Sinclair ZX81 Keyboard Membrane Replacement

The ZX81's keyboard: a printed membrane stuck into the recess of the case top

The keyboard of the Sinclair ZX81 is a self-adhesive membrane stuck into a recess in the top half of the case. Its printed overlay is part of the membrane, and two plastic tails, one with five stripes and one with eight, pass through a slot in the case and plug into the sockets KB1 and KB2 on the board. The tails are the weak point. They are brittle with age, a crack across one kills a row or bank of keys, and they are easily split while the case is being opened. A tail cracked near its end can sometimes be trimmed; otherwise the membrane has to be replaced, and new ones are made today.

Symptoms

• The cursor appears but the keyboard does not enter anything. Sinclair's first checks are that the tails are fully in their connectors and not twisted, and that the diodes D1 to D8 and the connectors are the right way round.
• One row or bank of keys dead: usually a crack across a tail.
• The whole keyboard dead: a short between two keyboard lines holds one key on and disables the rest. An open-circuit line or diode affects only the keys on that line.

Before blaming the membrane, test the board without it. With the board out of the case and the cursor showing, bridge one contact of the 5-way socket KB1 to one contact of the 8-way socket KB2 and watch for a character or keyword; check each KB1 contact against at least two KB2 contacts. If every combination works, the fault is in the membrane. Board-side checks are in the troubleshooting guide.

Tools and parts

• A No. 1 Pozidriv screwdriver; Sinclair warns that an ordinary flat screwdriver will almost certainly slip and may damage the board.
• A replacement ZX81 membrane, which includes the overlay.
• Isopropyl alcohol to remove the old adhesive.
• A sharp blade and a straight edge if a tail is to be trimmed.

Do not clean the tails with liquid solvent cleaners: Television magazine found that some of them make the plastic disintegrate.

Procedure

Sinclair's figures 10 and 11: the board is turned over onto the upside-down case top with the tails plugged in, and the case screws, long (black) in the base and short (yellow) in the base and board

Open the case

1. Unplug everything and remove any RAM pack.
2. Turn the computer over and remove the screws in the base. Some are under the rubber feet; Television counts four base screws, three of them under the feet (the two at the front and the one at the back left), while Sinclair's kit instructions use five screws to fix the base. Keep the screws sorted: the kit has long black screws and short yellow ones, and Sinclair warns of serious damage if a long screw goes in the wrong hole.
3. Lift off the base. The board is fixed to the top half by two short screws; remove them.
4. Turn the board over towards the front and ease the two tails out of KB1 and KB2. Handle them with great care. If a tail tears near the connector, cut it squarely at the tear and keep the rest; it may still be long enough to refit.

Repair a tail instead

1. If a crack is close to the end of a tail, make a clean square cut behind it.
2. Check that the tail still reaches its connector, and that the metallised contacts at the end are in good condition.
3. Refit it as below. If the tail cannot be saved, fit a new membrane.

Remove the old membrane

1. Peel the old membrane out of the recess in the case top.
2. Clean all the old adhesive off the recess with isopropyl alcohol.

Fit the new membrane

Sinclair's figure 9: the tails fed through the slot at the top right corner of the keyboard recess

1. Feed the tails through the slot at the top right corner of the keyboard recess. Leave the backing paper on; just locate the keyboard in the recess, and hold it in place for now with a rubber band or a little sticky tape.
2. Hold the board with its keyboard connectors next to the slot and the tails poking through, and plug the tails in: the tail with five stripes into KB1 and the one with eight into KB2. Television lays the case face down with the front towards you and the board component side up, edge connector at the front left, loops the tails over and pushes them in with a slight rocking movement. Pushing too hard buckles and splits them. The author of a 2016 composite-video write-up found it easiest to seat one side of a tail first and then press in the other pins.
3. Test the keyboard now, with the board resting on an insulating surface, before closing the case; see Testing.
4. Turn the board over so that its components face into the case top behind the keyboard, locate it on the pillars with the jack sockets behind their holes in the side, and fit the two short screws.
5. Turn the case the right way up and peel the backing paper off the keyboard. Locate its top edge against the top edge of the recess and stick it down carefully, working gradually towards the lower edge. Do not try to stick the whole surface down at once; Sinclair suggests a dry run first.
6. Check that the tails are still securely in their sockets, fit the base and its screws, and press the rubber feet back into their recesses over the screw heads.

Testing

1. Power up: the inverse K cursor should appear at the bottom left of the screen.
2. Press every key and the shifted keys, and check the screen entries.
3. If no keys work, check that both tails are fully home and not twisted, and that nothing is left in the sockets.

Related pages

• Sinclair ZX81
• Sinclair ZX81 Troubleshooting Guide
• Sinclair ZX81 General Maintenance
• Sinclair ZX81 Assembly Instructions
• Sinclair ZX Spectrum Keyboard Membrane Repair