Problems With Spectrum Plus Membranes

Since writing this article I’ve been able to test RWAP’s improved Spectrum Plus membrane available from sellmyretro.com – They have addressed the uneven pressure issue laid out below and I’m happy to report the membrane I tried worked 100% first time. So if you’re in the market for a Plus membrane, this is the one I would go for – kudos to Rich at RWAP for getting it sorted :) I’ll leave the original article below in case it helps others resolve any problems they may encounter.

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 just a clean and re-seating is in order).

I don’t stock modern replacement Spectrum Plus membranes at the store because in the past I’ve found them to be unsatisfactory  (and likely to generate  complaints / returns) – believe me I’ve tried VERY hard to get them working 100% but there has always been problems.

1. Random keys not working (properly). This is down to  alignment issues: holes not being  in exactly the right places, pad layout inaccuracies,  trapped air in the membrane etc. Small 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 (take 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 (?)

Note: 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).

(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).

2. The special/extra keys of the Plus require more  pressure to respond than the “normal” keys.  This
may or may not bother you but the original membranes do not exhibit this behaviour. The problem is due  to the Plus membrane having three layers of plastic  (instead of two in the rubber key type). With the modern plastic being stiffer and the special keys requiring three layers to connect – the extra pressure  required to do so is noticeable with some makes.

3. Rows/columns intermittently not working.  This can  be caused by:

a) 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.

b) 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.

c) 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).

d) 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.

e) 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.