Problems With Spectrum Plus Membranes

In my experience the best replacement Spectrum Plus membrane on the market is supplied by RWAP and available from sellmyretro.com. Other types are made of plastic that is slightly thicker than the original Sinclair membrane which causes extra pressure to be needed for the extra keys (Break, Delete, Edit, Cursors etc) compared to the others (this comes into play because 3 layers of contacts need to connect for these keys to function). The effect may not be a concern to everyone but the original membranes did not exhibit this behavior.

There were originally two kinds of membrane used in the Spectrum Plus / 128 – the early “parchment” type (these always fracture with heat/age) and the more plasticky Samsung type which  were far superior and often still work to this day – sometimes they just require cleaning and re-seating. Modern replacement membranes are made of higher quality materials but issues can arise, these are summarized below.

Random individual keys not working (properly): This is down to alignment issues: holes in the membrane not being  in exactly the right places, pad layout inaccuracies,  trapped air etc. Small alignment adjustments can often resolve such issues (but more often than not other keys then start misbehaving). It can be a very time consuming  and tedious exercise sorting these issues out since all the screws from the backing plate need to be removed each time (taking care not to over-tighten them). It does seem Sinclair themselves originally experienced issues – tiny pieces of paper tape can sometimes be found stuck to old membranes on some key positions,  presumably to compensate for irregularities (?)

Another factor related to dodgy keys that is not actually the fault of the membrane itself is the insulating sheet between the membrane and backing plate. Sometimes this was made of thin cardboard and through age / damp / heat becomes crinkled. It must be perfectly flat so if you have a problem sheet like this and cant replace it, you could try pressing it flat with an iron.

As part of the upgrade kit Sinclair provided for customers buying the Spectrum Plus case, a 22K resistor was supplied with advice to connect this in parallel with R68 on the motherboard if certain keys did not work, especially STOP (symbol shift + A). Later versions of the Spectrum motherboard had 5.6K resistor installed by default (instead of 10K) at R68 as a work around.

Whole Rows/Columns of keys (intermittently) not working.  This can  be caused by:

• The clamps not pressing the three conductive layers  together well enough (or not in the right places) – Adjustments  can resolve this but be careful not to over-tighten the  clamp screws as they are easily stripped.
• The membrane tails being pulled tight by the  clamps, warping the top of the membrane. Make sure there  is a little slack on the main membrane side when positioning  the tails in the clamps to avoid this.
• The membrane tails pressing against the heatsink or  modulator metal shells (a capacitance issue – easily resolved  by adding foam padding to them to keep the tails away).
• The ends of the tails not seating well in the sockets. I have not encountered this particular issue but some people suggest curling the end of the tails.
• Capacitance issues with the membrane material itself – mods  to the Spectrum PCB can be made to get around this but the work is not trivial.

I don’t stock modern replacement Spectrum Plus membranes at the store because – as outlined above – successful fitting can be a problem and I can’t be dealing with all the inevitable questions / complaints / returns.