SMK J-M0404 series
|This article requires additional photographic illustration — need low-friction variant, and tall stem vs long stem|
|Sense method||Metal leaf|
|Total travel||3.0±0.4 mm|
|Switch mount||Plate mount|
The base is branded "SMK", typically with the old SMK logo; there is no top branding. The switch was still in use in 1989, based on the PCB date of an Apple M0110A (Macintosh Plus) keyboard; in this example, the newer SMK logo is present.
The contact system is fairly simple. A metal "crossbar" is held in place by a groove in the upper shell, and a vertical leaf spring rests against it. When the slider is inserted, a ramp at the bottom holds the leaf spring away from the crossbar. As the slider is depressed, the leaf spring moves back against the crossbar and closes the circuit. The contacts are gold-plated. The contact system is essentially the same as that used by Omron B3G series.
"Operation Force" is cited as both "90 grams ± 20%" and "90 ± 30 g" in the same brochure, even though 20% of 90 is 18.
The series covers both momentary and alternate action switches. All switches found to date are SPST. There is technically no reason why the design should not support DPST, although the shell is specifically designed to hold only one pair of contacts. Straight stem sliders are however symmetrical and could operate two pairs of contacts simultaneously.
The low-friction variants use a blue-white viscous substance painted onto either side of the slider. These switches use brown sliders, and are intended for wider keys. The objective of the lubricant appears to be to reduce binding when hitting wider keys off-centre.
The "low friction" designation comes from Apple service documentation.
Low force switches use weaker springs. These switches are indicated by a cream or beige base. One use of these is for the return key in some Acorn BBC Microcomputer keyboards, where instead of a single switch and a stabiliser, a pair of low force switches were used. Currently no photographs are known to exist of the various SMK switch arrangements in the BBC Micro, which differed between batches.
The angled keystem low force switch has been found with both beige and ivory bases. The beige base version examined has a 16.5 mm long spring with a wire gauge somewhere between 0.18–0.20 mm. The ivory base version has a shorter, 15.6 mm long spring and 0.19 mm gauge wire. There is no readily discernible difference between the two switches and it is unclear whether the use of two separate colours is significant or whether SMK were not fussy about the shade.
Both sprung and unsprung dummy switches were used.
The keystem can be either short or tall. Short keystems can be upright (straight) or angled. Tall keystems are rare and so far only known from Apple keyboards; tall angled keystems have yet to be seen. Possibly to allow the slider to be inserted into the shell, the "inner" vane of angled keystems is tapered away at the top.
The table below shows all discovered variants. There are far more possible combinations than have been discovered. For example, tall stem angled has never been seen, and the tall stem switches have not been found in the same number of varieties as short stem switches.
|Short stem, upright|
|Tall stem, upright|
|Short stem, low friction|
|Tall stem, low friction|
|Short stem, angled|
|Short stem, low-friction, angled|
|Short stem, low-friction, angled, low force|
|Short stem, low-friction, low force, angled, pale version|
|Short stem, low-friction, low force, angled dummy|
|Short stem, low-friction springless dummy|
|Tall stem, upright, latching|
|Short stem, upright, latching|
|Short stem, upright, illuminated|
SMK-branded parts in this series appear on a number of websites, typically with bogus stock figures (as high as 65,000 per part), but attempts to determine any correspondence with known switches has proved futile. The following parts are confirmed (with corrections for transcription errors such as "(SHARP)" or "≠" for "#"):
|J-M0404#05||PCB Single Keys[witch]||ISO Parts|
|J-M0404#08||KEY SW MECH BTTM SEALED 5AMP||ICLocator|
|J-M0404#10||PCB Dummy Keysw[itch]||ISO Parts|
No Abtronics branches in any country ever respond to enquiries, and they have a high minimum order value; ISO Parts and Jotrin list bogus stock of nonexistent parts. Obtaining these parts may not be possible. Only J-M0404#10 may be identifiable, as it likely refers to the dummy switch sometimes found under space bars (e.g. some Type 2 Acorn BBC Microcomputer keyboards, and the SMK J-M 9031 keyboard).
Abtronics also lists J-M0409#01 with description "REF.042R" and no manufacturer; it is conceivable that the angled stem or tall stem versions were covered under a related series, but J-M0409 series may not exist. Retroamplis sell part JM-0404, intended for Boss pedals, which is a standard short straight stem switch. This apparently incorrect part number is derived from Boss, as noted in the Boss PS-2 Schematic (under "SWITCHES", as JM-0404 "Key Switch").
Switches similar to or identical to the design of the SMK vintage linear have been found from numerous brands, recognisable and unrecognisable. In some cases, these switches may have been made under licence, and it remains possible (albeit unlikely) that the switch was not originally designed by SMK. While SMK do have a patent for the SMK second generation series, no patents for the vintage linear switches from any companies have been found.
Some of the derivatives do appear to be clones.
The following are confirmed to have SMK branding on the PCB or switches:
- Some Acorn BBC Microcomputers (short upright and angled stem; low friction; low force; dummy switch)
- Some Apple M0110A variants (tall upright stem)
- Kaypro II
- Kaypro "New" 2
- Zentec Zephyr terminal (black cross mount)
The following have switches that resemble SMK vintage linear, but this is yet to be confirmed:
- Selcom/Jen Lemon II
Tall stem, upright
In situ, top view (Apple M0110A, ca. 1990)
- KBtalKing — ■鍵盤史的遺跡Apple M0110A (SMK軸)■ (Chinese) Dated 2010-11-18. Retrieved 2015-08-02.
- Internet Archive — Apple Documentation: DTCA2DOC-223 repair IIe 1989
- Deskthority — Vintage find: 2 NOS Maxi-Switch Kaypro replacement keyboards Posted 2013-04-11. Retrieved 2015-08-02.
- Bitsavers — Index of /pdf/zentec/Zentec_Zephyr_00-441-01/pictures
- Nightfall Blog — Selcom/Jen Lemon II (page 3) Dated 2014-03-05. Retrieved 2015-08-02.