General assembly steps are described in detail on e.g. this webpage: http://www.retrogamescollector.com/zx-spectrum-keyboard-membrane-replacement-how-to/
If you followed the above guide, and you’re still having problems installing a new Plus keyboard to your ZX Spectrum + or ZX Spectrum 128+ (toastrack) case, please check the following hints and solutions.
STOP (Symbol Shift – A) not working on ZX Spectrum 16/48K
Add a 22K resistor on top of R68 (the ‘R68’ print is readable on the board, it’s at the left side of the board).
Make sure there’s no contact with the modulator
If the membrane cable touches the modulator, this will cause issues, even if there is no electrical contact.
So put some (preferable thick) insulating tape to the edge of the modulator, to make sure there is no contact with the membrane.
Check ribbon clamps
When the ribbon clamps aren’t assembled properly, it often causes extended keys not working properly.
‘Extended keys’ are combination keys like Caps Lock, Extended Mode, Graphics Mode, cursor keys and others.
These combination keys are created by creating contact between 3 or 4 (2x 2 pairs) conductive layers on Plus keyboards – rubber-key keyboard membranes only have 2 conductive layers.
Make sure the ribbon clamps are present and screwed in good enough.
Do not overtighten the screws either: that can cause the screw holes to be damaged, after which screws won’t hold.
Also make sure the ribbon cables of the membrane are not pulled too tight, which can cause keys not to work properly and damage the membrane.
Check inner layer ribbon cable length
When a membrane appears to work and then goes bad again some random time after re-assembling everything:
Check that the short tails of the inner layer (between the cables going to the ZX Spectrum board) are actually the right length.
With some* membranes it seems the inner tails have not trimmed to the correct length at the factory.
The inner layer should -just- line up to the centre of the clamps, not more or less!
* Because this only applied to UK membranes so far, it’s important to know this does not seem to apply to ‘Tesla’ branded ones (offered by Sintech and ByteDelight) or the ones from China (offered by ZXRenew and ByteDelight).
Otherwise you might damage membranes that do not have this issue to begin with.
Check the ULA chip
We’ve see some ZX Spectrum 16/48K boards that were not ‘liking’ Plus keyboard membranes.
Rubber-key membranes seemed to work properly, but with Plus membranes random keys (we couldn’t track it down to a specific ‘row’ or ‘column’ in the keyboard) didn’t work.
What we discovered is that this was caused by a specific ULA chip.
Replacing the ULA chip with another solved these issues.
However the ULA that was swapped out may be 100% ok with rubber keyboard membranes, so do not discard that valuable chip!
Capacitor over diode fix (pending testing)
We recently (March 2020) got a hint that soldering capacitors over the keyboard diodes (D1 – D8) may solve issues with ULA chips that are picky to membranes.
We hope to test this solution soon (discussing it at this moment) and add this solution to this webpage.