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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 23rd Apr 2018, 12:15 am   #21
SiriusHardware
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Default Re: MK14 schematic revisions

I have to admit that sounds fantastically reasonable. Last time I looked at the possibility of having a PCB made in low quantities the cost was easily up into three figures.

This CAD package you've used to produce your project, is that freeware or commercial software, and for what OS(es) is it available?

Unfortunately I can't easily measure the key spacing on my MK14 as the replacement keypad is mounted millimetres above the main PCB and the wiring from it is tack-soldered onto appropriate points on the original keypad tracks and all hidden carefully underneath the keypad - which makes it difficult to remove the keypad far enough to perform measurements. (See attached image #1 - note the fairly haphazard alignment of some of the key legends, which were laid down using rub-down transfers and sealed in place with clear varnish).

I did this sometime early on in the life of the MK14. The keypad is not built on a PCB, but on ordinary stripboard / veroboard. To disguise that and make it look a bit nicer I placed a sheet of black modeller's 'plasticard' over the top of it and cut it to the same size before drilling holes to allow the pins of the switches to pass through it.

For the (as yet incomplete) Karen-O PIC14 emulator I used the same method of lettering the keytops but made a slightly better job of it (image #2), as I'm now older and rather more more patient.

As it happens the keyswitches I found for that emulator are nicer, with a better action than those currently fitted on the MK14 so I may consider swapping the keyswitches over to the keypad on the actual MK14 at some point.
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Old Yesterday, 3:10 pm   #22
Slothie
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Default Re: MK14 schematic revisions

In the last few years prices for PCB manufacture have dropped hugely as China has become more accessible to the "hobbyist".
The CAD software I am using is KICAD, its a bit quirky and has a few irritating but not show stopping bugs, but its unrestricted and its used and developed by CERN so it has reasonable support. It is supported on Linux (which I use), Windows and MacOS. I used to use Eagle CAD but its 80mm x 50mm board size limit for the free/cheap versions made it unusable for any substantial circuit with 40 pin DIPs! To get bigger boards you needed to buy a 1000+ licence which was a non-starter. All I can say is that having climbed the steep learning curve and the quirky interface its been worth it. There are possibly better free or cheap options but I didn't have time to evaluate too many
Thanks for even thinking of measuring the keyboard! I'm constantly conflicted between making an exact replica and a functional one. I suppose I'm trying to recreate the "look and feel" with perhaps period appropriate modifications to make it more usable (keyboard being the obvious example!) To be honest I'm hugely impressed with Karen Orton's PIC14 and it would have been good enough to satisfy my nostalgia until I stumbled across a SC/MP on eBay and this whole crazy project started... I love your PIC14, looks like you'll be able to take it on the bus or train to while away those boring trips!

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Old Yesterday, 5:49 pm   #23
SiriusHardware
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Default Re: MK14 schematic revisions

Thanks for the info about KICAD. I run a mixture of Windows and Linux here so it's good to have the choice.

Actually the reason I asked is that I have for years been interested in making an MK14 -keypad- PCB, as the task of wiring up a bit of veroboard full of switches is incredibly tedious, especially given the weird nonstandard keypad matrix adopted by S.O.C. It would make sense for it to be laid out for at least two alternative types of switches, maybe cheap 'tact' switches and more luxurious full-travel switches - if it is even still possible to buy those now, and at sensible prices.

As you've probably realised keypads are the bane of the original MK14, with many original MK14s having faulty or missing original keypads. I would like to be able to make an easily replicable version of the 'floating' keypad I have on mine, with 12 blind pads on the underside at the right hand edge - these could either be taken directly to the PCB via descending wires underneath the keypad for a neater look as they are on mine, or alternatively they could be carried out to the right hand edge of the PCB via a 12-way 0.1" spaced ribbon cable going to a 12 way edge connector for a 'fast fit' solution, in which case you'd just fit the keypad with four bolts and spacers, plug the edge connector in on the right hand edge and <Bam> you have a working MK14 keypad again.

As brilliant as Karen's PIC14 absolutely is, I'm not sure that it has onboard nonvolatile program storage, nor have I measured the current drain - probably a bit too heavy for a PP3 (9V) battery - so I'm not sure that it would be truly practical for portable use.

I'm informed (although I have not seen it) that there is a very faithful MK14 emulator for Android - that might be more practical for the commute.
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Old Yesterday, 6:50 pm   #24
Slothie
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Default Re: MK14 schematic revisions

I've seen "Cherry" MX keyswitches for sale for ~$0.95 each, but as pricey as that is they don't include a keycap and those seem hard to find....

For my board I created a footprint for 12mm square tact switches (because I have some) and 9mm domes (ditto) - the latter would be stick down with kapton tape or similar and then covered with a silicone overlay/metal frame like the originals. The footprint was easy to make although its not possible to tell Kicad that the pairs of pins are internally connected which means the "design rule check" pulls up "unconnected pin" errors if you make use of this feature, irritating but as I said, not a show-stopper.

Where did you get the keys for the emulator?
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Old Yesterday, 9:24 pm   #25
SiriusHardware
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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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Old Today, 2:03 am   #26
Slothie
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Default Re: MK14 schematic revisions

The dome switch part of the footprint was mostly experimental, since I had them after buying them on a whim from china for next to nothing... I took apart an old calculator as a teenager and saw how they were put together, but I dont recall how the PCB looked. The dome switches I have have small "legs" that locate in the hole,and (hopefully) the hole is big enough to allow them to skid enough, but its highly probable wear will be an issue on reflection.
In reality I'm going to use the 12mm switches unless the dome switches work incredibly well
I have discovered Rapid Electronics sell the D6 style switches (like the reset switch on the original MK14) which I believe from photo's I've seen are the type later MK14's were drilled for, I used the round-topped variant in an alarm clock and they were very positive, although not as good as a cherry key! They have the square top type SOC used, but there used to be clip on caps which they no longer sell. If I'd known that at the time I'd probably used that for the second footprint (all 3 would be a bit of a squeeze!)
If I ever invent a time machine I'll add those RS key switches to my (long!) list of things to buy and get you a couple of sets too!

Last edited by Slothie; Today at 2:03 am. Reason: spelling
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