Sinclair ZX Spectrum+ Keyboard Membrane Replacement

A Sinclair ZX Spectrum+ (1984), showing its moulded keyboard

The keyboard of the Sinclair ZX Spectrum+ is a moulded key assembly over a five-layer membrane, with a bubble mat that gives the keys their spring, held down by a reaction plate. The original membranes become brittle with age. DataServe Retro calls a Spectrum+ that reaches the copyright message but ignores some or all of its keys one of the most common Spectrum faults, and says the cause is almost always a worn membrane. The membrane is different from the rubber-key Spectrum's, so check you have the right part; the rubber-key keyboard is covered in Sinclair ZX Spectrum Keyboard Membrane Repair.

Symptoms

• No response from the keyboard, or groups of keys not working, on a machine that otherwise starts normally.
• A block of eight keys dead points to the KB1 circuitry or the 5-way tail; a block of five dead points to KB2 or the 8-way tail.
• A group of the extended keys (TRUE VIDEO, INV VIDEO, BREAK, DELETE, GRAPH, EXTEND MODE, EDIT, CAPS LOCK and the cursor keys) failing together: these also close row 1, column 6 on the top membrane, so a fault on that shared contact affects them all.
• Keys on the bottom row not working: Worsley often cures this without a new membrane (see below).
• One key printing repeatedly: the reaction plate is clamped too tightly.

Not every dead column is the membrane. In 1984 Ferranti and Timex found 6C001-7 ULAs whose keyboard inputs were marginal with the Spectrum+ membrane, which loads each input with about 100 pF; a ULA fault moves with the chip. If a new membrane does not cure a dead column, see the troubleshooting guide.

Tools and parts

• A replacement Spectrum+ membrane. Retroleum's experience is that the RWAP membrane works best; others are made of slightly thicker plastic, so the extended keys, which have to bring three layers of contacts together, need more pressure than the rest. Spectrum for Everyone lists membranes from RWAP, ZXRenew and Retro Radionics.
• A small cross-head screwdriver, and fine tweezers for fragments of old tail left in a connector.
• Thin foam or insulating tape for the top of the modulator.
• Packing about 0.75 mm thick and as wide as each tail, for under the tail clamps; Sinclair suggests Paxolin. Double-sided tape for the tail ends.

Before replacing the membrane

Two of Worsley's fixes can save a membrane:

1. For dead keys, especially on the bottom row: remove both tail clamps, clean between the layers of each tail around the clamped area with WD-40 on a cotton bud, and refit the clamps with some extra packing between tail and clamp.
2. For a key that repeats: slacken the reaction-plate screw nearest that key by a quarter turn.

Retroleum notes two older membrane types: an early "parchment" type that always fractures with heat and age, and a more plastic Samsung type that often still works after cleaning and re-seating.

Procedure

Open the case

1. Turn the Spectrum+ face down on a flat surface and remove the eight screws in the base: seven 8 mm pan-head screws round the overhang of the top and one longer 13 mm countersunk screw at the back next to the expansion slot, which may be hidden under a warranty sticker.
2. Turn the computer the right way up and lift the top of the case gently from front to back. Two flat ribbons connect the keyboard to the main board.
3. Grasp each ribbon as near the board as possible and pull it straight out of its socket without twisting. They are brittle and break easily. If one snaps, pull the fragment out with fine tweezers, opening the socket's small retaining legs with a jeweller's screwdriver if necessary.
4. Take out the plastic springs and feet from the top left and right corners of the base, noting how they go back.

Take out the membrane

1. Lay the top half key-side down. The membrane is covered by a grey metal or white plastic reaction plate.
2. Remove the screws holding the plate and lift it off. DataServe Retro counts ten screws; Worsley gives nine.
3. Remove the two small screws in each tail clamp and lift the clamps off without turning them over, so that they go back the same way round. Fitted the wrong way round, the keyboard will not work.
4. Lift out the old membrane.
5. Lift out the bubble mat, wash it in warm soapy water, dry it carefully and put it back.
6. Check the insulating sheet between the membrane and the plate. Some were thin card, which crinkles with age, damp and heat; it must be flat, and Retroleum suggests pressing a crinkled one flat with an iron if no replacement is available.

Fit the new membrane

Sinclair's matrix of the Spectrum+ top membrane: the 8-way tail (KB2) to address lines A8 to A15 through D1 to D8, and the 5-way tail (KB1) to the ULA's KB0 to KB4 with pull-ups R65 to R69

1. Lay the new membrane on the bubble mat, over the two locating pins.
2. Clean any dust off the reaction plate and refit it over the same two pins.
3. Refit the plate screws in Sinclair's order: the centre line of three first, the screws next to the tail clamps last. DataServe Retro works round the plate in a pattern from the centre to opposite corners, then goes round several times tightening a little more each time so the pressure is even.
4. Tighten each screw only to a firm fit, about a quarter turn back from fully tight.
5. Lay the tails in an S shape between the keyboard and the clamps, so the membrane is not under tension. Sinclair's guide is to let the ends of the tracks on the middle layer protrude 1 mm past the clamps; this is easy to see on the wide tail and harder on the narrow one. Leave a little slack between clamp and membrane.
6. Fit a packing piece about 0.75 mm thick under each tail at the clamp. Sinclair says this does not apply to a Spectrum+ whose serial number starts with S.
7. Refit the clamps the same way round as they came off, straight and flat, and tighten each evenly on both screws. These screws need to be a little tighter than the plate screws, because the clamps make the electrical contact between the layers inside the membrane. Retroleum warns that the clamp screws strip easily.
8. Stick the two surfaces of each tail together near the connector end with double-sided tape or a pair of tabs, to prevent short circuits on the board.

Reconnect and close

1. Push each tail into its socket without bending or buckling the end. Hold the wide tail with both hands so it does not bend as it goes in. Worsley eases each tail down into its connector with a slight side-to-side movement.
2. Protect the tails from the modulator and the heatsink. A tail trapped against the top of the modulator causes a short and can kill the whole keyboard; Retroleum blames capacitance where a tail presses on the modulator or heatsink. Sinclair's conversion procedure fits a foam pad on the modulator and, on Issue 3 and later boards, two on the heatsink.
3. Rest the top on the base and test the keyboard before refitting any screws.
4. Refit the springs and feet, then the eight base screws, without overtightening them; some cases are soft plastic and their posts strip easily.

Testing

1. Test every key, including the shifted and extended combinations.
2. If keys fail, check that the tails are fully home and that no debris from the old tails is left in the sockets.
3. Some replacement membranes are thinner than the originals, and the sockets cannot grip them. DataServe Retro sticks thin self-adhesive plastic to the back of the tail (not the contact side) to thicken it. If the sockets themselves are strained, they need replacing.
4. Single keys that misbehave usually mean a small misalignment between membrane and keys. Retroleum adjusts the membrane's position, which means removing all the plate screws each time, and has found tiny pieces of paper tape stuck to some original membranes at certain key positions, apparently to make up for irregularities.
5. Test STOP (Symbol Shift and A) in particular. If STOP does not work, Sinclair's conversion procedure connects a 22 kΩ resistor across R68 (on an Issue 1 board, between pins 1 and 5 of RP1 on the underside). Retroleum says later boards fitted 5.6 kΩ at R68 in place of 10 kΩ as a workaround, while Sinclair's Issue 6A parts list gives R68 as 6.8 kΩ.

Sinclair's matrix of the Spectrum+ lower membrane, which carries the extended keys

Related pages

• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum+ General Maintenance
• Sinclair ZX Spectrum+ Troubleshooting Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair ZX Spectrum ULA Replacement