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Keyboard Membrane Replacement - Rubber Key Spectrum



You pick up an old Spectrum and the keyboard is dead, or groups of keys mysteriously stop 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 membrane (although splits in the keyboard mats can also cause similar problems).

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

New keyboard membranes and rubber keyboard mats are usually available in the shop and fairly easy to fit provided you have strong fingernails and lots of patience - the keyboard plate has to be removed and 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 different (longer) to the others in some cases. 

 

Now turn the computer the right way up and gently lift the top of the case. You should see two flat ribbon cables linking the keyboard to the main circuit board.

Before ripping everything else apart it may be the case that the connectors on the keyboard PCB have scratched off the silver from the ribbon so an electrical contact can no longer be made, or that they are cracked or split near the ends. 


Gently grasp the ribbons as near to the PCB as possible and carefully pull them from their sockets without twisting them.  They tear easily, so be careful, you're trying not to damage them at this point.

If they do tear, try not to leave any bits behind in the sockets - if you do you will need to remove the bits with fine tweezers - see right.

 

 

If you manage to remove the tails without damaging them, cut off the end 5mm or so, then carefully push them back into their sockets, connect the Spectrum back up again and re-test the keyboard.

You may fall lucky and all is well – if not you will need a new keyboard membrane. These are usually available in my shop.

 


Replacing the membrane
First of all you need to remove the keyboard facia plate.
There were two versions of these plates.  Look beneath the keyboard - if there are any small metal tabs at the outside edges, gently prize them upwards.  This should now allow the top metal plate to be carefully pulled away.  However, these locating tabs were not very successful and tended to fall off.  They were soon dropped in favour of glue and indeed, the vast majority of the face plates are glued.  Getting them off can be a slow job if you wish to avoid bending the plate.

Starting at one side, 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 careful - the plate can get very hot, very quickly - don't send your claim for burnt fingers to me !  Also, a hot face plate bends more easily than a cold one, so again, patience is vital ! 

 

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 25 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's the time to 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 (it makes it swell and distort) and the Spectrum motherboard.
 

 

 

Now replace the membrane, 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 length ways to fit.


 

 

Apply double sided tape to the back of the keyboard plate and to the side edges of the top case (make sure you don't apply tape inadvertently to the rubber mat). 

 



 

All that remains is to carefully replace the ends of the flat cables into their sockets. Once this is done, lay the keyboard on the machine and test it.

Test all the keys and combinations.  If all is well, then the screws can be replaced.  If it isn’t, check that the cables are in the connectors properly, and that this is no debris in there from the old cables.

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.

A similar problem can be caused if the left membrane leg becomes trapped on top of the Spectrum modulator, but this time this results in none of the keys working at all.

The solution is to stick a thin piece of foam 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 2006 - 2011