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Futaba 1st generation

Contents

Overview

“First generation” refers to all of Futaba’s medium-profile and low-profile switches from the 1970s and 1980s introduced before the MA types. As has already been demonstrated with the discovery of a high-profile reed type from SMK, the fact that nobody has knowingly observed an old Futaba type does not mean that they do not exist. “First generation” encompasses both the mechanical and reed types, which are externally very similar and around the same size. While these were originally marketed by Futaba, keyswitch production later moved to Sejin Electron.

Owing to very limited official data from Futaba and Sejin, there is no clear indication on either series naming or part numbering. The part number system may be additive, and it appears that “C” may denote the cruciform keycap mount. “A” indicates alternate action, and in some cases 7 seems to denote illuminated (MD and the later MA). Scrutiny of many service manuals has yielded up only a single part number (ML-3-CM), and one other part number is known from surplus supply (MD-4PCS).

Subfamilies

Based on a single page from an unknown catalogue, the prior understanding of the series has been proven incorrect. In part this was due to ePartsHub and ECPlaza publishing the wrong photograph for MR-6C. The subfamilies appear to be as follows:

Family Method Type Lifetime Load Bounce time
MR Reed Momentary 30 million 24 V DC, 5 mA 1 ms
MD Sealed contact mechanical Momentary 10 million 24 V DC, 1 mA 2 ms initial; 5 ms end of life
Alternate action 100 thousand Unspecified
Unknown Momentary illuminated 5 million 24 V DC, 1 mA 3 ms initial; 5 ms end of life
ML Metal foil mechanical Momentary 3 million 24 V DC, 1 mA 5 ms end of life

MR

MR seems to be the reed switches. While reed versus mechanical is not known to be documented, the specifications of MR-6 and MR-6C (which seemed impossibly high) are a perfect match for those of GRI KBR. The MR-6C photo at ePartsHub is of a larger ML type, which is not documented there. Being reed, they have a higher carrying current of 5 mA over the 1 mA of the mechanical types, and they have a lower bounce time of 1 ms typical of reed switches.

US patent 4041427 filed in 1975 appears to cover these switches.

MD

MD switches use a sealed contact assembly similar to that of Alps “switchplate” types. Datanetics licenced their elastic diaphragm technology to Futaba, and these switches may be based on DC-50. However, patent S50-150878A published in 1975 is more likely to be that type.

Patent JP S59-44725 A filed in 1982 depicts MD, which seems strange. A subset of this range was sold as GRI KBM, and KBM is documented back as far as 1977, albeit inaccurately, and there is no proof that the KBM of 1977 is the same as the KBM of the 1980s.

Standard momentary switches have a lifetime of 10 million and an intial bounce time of under 2 ms. The alternate action version (confusingly listed as “4PA” instead of “MD-4PA”) has a reduced lifetime of only 100 thousand cycles, which is quite normal for alternate action. However, the illuminated type (MD-7) has a lifetime of 5 million and an initial bounce time of 3 ms. This longer bounce time suggests that these have a different contact mechanism inside, calling into question whether all MD types really are sealed contact. The alternate action type has been proven to be sealed contact, from inspection of Televideo keyboards.

The momentary types come in a number of colour arrangements that seem to denote weight; this can be seen clearly in a Hazeltine keyboard. Confusingly only a single weight is documented for MD-4P, which makes it hard to know whether MD-4P is a subseries or the base model. MD-7 is clearly a sub-series, because there are both momentary and alternate action variants that each come in either an incandenscent lamp or LED form. This would mean that MD-7 alternation action is 5 million cycles versus 4PA alternate action at 100 thousand; it seems that not enough care was taken with the documentation.

Known models:

ML

ML switches use fragile metal foil contacts with wiping action. These types cannot be disassembled safely: the movable contact is pressed into the shell and blocks the plunger from coming out, and it tends to tear if you attempt to remove it. Normally, one of the screw-in terminals functions as the stationary contact, against which the foil movable contact is pressed, but rare examples exist with dual foil contacts. The shells of smaller types are also prone to snapping on disassembly too, which compounds the misery considering that they are not particularly reliable.

ML switches are available in standard size with the full 3.1 mm travel, as well as reduced size with 2.5 mm of travel. The 3.1 mm version is the type found in later Acorn Atom keyboards and most BBC Microcomputer keyboards. The 2.5 mm version is more widely known, from Acorn, Atari and Memotech computers in particular.

A single model from the 2.5 mm range was sold as GRI KBM-LP.

Known models:

Sealed linear

Although these are also sealed contact, the term “sealed” in this instance indicates that the base of the switch is sealed in place, and it appears that the switch cannot be opened non-destructively. These switches are four-terminal; the additional two terminals are from an internal jumper that doubles as extra securing points to hold the switch. A step in the shell allows them to be held down by a metal plate in the same manner as covered Cherry M8 switches, even though the switches cannot be snapped into the plate. No part numbers are known for these, and they are fairly uncommon; examples include:

Internally they are almost the same as MD types, including the two-part plunger with a minuscule elastic band around the connecting peg. The jumper has a slot through the centre to fit over the spring nub. Considering that the series names seem to indicate the contact mechanism, these may also be MD types. The base is very thick, and secured so tightly that it will snap before it will come out. The following photo is of a standard switch from an AIM-65 keyboard; part of the base has been sliced away in order to get enough leverage to force out the base. As noted, the base snapped when trying to prise it out of the switch. The orientation of the base in the photograph is 90° out.

The ivory/ivory/cream type above is fairly heavy, possibly 85 cN actuation, and is distinctly heavier than MD-4PCS, SMK J-M0404 standard and Alps SKCC cream and green. The ivory/ivory/orange type found under space bar is heavier still.