!
!

Keyboard Membrane Replacement - 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 to some a better keyboard to use (I prefer the rubber key type), the Spectrum+ keyboard still had a membrane which perished just as quickly as the rubber key type.

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.

New keyboard membranes are usually available in the shop and fairly easy to fit provided you have patience and take your time.

Firstly, turn the Spectrum over 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 (and longer) to the others.   

Now turn the computer the right way up and gently lift the top of the case.  Take out the feet from the top left and right-hand corners of the base, but remember how they go back together!

You should see two flat ribbon cables linking the keyboard to the main circuit board.
Before dismantling everything else, 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 trimming of the legs has worked – if not you will need a new keyboard membrane. These are usually available in my shop.
 


 


Replacing the membrane

Turn the keyboard assembly onto its face and remove the two clamps towards the back of the case through which the flat ribbon connectors run.

Now remove all of the screws from the plastic or metal backing plate (could be either). 

When finished the backing plate can now be lifted off and the spectrum+ membrane exposed - remove and discard it. 


 

Replace the old membrane with a new one making sure that is located properly on the locating pins, then replace the backing plate and screws.

 

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 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.

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 carefully pass the flat membrane legs through the clamps making sure that they lay straight and flat - again tighten the clamps up evenly, a little at a time. 

You need to ensure that there is a little slack in the leg between the membrane base plate and the clamp itself before fully tightening - use a finger (see photograph right) to hold the leg in place as you tighten the screws. 

Your membrane will not work unless this stage is done carefully as the clamps are used to make connections between different layers of the membrane.

Again, I have had many perfect membranes sent back to me as non-working as people have attempted to test the new keyboard membrane assembly before passing them through and fixing the clamps.  This is usually by people who are used to replacing the membranes on rubber key Spectrums (which don't need leg clamps) but have not performed the task on a Spectrum+.

 

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.  Now the keyboard can be tested before screwing it back properly to the base unit.

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.

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.

 

By now you will probably have lost the feet of the Spectrum+ or forgotten how to put them back - see the picture to the right!

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