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Cherry Full-Travel Sealed Contact

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Overview

Cherry Full-Travel Sealed Contact (FTSC) is a type of membrane keyboard assembly using discrete plunger modules. Standalone keyboards with these switches have MY model numbers, leading to the switch technology also being referred to as “Cherry MY”. Most FSTC assemblies are placed into G81, but the oldest examples are G80, from when G80 simply denoted a German keyboard, in contrast with B80 full-size American keyboards (see keyboard codes).

Although “full-travel sealed contact” is a fairly generic description, it was not used for Cherry’s rubber sheet keyboard implementation, which was instead described as “NTK” (New Technology Keyboard), with keyboards being placed into RS (rubber sheet) series with G83 part numbers. Cherry’s The keyboard pros! catalogue from 2006/2007 confusingly describes FTSC as “mechanical individual keys with membrane contact switches”, presumably on the basis that every switch has a dedicated return spring and actuator spring within a discrete removable module.

History

Cherry FTSC history is not well understood. A number of patents have been discovered to date:

Patent Priority Filed Published Assignee Inventors
GB2141874B: Keyboard with membrane switch array 1984-03-29 1984-03-29 1987-04-01 Cherry Electrical Products Ltd
Cherry Electrical Products
Allan Ellson
EP0157035: Mechanical keyboard with membrane switch array 1984-03-29 1984-08-22 1993-01-27 Cherry Electrical Products Ltd Allan Ellson
DE3530050: Tastenmodul für Folientastaturen 1985-08-22 1985-08-22 1987-02-26 Cherry Mikroschalter GmbH Günter Murmann, Walter Mertel
EP0233926: Tastenmodul für Folientastaturen 1985-08-22 1986-07-31 1989-06-07 Cherry-Mikroschalter GmbH Günter Murmann, Walter Mertel
US4800245: Key module for key-actuated membrane switch panels 1985-08-22 1986-07-31 1989-01-24 Cherry-Mikroschalter GmbH Günter Murmann, Walter Mertel

The first two patents were filed by Cherry UK. The design of switch depicted in these does not match any known Cherry MY type, but the production design is not to different from that depicted in the original British patent. Cherry Germany then improved on the design, filing their own patent in 1985 for what is now referred to as Type 1 (specifically Type 1a), with follow-up patents filed in the United States and Europe the year after.

I spoke to my cousin, who worked for Cherry UK — he was responsible for setting up a polyester membrane production line in Harpenden around 1987 or 1988. He did not recall any further details of the keyboards themselves. sixty reported on the Deskthority wiki in 2011, without attribution, that MY was “first introduced in 1987”; this appears to now be corroborated. Nobody yet knows why it took several years to go from the first patent in 1984, to full scale production around 1987 or 1988.

sixty also wrote that MY “is based around a leaf spring on a vinyl membrane”; this was again posted without attribution. Normally membrane keyboards are thought of as using Mylar, which is a polyester film, and the production line in the UK was set up using polyester membranes. The Cherry website in 1998 made no mention of the specific membrane material.

My cousin felt that all membrane production took place in the UK. Another Cherry employee I contacted also feels that all membrane production (for both FTSC and NTK) took place in the UK before, being moved to Hungary, although he too could not confirm that no membrane manufacture took place elsewhere. Note that Cherry also subcontracts keyboard manufacture to the Far East.

Types

Cherry MY is divided into three types, of which one type has two variants. Current data is simply inadequate to get a clear picture of the distinction in date or usage between the three types. By the time that Acorn purchased from Cherry (which was before my cousin left in 1990 or 1991), Type 3 was already available, which can be see in the aforementioned data.

The type designations are not official, and simply serve as a way to refer to the different designs. Nothing is presently known about why the switch was redesigned twice.

Type 1a

Typically black plunger; UncleFan has found that—with the examples discovered to date—modules with upright keystems use black plungers, and those with angled keystems use clear plungers. Per the patent, the stationary tip of the leaf spring is visible in the retaining post. The upright and angled types can be seen together in the TA Gabriele 7007 typewriter; UncleFan has also found photos of a G80-0761 that matches the example posted to Cherry on Instagram (allegedly from 1989), which also appears to be from a Triumph-Adler typewriter.

Type 1 modules are heat-staked to the membrane backplate, meaning that the modules cannot be removed without damaging them, and they cannot be reattached after removal.

Type 1b

This is the typical form of Type 1, with the leaf spring retaining post fully enclosed, as seen also in the later types. These so far have only been seen with 0° keystem black plungers.

Type 2

This is a rare form with clear plungers of a different design. The modules are secured to the membrane backplate by feet that are spread apart by small pegs inserted to splay them out. This allows the modules to be removed and replaced without damage.

Type 3

The vast majority of FSTC keyboards use Type 3, with vaguely cylindrical plungers. As with Type 2, the plungers are clear instead of black, and the modules are safely removable from the assembly.

Parts

Part Type Source
MY1A-21NC Currently associated with the Type 3 actuator Cherry

See also