MEI T-5 Series

Contents

• Overview
• Specifications
• Part number schema
• Subseries
• Variations
• Parts
  • Confirmed parts
  • Observed variants
• Occurrences
• See also
• Documentation

Overview

T-5 is a series of mechanical keyboard switches from Mechanical Enterprises. Unlike most Mechanical Enterprises (MEI) keyboard switch series, T-5 appears not to have been patented, so there is no suggested date of introduction. The earliest known mention of the series (as “T5”) is in Don Lancaster’s 1976 book TV Typewriter Cookbook (fourth printing 1977). An MEI advertisement in the June 1976 issue of Computer Design depicts the internal construction of a switch that appears to be T-5 (the accompanying photograph seems to be a different series, possibly LFW). The earliest complete advertisement showing both the series name and illustrations of the switches is from 1978.

The switches remained in production at Datalux (the successor to MEI). An American manufacturer reported in July 2021 that (following considerable difficulty in communication) Datalux had agreed to a “last time buy” of T-5 switches, after which it seems that T-5 will be discontinued.

T-5 switches are comparatively tall; the shell height is 18.2 mm including the stand-offs, which is fractionally shorter than Cherry M6. Surprisingly for such a large switch introduced well into the 1970s, T-5 switches are PCB-mount only: there are no provisions for fitting them to a mounting plate.

Specifications

Part number schema

According to the catalogue excerpt, the part number schema is as follows:

T-5S-A-CC(-HS)

“T-5”

T-5 series (also written as “T5”)

S

Sub-series (e.g. C, D, H, J etc; see table below for known subseries)

A

Action: M = momentary; A = alternate

CC

Contact arrangement: NO = SPST NO, OO = DPST NO, OC = SPST NO + SPST NC

“HS”

The exact meaning is not given, but it can be treated as denoting “heavy spring” as this suffix indicates the use of a 5 oz return spring (single pole momentary action types only)

Subseries

T-5 series is divided up into subseries based on design. Each catalogue or advertisement discovered to date contains details of a small portion of the series, listing only a small number of the different subseries. Some subseries are known only from NSN records. The subseries found to date are listed below. The general descriptions below are unofficial, while the descriptions marked “1982” come from the 1982 advertisement in Electronic Design magazine.

The illuminated type sold by STRONIC 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”).

Subseries T-5L can be used with either a box lens (cuboid keycap) or a truncated lens (standard-shaped keycap). In 1982 it was advertised only with a box lens, but in 1985 it was advertised as being available in either configuration, just as it was in the catalogue.

The descriptions of T-5B and T-5C are the same, but T-5B is depicted with a considerably shorter keycap, somewhere around half the height of the full-size keycaps. This would suggest that T-5B has a shorter keystem.

Variations

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 their production models used 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.

T-5 switches sold by RS have black plungers and both terminals gold plated; this can be seen for example in a now out-of-stock listing at Silicon-Ark for T5L-M-NO (RS part 336-191).

A red plunger stemless switch also exists. The terminal plating colours suggest T-5 (photos from Deepak Kandepet):

This example is assumed to be a T-5P variant.

Parts

Very view part details have been discovered. A number of discovered types presently have no known part number.

Confirmed parts

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.

By March 2019, only the following types remained available; for a small one-off order, they were $10.50 each, and $4.95 each for an unspecified larger quantity.

• T-5C-M-NO
• T-5D-M-OO
• T-5K-M-NO
• T-5L-M-NO

Alternate action and DPST NO+NC were no longer produced by this time.

Observed variants

The following table lists directly observed examples (rather than catalogue listings), and their specific characteristics.

Occurrences

• 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.

See also

• T-5 entry, Deskthority wiki (prior notes)

Documentation

All literature scanned by Bitsavers unless otherwise noted.

• Keyboard switches advertisement, Computer Design, June 1976, page 149
• T-5J advertisement, Computer Design, July 1978, page 177
• T-5 advertisement, Electronic Design 1982, collected by Marcin Wichary; depicts T-5B, T-5C, T-5J, T-5K, T-5L, T-5Q and T-5S
• T-5 Series Lighted Keyswitches advertisement, Electronic Engineers Master 1985–86 Volume B; depicts T-5J, T-5K, T-5L and T-5Y
• T-5 Series Lighted Keyswitches advertisement, Electronic Engineers Master 1989–90 Volume B; omits T-5Y, briefly showing instead T-5B, T-5C, T-5H, T-5P and T-5Q (the same advert was also run in the 1988–1989 issue but this scan is clearer)
• Series T-5 catalogue excerpt, covering only T-5C and T-5L (no date), obtained from RS in 2016