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Old 24th Apr 2018, 10:24 pm   #25
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: MK14 schematic revisions

Ah, unfortunately they were bought as part of a full QWERTY legended set for a couple of pounds at a radio rally (an amateur radio junk fair) in the 1990s, so they are not a repeatable purchase. I wish I had bought more sets at the price. They have a lovely light action, are proper switches and are designed to fit snugly, side by side, into a 0.1" matrix grid (like veroboard or similar).

If you look at original dome switch keypads like those on a cheap late 1970s calculator, the areas where the three 'feet' of the domes rest are quite broad / wide, smooth and heavily plated without any depressions or holes in them. The tape alone keeps the domes centred where they are supposed to be.

I think this is because when you press the centre of the dome it flattens and the three feet skid outwards a little bit each time. If that also happens on your keypad the relatively small contact areas surrounding the three holes will tend to suffer chew damage / wear and tear.

I have the keypad for a late 70s Texet 880 calculator which, sod's law, I can't find just now, otherwise I'd take a photo to show you how dome switch PCB pads typically looked back then. I think I have another at work, I'll try to remember to bring it home.

In any case, if you can fit actual switches (even tact switches) rather than PCB mounted dome switches, I would definitely do that. I would have been very grateful for that option back in the day, but S.O.C didn't include the option to fit proper switches until around issue IV or V of the MK14. The fact that they eventually did means it's perfectly OK / perfectly authentic for your replica to have the option to fit proper switches as well.

I have (hoarded, naturally) a set of 20 grey keycaps with a large square hollow in the base of each cap which would very likely fit those cherry switches if they have a square topped (rather than cruciform topped) actuator: I just don't have the switches to put them on.

Mine are all blank, but until quite recently (sometime in the last ten years or so) RS sold sets of 20 of the same or similar keycaps legended with 0-9, A-F plus four blanks, ideal for an MK14 keypad. I only found that out about a year after they were discontinued.
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