Spectrum+ Keyboard Membrane

The Sinclair Spectrum+ has a membrane keyboard, and the electrical contacts are on a pair of flex circuits, which are sandwiched together. Unfortunately, the integrated ribbon cable section of this circuit passes over the voltage regulator heatsink, and over time, the heat can degrade the plastic making it brittle and crack. When that happens, the electrical connections will break and the keyboard no will longer work.

The rubber membrane pushes the plastic key tops back up. It is also responsible for producing the correct amount of pressure on the electrical contacts.

Cleaning oils and lubricants may cause damage and melt the rubber plastic.

After cleaning with water, the rubber membrane layer returns in its place.

The membrane layer has these "wells" for each key. The dimple at the centre presses down on the electrical contacts.

The flex circuit is the next layer to return in its place.

The ribbon cable section of the flex circuit goes between these neat plastic pillars and a plastic strip screws over it to clamp the ribbon cable. Obviously, do not over tighten this, as it will damage the thin ribbon cable, which may have become brittle after 30 years.

A rigid plastic panel goes over the whole assembly. The purpose of this panel is to ensure uniform pressure over the whole keyboard membrane assembly.

Here is the plastic strip to clamp the ribbon cable. Under the ribbon cable, there is a thin layer of rubber to help secure it.

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