MEI T-5 series
T-5 or T5 is a series of mechanical keyboard switches from Mechanical Enterprises. This series is notable for being one that was seemingly not patented, so we have no suggested date of introduction. It was available by 1977, and remains in production to this day, although very few types remain available.
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||⅝ʺ-spacing illuminated (inside-fit keycap lens)|
|T-5L||¾ʺ-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 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.
- 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)