Eric L Long (momentary)|
J Habecker (alternate action)
|Sense method||Metal leaf|
|Switch mount||Plate mount|
US 3715545 (1971)|
US 3770923 (1972)
Serie M7 is a series of tall keyboard switches from Cherry.
- 1 Nomenclature
- 2 History
- 3 Part numbers
- 4 Description
- 5 Design
- 6 Keycap mounts
- 7 Hack value
- 8 Variants
- 9 Keyboards
- 10 Specifications
- 11 References
Being Cherry's first dedicated keyboard switch modules, the series originally had no name. Cherry described them as the 'Gold "Crosspoint" Contact Switch' or 'Key Module', but the descriptions varied. Part numbers began "M" (perhaps for "keymodule") and were divided up into subtypes based on characteristics, including M4, M5 and M6, and seemingly also M7. At some point in the late 70s, Cherry found cause to introduce additional keyswitch types, and the original switches were regrouped under M7 (perhaps as this was the highest subtype in use so far), gaining a series name of "Serie M7".
All Serie M7 switches have part numbers beginning M7, but for the purposes of classification, M4, M5 and M6 parts are being considered "M7" simply because the series had no concise name up until additional switches were introduced.
Cherry catalogues C-73 and C-74 describe the design as having "provided highly reliable keyboard switching for several years". Catalogue KB79-2 (assumed to be from 1979) notes that the design has "provided highly reliable keyboard switching for nearly 10 years". This gives an introduction date of 1969 or 1970. US patent 3715545, filed on the 18th of June 1971 describes the momentary switch, and US patent 3770923 filed on the 14th of January 1972 describes the alternate action version.
By the Keyboards and Switches Catalogue 1982, the various part number prefixes (M41 to M71 in catalogue KB79-2R) had been consolidated under Serie M7, with all part numbers now beginning M7. Where the second digit had largely been '1' (M41 to M71), instead the first digit was held at 7, and the second digit now ranged from 1 to 8 (excluding 2). At least one trend remained, that of alternate action switches having 8 as the fourth digit (e.g. M73-0800, M76-0890). Grouping the series at M7 meant that "M" was now a generic prefix, used not just for M7 but also the then newly-introduced Cherry M8 and Cherry M9 switches (along with the seemingly undocumented Cherry M10 and Cherry M11).
The switch design went through at least two generations of external design, and the internal mechanism also saw at least one change in design.
US patent 3849621 from 1974 depicts what could be an illuminated version; the design differs from the illuminated types found in the catalogues however.
- M4n-nnnn: lighted
- M5n-nnnn: space bar
- M6n-nnnn: other keys including the shift–shift lock mechanism
Once the series was consolidated under M7, it becomes a little less clear:
- M71-nnnn: lighted
- M73/M74-nnnn: typical keys
- M75/M76/M77/M78: a complex design that allows up to four poles and/or illumination
Standard M61 keys moved to M73 during this process; the distinction between M73 and M74 is not yet clear, nor of the distinctions within M75, M76, M77 and M78: these two groupings each share one brief set of diagrams within the 1982 Cherry catalogue.
The M51-0131 switch depicted below appears to be clear exception to this, as it should fall within M61. However, M51-0182 is another non-space bar M51 switch. Study of Cherry part numbers is still ongoing.
Part number equivalence
The following are confirmed part number replacements:
|Old part number||New part number||Type|
|M61-0120||M73-0120||Momentary, 70 cN, tee mount|
|M51-0182||M73-0110||Momentary, 70 cN, 10°, bar mount|
The switch has many design variations, including different keycap mounts (tee, bar, and cross), upright and angled stem, multiple contact types, latching action, integrated illumination, and various pole/throw combinations.
Travel is 4±0.5 mm, with actuation at 2.4±0.8 mm. Actuation force for a basic switch is given as 71±15 cN in a Cherry specification (based on US to metric conversion), and 70±15 cN in a catalogue. Other variants of the switch used different actuation forces.
The switch shell is sealed during manufacture; the specific process used is unknown. The switch cannot be opened up without damaging it: in some cases, the base can be pulled clean off the switch, but otherwise, it must be sliced apart with a knife. Reassembly of an opened switch involves gluing it back together. The contemporary Futaba complicated linear switch sealed only the contact mechanism, but the switch required significantly more complexity to achieve this.
The switch is only suitable for plate mounting.
A single pole single throw (SPST) switch consists of 6 parts:
Small nubs on the base of the slider hold the return spring onto the slider; the spring can be pulled clear of these easily, and returned onto these nubs using tweezers.
Double pole single throw (DPST) switches feature a second set of contacts on the opposite side of the switch.
"Wings" on either side of the slider hold the contact leaf away from the static contact. When the slider is depressed, the leaf spring is able to make contact with the static contact plate. SPST switches typically have symmetrical wings designed to operate two sets of contacts, but they can also be found with asymmetrical sliders that operate only a single pair of contacts.
The design of the switch changed a number of times throughout its lifetime. Some of the differences are shown below; it is assumed that the right-hand switch shown is the newer design.
The design can broadly be categorised into four styles:
- Style A
- Cherry branding on the near side of the slider (along the long side)
- Style B
- Cherry branding on the far side of the slider (along the long side), discrete retention prongs, and a "wall" around the slider set off by a 1 mm gap
- Style C
- Per M51-0131, with terminal numbers on the top, and the Cherry branding along the short side
- Style D
- Per Sandy's Sagem keyboard, with graphical Cherry branding
Tee mount appears to be most common mount. In catalogues, it is described as "T".
The bar mount is a subset of the tee mount, and keycaps should be fully compatible between the two mounts. In catalogues, it is described as "straight" or "–".
Cross mount is not common, and it does not appear in catalogues. The mount used on M51-0131 switches destined for Burroughs has different dimensions to tee or bar mount, likely requiring different keycaps. The M51-0131 mount is similar to MX but a bit different. It is not symmetric in both directions. One 'line' of the + is a bit longer and a bit thicker. However, the difference is so small, that it is possible to fit keycaps which where designed for MX switches. After the keycaps had been put on, the plastic around the switch mount of the keycaps appears to be a bit white around the longer and wider line of the cross.
It is possible to drill a hole into the switch from bottom to top at the opposite side of the leaves, without destroying the switch. If precisely done, this could be used to mount the switch without the need of a separate plate.
The depicted switches were sourced from Electronic Surplus (SKU 2074) with no information about their age or origin known. Based on their characteristics of standard weight, 1 form A momentary with a bar mount, this would make them M61-0100. If they were produced in the 1980s onwards, the part number would instead have been M73-0100.
This appears to be the standard keyboard switch. According to the service manual, it was the primary switch in the ADDS CONSUL 980. The switch depicted below is from a CDC IST II ca. 1981. Standard weight, tee mount, 0° stem corresponds to part M61-0120 (1979) and M73-0120 (1982) so the depicted switch could be either.
Notable design characteristics include:
- "CHERRY" legend inverted compared to M61-0100 above
- The slider is only capable of operating a single pair of contacts (SPST instead of the typical DPST-ready design)
- Distinct plate retention clips instead of raised areas of plastic
- The ridge around the slider aperture is set off by a clear gap
Also of note is the use of gold-coloured and copper-coloured materials for the stationary and movable contacts respectively, instead of the typical grey metals.
Defaced tee mount
The following switch is from an unidentified batch sold by RS; the switches have the RS logo printed onto the side, and the Cherry branding poorly machined off. Compared to all other examples discovered to date, the chief difference is holes cut in the base of the switch. They are otherwise virtually identical to normal Cherry switches from around 1981.
The following is a comparison between the bar mount switch depicted above, and the RS defaced switch (right):
The depicted switches appear to be from a batch of M51-0131 switches manufactured by Cherry for Burroughs. However, the corresponding Cherry design drawing for M51-0129/0131 shows tee mount. The use of M51 instead of M61 for a standard weight momentary switch is a mystery. Differences between the examples in the 1973 and 1974 catalogues and the 1979 catalogue suggests that the numbering scheme may have changed more than once. For example, 0°/10° shift and shift lock in 1973 and 1974 were M61‐0300/310 and M61‐0500/510 respectively, but M61-0025/0026 and M61-0027/0028 in 1979.
Note that the top of the switch is marked with the numerals 1 to 4, possibly corresponding to the four terminal positions (of which only two are in use here).
The box of switches manufactured for Burroughs; these switches are reported to be identical to the switches depicted above:
The following switch is from Sandy's Sagem keyboard, of unknown year.
Stem comparison with Cherry MX Brown
- ADDS CONSUL 980 (M41-0100, lighted; M51-0106, space bar; M61-0120, tee mount; M61-0300, shift; M61-0500, shift lock; M61-0800, lighted alternate action; per computer service manual findings)
- Blaupunkt CEPT-Btx-C1 Keyboard (style C, tee mount, grey slider)
- CDC IST II (style B, presumed to be M61-0120)
- Cherry B 65-1712 (style C, tee mount, black slider)
- HP 9830A (style A, bar mount, 1976)
- Sagem keyboard (style D)
- HAL KB-2100 (M61-0120 tee mount)