Nelno Posted April 27, 2021 Share Posted April 27, 2021 (edited) I've been working intermittently on a project to create a replacement for the TI-99/4A keyboard using modern keyboard switches. So far I have the PCB shape and key placement working well enough to slot into the case. In the attached photos you can see the 3D printed part representing both the PCB (bottom layer) and the keyboard switch substrate that the Kailh low-profile switches plug into. In its final form there will be a PCB on the bottom layer and a 3D-printable layer on top of that which the switches plug into. Some additional clips that are screwed into the original screw holes on the case should hold it in place, though I haven't designed those yet which is why you see a bunch of tape holding the prototype onto the case. This is an earlier prototype. It's printed in two pieces and joined because my 3D printer isn't large enough to print the 24 cm wide piece in one go. The raised area is the substrate for inserting the keyboard switches. They just snap in. The lower flat area would be the separate PCB, secured to the substrate with some screws. I chose the Kailh low-profile brown switches because: a) I had some of them b) the low-profile switches with low-profile keycaps match up nearly perfectly to the height of the original keys with just the PCB + switch substrate. Using full-size switches would require a much thicker substrate, which is not the end of the world, but would take a lot longer for me to print prototypes. c) I think browns are probably the closes modern approximation to the TI's Hi-Tek keyboard. I've also designed 3D-printable low-profile keycaps for these Kaihl low-profile switches, but I am expecting that some injection-molded keycaps would be used in the final build. One issue with this is the tilt of the keyboard in the case doesn't leave much option for adjusting the angle of the top of the keys. In the original Hi-Tek keyboards the tops of the keycaps are level. In order to do that with modern key switches, the only real option is to make the keycaps themselves attach to the switches at an angle, because the switches themselves must all plug into the single, slanted PCB. This would mean custom keycaps, or some sort of adapter for original keycaps. I'm not too worried about this, though, because in modern keyboards its natural for the keycaps to not be level when the keyboard is tilted at an angle (i.e. by the keyboards back feet). Ideally anyone building this could just use standard low-profile Kailh keycaps. In retrospect, using the Kailh low-profile switches may not be the best choice because keycaps are much harder to find than they are for Cherry or Kailh standard switches. The main challenge I will have is PCB design. I have exactly zero experience with PCB design, so I'm learning as I go. Currently trying to get a good workflow for FreeCAD to PCB design software. Fusion360 + Eagle is no good because it only allows 80 cm^2 PCBs in the free version and I'm not paying Autodesk's insane software rental fees for a project I intend to open source. At any rate, the PCB for a keyboard seems like one of the easiest things to start with and a good way to learn. I started this due to my experience replacing the plungers on my Hi-Tek keyboard and realizing these keyboards are relatively rare compared to the Mitsumi membrane keyboards. I don't know if this would fit in a case that takes Misumi keyboards, since both TI-99/4As I own have Hi-Tek keyboards. Are the various keyboards interchangeable? My end goal is to provide a replacement keyboard with a modern feel, using modern parts that are easier to source, but hopefully something that looks close to and has the option to be, at a glance, difficult to distinguish from the original keyboards. Interested to hear any feedback. Am I wasting my time with this? Would anyone else find it useful? Has this already been done? (I couldn't find anything indicating it has when I was working on my keyboard plungers about 6 month ago). Edited April 27, 2021 by Nelno Grammar. 13 1 Quote Link to comment Share on other sites More sharing options...
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