Sinclair QL Keyboard Membrane Replacement

A QL opened up: the keyboard's metal backing plate is in the upper case, with the two keyboard ribbon tails running down to the main board (photo: Ewx, CC BY-SA 3.0)

The keyboard of the Sinclair QL is a membrane under a rubber bubble mat, wired as an 8 × 8 matrix and connected to the main board by two ribbon tails at J11 and J12. Old membranes go brittle. DataServe Retro, which sells new ones, puts groups of dead keys, or a QL that will not get past the start-up screen, down to a membrane made brittle by years of heat from the computer. Sinclair's own service manual calls a break in the membrane a common fault.

Symptoms

• A group of keys does not work; on one QL bought from Australia about a quarter of the keys were dead, mostly in one area of the keyboard.
• Keys that work with the case open and the tails straight, but fail when the tails are bent, or the other way round.
• The QL locks up while an on-screen clock keeps running, or a few keys do not work: Tony Firshman of TF Services finds dirty or damaged tail ends behind this.

Sinclair's fault table separates the membrane from other causes:

Keyboard symptoms and remedies (Sinclair)

Symptom	Remedy
Keyboard does not respond at all	Check connectors J11 and J12 and the ribbon cable
Multi-character printing, key bounce, keys sticking down, a key does not register	Renew the membrane
A key does not always register	Renew the bubble mat
Continues to fail the keyboard test	Renew IC24 (IPC), then IC18 (CPU)
Keyboard locks up after prolonged use	Renew IC23, then IC24, then IC18

Sinclair notes that keyboard faults can come from the ZX8302 or the IPC, but are more often on the membrane or its connectors. A whole row or column that is dead points to a connector, and a pair of keys or two rows misbehaving together point to a short; check with an ohmmeter.

Safety

The QL has no mains voltage inside; the power pack is external and the highest voltage on the board is its 44 V peak-to-peak AC feed. Unplug the power pack before opening the case.

Tools and parts

• A new QL membrane. DataServe Retro and RWAP Software both sell newly made QL membranes.
• A cross-head screwdriver for the case screws and the six keyboard plate screws.
• A small flat screwdriver for the key sleeves, if keys are to be cleaned.
• Contact cleaner for the tail sockets.

Procedure

Opening the case

1. Unplug all leads and turn the QL upside down.
2. Remove the eight self-tapping screws: four 5/16-inch screws along the front edge under the overhang, and four 1¼-inch screws along the rear edge. Do not remove the two screws on the base directly below the Microdrives.
3. Holding the halves together, turn the QL right way up and lift the top half. It stays connected by the two keyboard ribbons and the leads of three LEDs.

Testing the old membrane

1. With the case open, squeeze the membrane between finger and thumb over a dead key to imitate a key press, and see whether it registers.
2. Try the dead keys with the tails straight and again with them bent as they are when the case is closed. Keys that come and go with the tails point to cracked tracks in the tails.

If the only break is close to the connector end, the membrane can be saved: pull the tail out, cut off a short section above the break and plug it back in. Firshman cuts about 5 mm off a damaged tail, but only on the later membranes with straight signal lines, not the old type. A break anywhere else needs a new membrane.

Separating the case halves

Sinclair's Issue 6 layout: the keyboard connectors J11 and J12, with pin 1 marked, to the right of the RAM

1. Free each membrane tail from its socket on the board: pinch the tail next to the socket between finger and thumb and pull straight up.
2. Release the six LED leads from the snap-action wire post socket beside the TV modulator can. Pinch its black plastic moulding and lift it until you feel resistance, then pull the leads free.
3. Note which lead goes where, or use the table below when refitting.

Changing the membrane

1. Remove the six ¼-inch pan-head screws that hold the keyboard backing plate to the upper case.
2. Lift the plate out with the membrane attached. The bubble mat is underneath it.
3. Separate the old membrane from the plate by carefully breaking the adhesive seal that holds its ribbon tails in place.
4. If keys need cleaning, hold a key down and prise the retaining sleeve off its underside with a small screwdriver under the rim. Sinclair's remedy for sticky keys is to clean the key surfaces or renew the bubble mat.
5. Lay the bubble mat back in place. The bubble mat, membrane and backing plate are perforated to fit over six locating dowels moulded in the upper case.
6. Fit the new membrane and the backing plate over the dowels. Sinclair notes that it can help alignment to release the adhesive bond between the membrane and the plate.
7. Refit the six screws.

Reassembly

1. Clean the tail sockets on the board with contact cleaner.
2. Push each tail fully into its socket. Make sure it is fully inserted and not kinked.
3. Reconnect the LED leads to the wire post socket before screwing the halves together:

QL LED connections (Sinclair, Section 2 para 2.6)

Pin	Diode	Wire colour	Function
1	D21k	Black	MDV2 (red)
2	D21a	Grey
3	D20k	Black	MDV1 (red)
4	D20a	White
5	D27k	Black	Power (yellow)
6	D27a	Red

1. Close the case and refit the eight screws in their positions.

Testing

1. Switch on and check that the power LED lights and the start-up screen appears.
2. Type every key at the BASIC prompt, including SHIFT, CTRL and ALT combinations. Sinclair's keyboard test, run from its test tape, asks for every key in turn and warns that each must be pressed individually, not skimmed over.
3. Close and open the case once more and recheck the keys that were dead, to make sure the tails are seated.

Related pages

• Sinclair QL
• Sinclair QL Maintenance Guide
• Sinclair QL Troubleshooting Guide
• Sinclair QL Service Manual
• Sinclair QL RGB Port Repair
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair QL Voltage Regulator Replacement
• Sinclair QL Serial Port Repair
• Sinclair QL RAM Replacement