Futaba ML series
|Family||Futaba linear switch|
|Introduced||1983 or earlier|
|Sense method||Metal leaf|
|Pretravel||0.045±0.01″ (1.14±0.25 mm)|
|Total travel||0.1±0.02″ (2.54±0.5 mm)|
|Keycap mount||Custom cruciform|
|Switch mount||Plate mount|
Futaba ML series (formerly Futaba low-profile linear) is a provisional name for a lower profile version of the Futaba linear switch family. The switch remains plate mount, but the height of the switch shell is lower than that of a normal Futaba linear switch, allowing for a shallower keyboard. These switches were introduced by 1983, as they were used in the Acorn Electron and some Atari 800XL computers, both introduced at that time.
Futaba ML series switches share the same keycap mount as the larger Futaba linear switches (MD and MR series).
This switch has been found as both ML-3-CM in the Memotech MTX 512 technical manual, and listed online as ML88. As the common portion of the codes, "ML" is being taken to be the series name until such time that documentation surfaces.
Futaba ML switches are a development of the Futaba MR-6C series. The switch has the same keycap mount as its larger predecessors, but the shell is reduced in size. Travel is also reduced from around 3.5 mm, to around 2.5 mm (the GRI KBM-LP specification cites 0.01±0.02″ but they appear to mean 0.1±0.02″ as that comes to 2.0–3.1 mm, while travel was measured as around 2.7 mm).
Where the MR series switches have two overlaid contact leaves, ML series switches have only one. The small black rubber disc inside the slider is also omitted. As such, the number of parts per switch is reduced from 9 to 7.
Experience with the BBC Master suggests that these switches may also have reduced durability; this supposition seems to be corroborated by the 3 million cycle lifetime given in the GRI KBM-LP specification.
|Characteristic||GRI KBM-LP||Sejin ML88|
|Operating force||3.5±0.8 oz (99±22 g)||100±25 gf|
|Travel||0.1±0.02″ (2.54±0.5 mm)||2.5±0.5 mm|
|Pretravel||0.045±0.01″ (1.14±0.25 mm)||≥ 1.0 mm|
|Overtravel||≥ 0.5 mm|
|Bounce time||5 ms|
|Contact resistance||Initial, 250 mΩ; 5 Ω max||Initial, 100 mΩ; 5 Ω at end of life|
|Specification||GRI Bulletin S-20||FS-5110L5|
Until around October, 2014, George Risk (GRI) were selling this switch as the KBM-LP.  It is not known whether these were old stock, or whether the switches are still manufactured. GRI KBM-LP switches were not branded at the time.
This series is not well understood. No measurements have been taken to determine whether the switches found in various home computers are of the same specification. The service manuals for the Memotech MTX 500/512 service manuals cite the switch part number as "ML-3-CM". This is similar to the format of MD-4PCS, a part number for complicated linear.
By comparison, Atari 800XL and CX85 service documentation only lists the keyboard as a whole unit.
The two legs are threaded, as with the larger Futaba linear switches. To open the switch, first unscrew and remove the two legs. Then, gently insert a screwdriver or one leg of a pair of tweezers inside the retention recess and lever away the retention clip.
The contact leaf should detach cleanly from its mounting lugs when the slider is depressed; unlike the simplified linear, the leaf comes off fairly easily and without becoming mangled or torn. Removal of the leaf should not be attempted without good reason as it remains a risky operation.
When reassembling the switch, the legs should first be placed inside the base. Then, insert the base very slowly, making sure that the legs are passing down the correct side of the contact leaf prongs; failure to do so may result in a crushed contact leaf that will need straightening, although the reason for the large static prong is unknown.
- Acorn Electron
- Earlier Acorn BBC Master Series Microcomputer machines (not the Compact)
- Some Atari 800XL computers
- Memotech MTX 500/512 (part ML-3-CM)
Care should be taken when reassembling the switch to ensure that the shell doesn't snag on the leaf and buckle it. The leaf can however be straightened out successfully.
- GRI KBM-LP specification (erroneous)