Replacing the Keyboard Membrane - Sinclair Spectrum+ and 128

You pick up an a Spectrum+ or Spectrum 128, connect everything up and turn it on and can't get past the  "© 1982 Sinclair Research Ltd" screen, or Spectrum 128 start up menu. There is no response from the keyboard, 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.

Although to some a better keyboard to use (I prefer the rubber key type), the Spectrum+ and 128 keyboard still had a membrane which perished just as quickly as the rubber key type.

New keyboard membranes are available in the Sales pages of the Sinclair section of the website here. They are fairly easy to fit provided you have patience and take your time.

The procedure below describes how to change the membrane on a 48K or 128K Spectrum+. Follow the instructions carefully, some important points are easy to overlook.

Firstly, turn the Spectrum over and place face down onto a flat surface and remove all the screws in the bottom of the case.  There are 8 screws to remove in total. Note that the screw in the top centre is different to the others (it is counter sunk and and longer).  It may also be hidden behind a warranty warning sticker.

Now turn the computer the right way up facing you in the normal working position.and gently lift the top of the case from front to back. 

You will now see two flat ribbon cables connecting the the keyboard to the Spectrum motherboard.

Gently grasp the ribbons as near to the PCB 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 itself.

If they do snap off, pull out the fragments of membrane leg from the socket 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.

Now take out the plastic springs and feet from the top left and right-hand corners of the base, but remember how they go back together!

You can now separate both parts of the case for easier working.

The keyboard membrane is housed in the top part of the top (keyboard) part of the case. Lay it key side down and you will see that the membranes is covered with a grey metal, or white plastic plate which is secured in place with 10 screws.

Remove all 10 screws and lift off the backing board and put to one side.

Now remove the 2 smaller screws in each leg clamp which hold the membrane legs to the case.

Lift off the clamps with turning them over, then place to one side with the screws back resting inside them.  Do this so that you don't forget which way around the clamps are attached - if you put them back on the wrong way around, the keyboard will not work.

The old membrane is now free and can be lifted off and thrown away.

Beneath the membrane you will find the "bubble mat". This mat provides the "feel" behind the key action when you are typing.  Lift this out and wash it in warm soapy water.

Dry it carefully then put it back in place.

Now put the new membrane in place on top of the bubble mat, ensuring that it is in the correct position over the two locating pins

Clean any fluff or dust from the metal or plastic backing plate, then fit back in position on top of the membrane. It should also fit over the two locating pins.

Now replace the 10 screws you removed earlier.

It is particularly important to follow a sequence to ensure that the plate is secured firmly, but evenly:

  • start with a central screw and screw in until you start to feel some resistance
  • then move to top left screw hole and insert the next screw tightening in the same way
  • next, move to the bottom right and repeat the process
  • now move up to the top right, then bottom left and a complete "circuit" will have been done
  • go back to the centre row and insert another screw, then follow the same sequence around the outside until all the screws have been replaced.

Now follow the same sequence around the screws and this time tighten just a little more, gradually working your way around them all several times to ensure that the pressure exerted on the backing plate is even. The screws do not need to be too tight.

Now replace the two leg clamps over the membrane legs, making sure that they are straight and flat. Tighten in place with the 2 screws for each clamp. Leave a little bit of slack in the membrane leg between the clamp and the membrane body. Tighten the clamps evenly on each screw so the clamps fit flat and not angled or "lop sided". These screws do need to be a little tighter than the membrane back plate screws as they cause an electrical contact to be made within the membrane. This is why it is essential to have the bracket mounted the right way around!

If this series of steps is not followed carefully, you may find groups of keys which do not work (or the key has to be pushed harder) when you have finished re-assembling the Spectrum. I have had many completely functional membranes sent back to me because people have not had the patience to follow this sequence of re-assembly.

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.

Be careful when replacing the base screws which hold the computer together.  These cases were manufactured in various countries and locations and quality wasn't always consistent. The plastic of some cases is quite soft and it is very easy to strip the fixing posts when tightening the screws - don't over do it!

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