Replacing the Keyboard Membrane - "Rubber Key" Spectrum

You pick up an old "rubber key" Spectrum, connect everything up and turn it on and can't get past the  "© 1982 Sinclair Research Ltd" screen.

The keyboard is dead, or groups of keys mysteriously aren't working - probably one of the most common Spectrum faults and fortunately one of the easiest to put right. The cause is almost always a worn out keyboard membrane.

The procedure below describes how to change the membrane on a 16K or 48K "rubber key" Spectrum - take your time and be patient!

Removing the old membrane
If you don't yet have a new replacement keyboard membrane, they are available in the Sales pages of the Sinclair section of the website here.

The replacement process is fairly straight forward, but you will need strong fingernails and lots of patience - the keyboard plate has to be removed and it is usually stuck down pretty solidly!

Firstly, turn the Spectrum over onto a flat surface and remove all the screws in the bottom of the case. Note that the screw in the top centre is usually different (longer, with a tapered head) to the others, so don't put it back in the wrong hole. 

Now turn the computer the right way up and facing you in the normal operating position. Gently lift the top of the case. You should see two flat ribbon cables linking the keyboard to the main circuit board.

Gently grasp the ribbons as near to the main board as possible and carefully pull them from their sockets without twisting them.

 They are delicate and usually quite brittle due to prolonged heat and old age, so be careful as they will break very easily.

Try not to damage the ends as bits of membrane leg left inside the leg molex socket can be tricky to remove without damaging the socket.

If they do snap off, pull out the fragments of membrane leg with a fine pair of tweezers, you may need to open the little legs which hold the membrane leg in place with a very small, flat bladed jeweller's precision screwdriver.

Replacing the membrane
First of all you need to remove the keyboard facia plate.

There were two versions of these plates. Look carefully at the underside of the keyboard - if there may be four small metal tabs at on the outside edges of the plate, gently prize them upwards. This should now allow the top metal plate to be carefully pulled away.

Keyboard plates with these securing tabs were used on issue 1 and some issue 2 spectrums, however, they were not very successful and tended to fall off. They were soon dropped in favour of incredibly strong glue and getting these plates off can be a slow job if you are to bending the plate.

Starting at one side, try and find a point where the face plate might be a little loose and using your finger nails, slowly pull the plate with even and constant pressure (to avoid kinking it) until it starts to lift from the case.

Now move to the top of the plate, releasing a little at a time, working around all sides of the plate little by little.

If the plate is stuck very tightly, then careful heating with a hair dryer will soften the glue and make removal easier. However - be very careful - the plate can get very hot, very quickly.

Also, a hot face plate bends more easily than a cold one, so again, patience and care is vital.

If you are replacing the plate as well, you can use a bit more brute force!

Once free, remove the rubber mat, then lift and pull the membrane free through the top case.

The keyboard mat will be full of all sorts of unspeakable rubbish and debris collected over the past 35 years or so.

Take this opportunity to wash out all the old pizza, spiders and dried coffee - a rinse in warm soapy water does the trick (make sure to dry it properly afterwards).

Also check for any split keys, if there are any, the only solution is a new mat.

Now clean off the old glue residues from the top case and the back of the keyboard plate - I use a small screwdriver for this.

A cloth with a little white spirit helps to soften the old glue first, but keep this well away from the rubber keyboard mat as it makes it swell and distort and the spectrum motherboard to avoid losing your coloured key legends.

Now remove and replace the old membrane with a new onee, making sure it is the correct way around with the flat cables poking through the slots as before.

 Make sure it is correctly seated on the Spectrum top case with the locating pins through the appropriate holes in the membrane.

Next, lay the keyboard mat over the top of the membrane - again, there are small locating pins to hold the mat in the correct position.


The keyboard plate now needs to be re-secured to the top case to hold the mat and membrane in place.

The easiest way to do this (which will also allow easier future removal) is to use double sided adhesive tape.

 Most tape is a little too wide for the top and bottom of the plate, but it is easily cut lengthways to fit.

Once cut to the correct width and length, apply the double sided tape to the back of the keyboard plate and to the bottom and side edges of the top case (make sure you don't apply tape inadvertently to the rubber keyboard mat).

Peel off the backing tape to reveal the sticky reverse side of the adhesive tape on all of the strips which have been applied to the top case and face plate.

Carefully stick the top plate down over the keyboard mat and hold firmly in place for at least 60 seconds.  The glue is fully activated by pressure, so push down as evenly as possible and hold in place.


Now replace the ends of the flat cables into their sockets on the main board, again taking care not to bend or buckle the ends.

The wider leg will need to be held with both hands to prevent it bending too far as you try to push it into the molex socket. This is a little bit awkward as you do not have much room to work. You will need to rest the upper part of the case against your hands, or some other object to stop it moving around too much.

Now the keyboard can be rested on top of the base and tested before screwing it back properly to the base unit.

Test all the keys and combinations. If all is well, then the screws can be replaced. If it isn’t, check that the ends of the membrane legs are securely in the molex sockets on the motherboard, and that this is no debris in there from the old cables.

Some replacement membranes these days are thinner than the originals and it be difficult to get the old ribbon cable sockets to grip the end contacts. This can cause the contact to break when the lid is replaced and keys may not work despite the new membrane.

Carefully attaching some thin sticky back plastic to the back of the ribbon (not the contact side) helps to “thicken up” the ribbon a little so that the connector sockets can get a better grip. If even this is not enough, it may mean that the molex connectors themselves have been "strained" and cannot grip the legs sufficiently to make a good electrical contact.  If this is the case, they will need to be replaced. Spares are available here.

A similar problem of non-working keys can be caused if the left membrane leg becomes trapped between the top case and the top of the Spectrum TV modulator when the case has been screwed back together. This causes a short circuit which results in none of the keys working at all and the false belief that the new membrane is faulty.

The solution is to stick a thin piece of foam, or a couple of strips of insulation tape on top of the modulator which protects the membrane leg, or carefully bend the leg in the other direction away from the modulator as the lid is closed.

 I have also seen technicians sellotape the legs to the top case to keep them out of the way. This requires the dexterity of surgeon though and I find the thin foam the best method.

Remember on all spectrum repairs that when re-assembling you are only screwing into plastic - you are not screwing a shelf to a wall. You only have to over tighten once to strip the thread or crack a pillar - be patient and take care!
 
!

© DataServe Retro 2004 -  2021