In search of 4B3A Microswitch Keyswitches (for a system restoration, not keyboarding)

Brent Hilpert bhilpert at
Fri Jul 10 12:55:56 CDT 2020

On 2020-Jul-09, at 10:12 PM, Ian Finder wrote:
>> On Jul 9, 2020, at 21:58, Brent Hilpert <bhilpert at> wrote:
>> On 2020-Jul-09, at 9:39 PM, Ian Finder via cctalk wrote:
>>> I know what you guys are thinking- no, this isn't for a keyboard collection
>>> or some modern build or some other nonsense like that.
>>> I have a friend who is restoring a fairly interesting and historically
>>> significant vintage computer-
>>> The correct SD-series replacement switch would be the 4B3A-
>>> *** These can allegedly be found on some of the Diablo printing
>>> terminals.***
>>> Subject to what /appears/ to be a batch-related encapsulation failure in
>>> the glue on the proprietary hall effect sensors, around a little over half
>>> of the switches on the current keyboard are bad.
>>> It is possible other switches ending in ***A could be made to work with a
>>> bit of labor and disassembly (swapping the fairly brittle sensors).
>>> I am not a keyboard expert but I have learned that you can remove a key on
>>> some of these microswitch keyboards and read the model fairly easily on
>>> each switch.
>>> Please let me know if you have a lead on a donor for these switches. They
>>> will be put to good use, and you can reply to me off-list for more details.
>> Are these the key-switch model which snap into a thin-gauge springy stainless-steel U-channel to form the rows of the keyboard?

> No, as I understand it that is the predecessor

OK, too bad, in my parts stash I have an orphan (caseless) made-by-Microswitch keyboard from 1975, using the key-switch type as I described.

I had a couple of the key-switches open years ago. According to my notes there are two types of key-switch, one for characters (black plunger) and one for modifiers (shift,ctl) (green plunger), the chip IDs (inside the key-switches) are 42B and 40H respectively.

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