In search of 4B3A Microswitch Keyswitches (for a restoration, not a keyboard collector!)

Santo Nucifora santo.nucifora at
Fri Jul 10 15:19:20 CDT 2020

Hi Ian,

What type of computer is your friend restoring?   I can't help as I have a
full keyboard that I'd like to keep intact but this is a home made
terminal/keyboard made from Diablo 1620 parts including the keyboard and I
believe the HyTerm main board.  It has SD switches but they are mostly
4B3Bs.   These are said to be the same hall effect switches as at least one
of the famous Symbolic keyboards so that's likely a dead end.  I do have
the schematic for this board if it's helpful.

Homemade keyboard pics:


On Fri, Jul 10, 2020 at 1:26 AM Ian Finder via cctalk <cctalk at>

> 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- a and subject to what appears to be a
> batch-related encapsulation failure on the proprietry hall effect sensors,
> but could have been some other trauma, around a little over half of them
> are bad.
> The correct SD-series replacement switch would be the 4B3A-
> *** These can allegedly be found on some of the Diablo printing
> terminals.**
> 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.
> 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.
> Thanks,
> - I

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