, I, K, M |
| KB3 (D3) | C, F, R, 4, 7, U, J, N |
| KB4 (D4) | V, G, T, 5, 6, Y, H, B |
Step 2: read the pattern
Sinclair’s own fault-finding rules, from the service manual as quoted by RetroTechCollection, match the patterns above:
| What you see | Most likely cause |
|---|---|
| A group of five keys dead (one row of the first table) | A broken track on the 8-way tail, or that line’s circuit on the board (its diode) |
| A group of eight keys dead (one row of the second table) | A broken track on the 5-way tail, or that line’s circuit on the board (its pull-up resistor or ULA input) |
| Random single keys dead or needing a hard press | Worn or dirty contact pads inside the membrane |
| Keys that work cold but fail when the machine warms up | Membrane (if the ULA is known good) |
| Wrong characters, or two keys giving the same result | Shorted tracks on a tail, so check for cracks and contamination |
| Whole keyboard dead, but the machine boots to the © message | Check the board first: +5 V, the pull-up resistors and the ULA |
In practice, the membrane causes the first four by a wide margin. The two board-side checks are only worth doing when the pattern doesn’t fit, or a new membrane hasn’t helped.
Optional: a BASIC keyboard tester
If enough keys still work to type a few lines, this short program reads all eight half-rows and shows the raw values live. It uses the port formula given in the Spectrum manual (254 + 256 × (255 − 2^n)):
10 FOR n=0 TO 7
20 LET p=254+256*(255-2^n)
30 PRINT AT n,0;p;" ";IN p;" "
40 NEXT n
50 GO TO 10With nothing pressed, note the value on each line. It varies from machine to machine, because bit 6 follows the EAR socket. Pressing a key in that half-row should reduce the value by 1, 2, 4, 8 or 16, depending on its position. A key that never changes its line’s value is not making contact. If too many keys are dead to type the program, use the tables above, or test with a known-good membrane.
Step 3: inspect the tails before buying a new membrane
Open the case (see below) and look closely at the last few centimetres of each tail where it enters the connector. Cracks, creases or dark breaks across the tracks are the classic failure. Light tarnish on the contacts can be cleaned off with isopropyl alcohol on a swab.
Ian Worsley’s repair notes, republished by World of Spectrum and summarised by RetroTechCollection, describe a temporary fix for a cracked tail: cut it back past the damage (by up to about an inch) and carefully strip the insulating layer back by about another quarter of an inch, so the tracks can make contact in the connector. It only works if the tail has enough slack, and old plastic often cracks again, so treat it as a way to get a machine running while a new membrane arrives.
If the tails look perfect and a whole group is still dead, test the membrane itself. Use a multimeter on continuity: hold one probe on the 8-way track and the other on the 5-way track for a dead key, and press the key. No continuity confirms the membrane is the problem. Our multimeter guide covers suitable meters.
Step 4: rule out the board (only if the membrane tests good)
These checks come from Sinclair’s manual and the T.I.S. field-service notes, as summarised by RetroTechCollection:
- +5 V present: a whole dead keyboard can simply be a power fault. Check the regulator output first, see our Spectrum voltage regulator guide.
- Pull-up resistors: on an Issue 3 board, the five 10 kΩ pull-ups are R65 to R69. A failed one kills one group of eight. Values and numbering differ between board issues, so check against the schematic for your issue.
- Address-line diodes: D1 to D8 (1N4148) sit between the 8-way connector and the address lines. An open diode kills one group of five.
- ULA: Sinclair’s fault table lists the ULA (IC1) for a completely dead keyboard or a wrong or too-fast response. That’s a last resort, once the cheaper parts are cleared.
Which replacement membrane do you need?
Membranes are not interchangeable across the range, so match the listing to your exact model. If you’re not sure which you have, see our guide to identifying ZX Spectrum models.
| Model | Membrane notes |
|---|---|
| 16K and 48K (rubber keys) | Two-layer membrane with a 5-way and an 8-way tail. The most widely reproduced type. |
| Spectrum+ (48K) and Spectrum 128 | Different, multi-layer membrane (Sinclair’s manual says five layers) that handles the extra single-press keys. The Spectrum+ and 128 share it. Not interchangeable with the rubber-key type. |
| Grey +2 | Same electrical interface as the Spectrum+, but the tails exit on the side, so it’s a different part (per the Sinclair Wiki). |
| Black +2A, +2B and +3 | A different keyboard arrangement again. Buy a membrane listed for these models. |
Fitting a new membrane: rubber-key 16K/48K
This is a summary of Sinclair’s procedure, as given in the service manual:
- Unplug everything, turn the computer over and remove the five case screws.
- Lift the top half carefully. It stays attached by the two tails, so pull them gently out of their connectors and note how they were routed.
- The metal keyboard template (escutcheon) is held on by double-sided tape around its edges. Sinclair notes it usually can’t be removed without some damage, so lever it off slowly from one end and expect to straighten it.
- Lift out the rubber key mat and the old membrane. Fit the new membrane over the locating projections, then refit the key mat and make sure every key is seated, or keys can get trapped under the template.
- Stick the template back with fresh double-sided tape on all four edges. Sinclair specified 12 mm tape for the sides and bottom and 6 mm for the top.
- Push each tail fully and squarely into its connector without kinking it, and make sure neither tail is trapped or folded when you close the case. A tail bent double will fail again.
Fitting a new membrane: Spectrum+ and 128
The Spectrum+ keyboard is a different job. The tails are held by two clamps, and a reaction plate is held by a set of screws (ten on the 128, according to its service manual). Sinclair’s assembly notes say:
- Lay the tails in a gentle S shape between the keyboard and the clamps, so that the membrane is not under tension.
- Fit a thin packing piece (about 0.75 mm) under each tail at the clamp.
- Tighten the reaction-plate screws to a firm fit only, centre screws first. Worsley notes that a key that repeats on its own usually means the plate is clamped too tightly near that key, and backing the nearest screw off a quarter turn cures it.
- Test every key, including the shifted and extended single-press keys, before closing the case.
Worsley also notes that dead bottom-row keys on a Spectrum+ can sometimes be cured without a new membrane, by cleaning between the tail layers around the clamp and adding extra packing.
If you fitted a Spectrum+ keyboard to a rubber-key board
Sinclair’s Spectrum-to-Spectrum+ conversion instructions include a specific fix: if the STOP key doesn’t work after the conversion, fit a 22 kΩ resistor across R68 (Issue 2 and later boards). This applies only to that conversion. On an unmodified 48K with its own membrane, check the tails first.
Related ZX Spectrum guides
- ZX Spectrum common faults
- How to repair a ZX Spectrum
- Sinclair ZX Spectrum+ keyboard layout
- ZX Spectrum tape games not loading
Sources: Sinclair Research ZX Spectrum and Spectrum 128 service manuals, Ian Worsley’s Spectrum repair notes, and the T.I.S. field-service notes, as compiled by RetroTechCollection; Sinclair Wiki: Keyboard; ZX Spectrum BASIC manual, chapter 23.
FAQ
Why have only some keys on my ZX Spectrum stopped working?
The keyboard is a matrix of eight groups of five keys and five groups of eight. When one track on a membrane tail cracks, every key on that track dies together. A dead group of five points to the 8-way tail, and a dead group of eight (spread across the keyboard) points to the 5-way tail. Both usually mean a new membrane.
Can I repair a cracked ZX Spectrum membrane tail?
Sometimes, temporarily. If the crack is near the end and the tail has slack, it can be trimmed back past the damage, with the insulating layer carefully stripped back so the tracks can make contact. Old membranes tend to crack again, so a replacement membrane is the long-term fix.
Is a 48K Spectrum membrane the same as a Spectrum+ membrane?
No. The rubber-key 16K/48K uses a two-layer membrane, while the Spectrum+ and Spectrum 128 use a different multi-layer membrane. The grey +2 and the black +2A/+3 use different parts again, so buy one listed for your exact model.
How do I test a ZX Spectrum membrane with a multimeter?
Set the meter to continuity. Using the key tables, find which 8-way and which 5-way track a dead key sits on, put a probe on each of those contacts at the tail ends, and press the key. A working key shows continuity, and no reading means a broken track or a worn pad.
My whole Spectrum keyboard is dead. Is it the membrane?
Less likely. One cracked track kills five or eight keys, not all 40. If the machine boots normally but no key responds, check that the tails are fully inserted, then +5 V, the keyboard pull-up resistors and finally the ULA.