MEI T-5 series
- Part number schema
- See also
T-5 or T5 is a series of mechanical keyboard switches from Mechanical Enterprises. Unlike most MEI keyboard switch series, T-5 appears not to have been patented, so there is no suggested date of introduction. It was available by 1977, and remains in production to this day, although very few types remain available.
T-5 switches are comparatively tall; the shell height is 18.2 mm including the stand-offs, which is fractionally shorter than Cherry M6.
Part number schema
According to the catalogue excerpt, the part number schema is as follows:
- T-5 series (also written as “T5”)
- Sub-series (e.g. C, D, H, J etc; see table below for known subseries)
- Action: M = momentary; A = alternate
- Contact arrangement: NO = SPST NO, OO = DPST NO, OC = SPST NO + SPST NC
- The exact meaning is not given, but it can be treated as denoting “heavy spring” (single pole momentary action types only)
Sadly, I have never laid my hands on a complete catalogue of T-5 series. The series is divided up into sub-series based on design; the following sub-series names have been found to date:
|T-5C||Standard tall stem non-illuminated|
|T-5D||Unknown (absent from catalogue excerpt)|
|T-5H||Unknown (absent from catalogue excerpt)|
|T-5J||Rubber-sealed version of T-5K (¾″ spacing)|
|T-5K||⅝″-spacing illuminated (inside-fit keycap lens)|
|T-5L||¾″-spacing illuminated (outside-fit keycap lens)|
|T-5S||Unknown (absent from catalogue excerpt)|
|T-5V||Unknown illuminated (absent from catalogue excerpt)|
|T-5Y||2-unit version of T-5L, with strut-stabilised box lens keycap|
My own illuminated type does not match any of these drawings sufficiently, and appears to be another variant still. (It appears to be a modification of T-5L.)
Subseries T-5V (written in the alternative form as T5-V) is taken from the description of a set of keycaps made by Honeywell (CAGE code 27914) all described as “Fits with mechanical enterprises T5-V key switch”. NSN 7025-01-461-8550 is described as “Special Features: Lighted keytop with blue skirt and white”, while other items in this series (48 NSNs, including 5930-01-421-4908 to -17, -20 to -28 and -31 to -48) are all described as “Special Features: Blue skirt with white top, fits mechanical enterprises key switch T5-V”).
The series has a number of unexplained variations. Most prominent is the use of the term “WEAB”, which can occur on any of the top, bottom or plunger. Some switches are branded only “MEI”, some are branded both “MEI” and “WEAB”, and some are branded solely “WEAB”. The term “WEAB” has so far not been found to correspond to any particular characteristic, be that terminal plating/tinning, plunger pigmentation or keystem. Datalux themselves do not recognise the term, nor do former importers Devlin.
Plungers can be either black or dark grey; the dark grey pigmentation appears to be metallic based on the swirls and the way it reflects light, but it is non-conductive (thus seemingly not related to electrostatic discharge protection). The grey colour does not directly correspond to any other characteristic, be that keystem, weight or “WEAB” marking.
Datalux confirmed that current production models use gold-plated terminals; the contact wire is also gold-plated. However, some variants instead have only one gold-plated terminal, specifically the terminal used as the stationary contact. In these variants, the other terminal has a grey finish that is too dark and dull to be silver plating (as used in some Cherry M9 variants); it is possible that instead the terminal is tinned. The grey colour continues all the way inside, where it is distinctly matt finish. Having both terminals gold plated does prevent oxide build-up between the contact wire and its terminal.
A red plunger stemless switch also exists. The terminals suggest T-5, but Mercutron M-5 cannot be conclusively ruled out.
Very view part details have been discovered. A number of discovered types presently have no known part number.
The following table shows all recovered part numbers, either from their NATO Stock Numbers or from the catalogue excerpt retained by RS that was annotated with their own part numbers. Thus far, no service manuals have been obtained with part numbers in.
As of March 2019, the following types are still available; for a small one-off order, they are $10.50 each, and $4.95 each for an unspecified larger quantity.
Alternate action and DPST NO+NC are no longer produced. Heavy spring (-HS) versions may be available.
The following table lists directly observed examples (rather than catalogue listings), and their specific characteristics.
|Variant||Part||Branding||Plunger colour||Terminal colours||Keystem||Known from|
|Momentary non-illuminated||T-5C-M-NO||—||MEI||—||Black||Gold||Standard tall||Sold by RS via Devlin|
|Momentary non-illuminated heavy||T-5C-M-NO-HS|
|Momentary non-illuminated||?||MEI||MEI||WEAB||Dark grey||Gold and silver||Standard tall||UK101; unknown keypad|
|Momentary non-illuminated||?||WEAB||?||WEAB||Dark grey||?||Standard tall||Digicomp DR-70|
|Momentary illuminated||?||—||WEAB||—||Dark grey||Gold and silver||Standard tall||Sold by STRONIC|
|Momentary non-illuminated||?||WEAB||WEAB||—||Dark grey||Gold||Custom||Scanvægt 810422|
|Momentary non-illuminated||?||WEAB||MEI||—||Black||Gold||Short angled||Volker Craig|
|Alternate action non-illuminated||?||—||Gold and silver|
- Don Lancaster’s 1976 book TV Typewriter Cookbook (fourth printing 1977) mentions “T5” on page 137 giving us a low water mark date for the switches.
- T-5 entry, Deskthority wiki (all my prior notes)