Friday, 27 February 2015

ZX81 and ZX Spectrum keyboard membrane repair

One of the frequent problems with the Sinclair computers of the early 1980's in the keyboard membranes turning brittle. This results in loss of one or more rows or columns in the keyboard. These are split into 5 rows of 8, so if some columns are out, you may have keys working and some not. For example, keys 3,4 and 5 working and also 6, 7 and 8, but 1,2, 9 and 0 don't respond, or if rows are out, 1,2,3,4 and 5 work, but 6,7,8,9 and 0 don't etc. It's rarely a single key. In this case, it's all of them.
Sometimes this happens right by the connector, as this this one, the entire row tail is detached. If this is the case, and the rest isn't too brittle, it is recoverable. It often happens when someone decides to have a peek inside, normally just before it is consigned to the attic, or just after it is retrieved from there. This usually leaves a small piece of membrane in the connectors, a rough edge on the membrane, and an ebay listing which states 'untested, worked before it was put in the loft'.
It can also happen if they tails fold sharply when the case is closed. Sometimes there are cracks further up, as with this ZX81 membrane, it would need to be cut back to before this point.
I have considered repairing this sort of thing by clamping another piece of membrane with tracks on over the top of this, so it makes good contact and bridges the gap. I don't know how reliable it would be though, or how best to clamp it. If it's too far gone, the best option is to get a replacement membrane. The are some excellent ones available from RWAP.
This is also the point where I line up candidates for my USB keyboards. I've designed the boards to be able to cope with short membrane tails that would otherwise be to short and would have to be replaced.
These wouldn't be long enough to reach the normal ZX81 or ZX Spectrum board.
However, when there is just damage to the end, they can be recovered if there is enough non-brittle tail left.
First cut back the tail until there is a flat edge with no damage. There are two parts to each tail, one with traces on, and one clear. Cut back the clear one by about 10-15mm. I then carefully wrap the remaining parts (apart from the last 10-15mm) with clear tape to support what is left and stop it being damaged by bending.
If both parts are still long enough, it can be reinstalled. The 5 way tail on this one is a bit short, but should still reach.
To reassemble, first give the case a good clean.
Then install the membrane with the tails poking through the slots.
The new Spectrum membranes look much the same if you're using one of those.
Cover that with the rubber mat, these could sometimes benefit from a wash before refitting.
Next comes the metal faceplate. Replacement for these are available (also from RWAP) if yours is a bit scratched or dented. Remove the existing adhesive or tape, clean well and apply new double sided tape.
Carefully position that and press down along the edges and the lid section is complete.
Underneath, the repaired tails will be shorter, so it's sometimes fiddly to reattach them,
Remove any remnants of the old membrane tail end in the socket and then gently connect the new (or at least newly prepared) tails. The trick is to apply as little pressure as possible, and to always press directly into the socket, not at an angle, or it may bend and snap again.
With those attached, it is time to test again.
All keys are now working, just be careful when opening the case in future.