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Cherry MX compatible upgrade for Mitsumi 99/4A keyboards


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Hi all,

 

Thought I'd share a project I've been working on.

 

I recently bought a beige '99 which had the infamous Mitsumi mylar keyboard which wasn't working at all. :(

 

I did manage to sort it out to work *relatively* well, but in the meantime, decided (with some prompting from Derek Christenson) to design a replacement keyboard which uses modern (Cherry MX compatible) switches. Additionally, it uses the original Mitsumi TI-99/4A keycaps (via 3D printed adapters) for that original look.

 

I've tested my first revision (v0.1) board and it's working great. A couple of minor adjustments for v1.0 and then I'll release all the details for general consumption.

 

The project could be adapted to use new (or perhaps other keyboard types) keycaps at a later time, but I created it for the very specific use-case of upgrading a Mitsumi board.

 

If you'd like to follow the project:

 

Hackaday: KEYBOARD/4A-99 | Hackaday.io

GitHub: visrealm/keyboard4a99: Modern keyboard replacement for a Texas Instruments TI-99/4A (github.com)

 

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Edited by visrealm
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18 minutes ago, chris36 said:

Would this work with non-beige units?

depends on the keycaps -- this is just backplane replacement, you still need your old keycaps to fit into the new cherry switches -- it might be possible to make an adapter like he did for the beige ones, but there is two types of 'black' keycaps that i know of.

 

i still would love to see a full complete keyboard replacement with all the keycaps as well and printed with the right symbols on them, but that would greatly increase the cost.

 

would be neat if at the same time this new pcboard replacement is made, to squeeze in somewhere a ps2 or usb adapter as well, so not only you replacing the keyboard you are gaining the option to use another one. but there is not much open space on pcboard unless all the ics and parts can be squeeze into the empty spaces on both sides of the long space bar.

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7 hours ago, Gary from OPA said:

depends on the keycaps -- this is just backplane replacement, you still need your old keycaps to fit into the new cherry switches -- it might be possible to make an adapter like he did for the beige ones, but there is two types of 'black' keycaps that i know of.

As far as I know, if the black keyboard is also a Mitsumi mylar piece of crap, the keycaps should work here. I also have a Black TI, but fortunately it has the mechanical switches. The keycaps from that keyboard, however, will not fit on this. They're much too tall. You could create adapters for them and adjust my 3d printed PCB mounts to push it a bit deeper into the case, but they would still protrude more than stock.

 

7 hours ago, Gary from OPA said:

i still would love to see a full complete keyboard replacement with all the keycaps as well and printed with the right symbols on them, but that would greatly increase the cost.

Agree, this would be fantastic! But, as you said, it would cost a small fortune. I'm sure there are many unused Mitsumi keycaps around that can be reused.

7 hours ago, Gary from OPA said:

would be neat if at the same time this new pcboard replacement is made, to squeeze in somewhere a ps2 or usb adapter as well, so not only you replacing the keyboard you are gaining the option to use another one. but there is not much open space on pcboard unless all the ics and parts can be squeeze into the empty spaces on both sides of the long space bar.

Someone else mentioned this in a Facebook post in the TI 99ers group. It would require a power connection to be brought in (probably directly from the internal PSU). Using an RP2040 or similar, it probably could fit on the PCB, but might be better as a daughter board? Something to consider for the future perhaps.

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@visrealm   I thought I wasn't paying attention.

 

Looking at the first photo of the board it looks like the ribbon cable is on the top (like TI's), then looking at other photos of the board on github, the connector seems to be in the middle of the board.

 

Did you recycle a ribbon cable from the original keyboard?

 

I really like how you labeled the pins - that's a big help for us hobbyist.

 

Thank you for creating this, good working keyboards for the ti-99 are getting scarce.

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3 hours ago, visrealm said:

v1.0 boards came in. Some teaser shots :)

 

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I'm still working on fully documenting the build procedure. Once that's done, I'll make all the details public. 

 

 

One question about the photo with the metal brackets across it, not all ti99 keyboards had those, are they really still needed now that it is a stronger PCB compared to the original design and no longer a membrane layer?

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7 hours ago, dhe said:

@visrealm   I thought I wasn't paying attention.

 

Looking at the first photo of the board it looks like the ribbon cable is on the top (like TI's), then looking at other photos of the board on github, the connector seems to be in the middle of the board.

 

Did you recycle a ribbon cable from the original keyboard?

My v0.1 prototype board had the ribbon cable mounted at the top. For the V1.0, I placed it closer to the Mitsumi placing for easier routing and so you could re-use the Mitsumi ribbon cable if you wanted to.  I did re-use the Mitsumi ribbon cable in the shots above, but there are new cable options available too.

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6 hours ago, Gary from OPA said:

One question about the photo with the metal brackets across it, not all ti99 keyboards had those, are they really still needed now that it is a stronger PCB compared to the original design and no longer a membrane layer?

Some support is definitely necessary. She's a little flexy without. Perhaps a 2mm+ PCB might help - I'm using 1.6mm.

 

In my v0.1 prototype, I created a facia/overlay on the top of the keyboard with brackets which held onto the PCB. But, for v1.0, I added the mounting holes for the original Mitsumi metal brackets in order to reduce 3D printing part count and costs. If you don't have the metal brackets, going the v0.1 approach is still an option.

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I've now released the build details for my KEYBOARD/4A-99 keyboard upgrade. If you're interested in this, have a read of the documentation in the repository.

 

Also note: arcadeshopper.com has already ordered a batch of PCBs and plans to make these available over the coming weeks. If you're after a kit (or similar), please hold off until @arcadeshopper has had a chance to work things out. That will certainly be the easiest option for those in the US.

 

I'll be releasing a build video at some point too.


https://github.com/visrealm/keyboard4a99

 

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This is awesome. I've been learning kicad and honing my CAD skills to for months to do what you have done. Makes me wanna buy some more Beige Ti's in the hopes of getting some bad keyboards to replace.😀 I've got enough keys in a variety of 'clicks' to build several of these and will refine my efforts to making this work on other types of caps from some bad black Ti's I have around. Thank you SO much for the efforts you have put into this.

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Well, so far...

I've been doing some dry fitting of the keycap adapter. I had to adjust my "Hole Horizontal Expansion" up to +0.1 from -0.1. I'm using Kaihl Black (and a couple of Blue) switch's. They have a dust collar that goes all the way around the post so the fit is a little tighter but doable. They are spares from another keyboard project where I used Hot Swappable switches and a mounting plate verses a PCB.

 

I use an Ender 3 Pro with direct extrusion, dual Z stepper drives, through an updated hot end on a glass bed. Does great for one adapter at a time but the occasional booger and string tends to mess up doing two or more. Still have to stay on top of it after Auto bed leveling or it will extrude a big blob that curls up to the side of the nozzle and comes off at the wrong time during print.

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Posted (edited)
11 hours ago, OLD CS1 said:

Need to show those damnedable Mylar membranes.

🤣 you're so right!

 

11 hours ago, dphirschler said:

I think the HI-TEK keyboards are also a good candidate for this upgrade.

 

Darryl

I only have access to the Shitsumi and a (Black) Stackpole. My Stackpole works perfectly, so I don't want to mess with it, but I can say the key caps on it are very tall. To adapt them would require different PCB mounts to push it deep into the case. Probably doable though.

 

15 hours ago, Duewester said:

This is awesome. I've been learning kicad and honing my CAD skills to for months to do what you have done. Makes me wanna buy some more Beige Ti's in the hopes of getting some bad keyboards to replace.😀 I've got enough keys in a variety of 'clicks' to build several of these and will refine my efforts to making this work on other types of caps from some bad black Ti's I have around. Thank you SO much for the efforts you have put into this.

Thanks! I did see your keyboard project prior to starting mine but saw there haven't been any updates on it for some time. I think I even had a question there about key caps.

 

PCB design isn't so hard. I just use EasyEDA. It has "Easy" right there in the name. 🤣  I've been doing that for a while (a few years). I have designed a homebrew 6502 / TMS9918A system on a backplane and more recently that same system emulated using a Pi Pico

 

I've only just started with CAD, around 5-6 weeks ago (when I bought my first 3D printer). I've been using Onshape since day 1 (5-6 weeks ago) since it's "free", has great features and seems to work very well (so far). I do have some 3D modelling (and 3D game programming) experience prior to that though.

16 hours ago, FALCOR4 said:

Thanks for doing this, @visrealm.  A solution like this has been needed for long time for all of the broken Mitsumi keyboards out there. You've done us all a great service.

No worries!

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Posted (edited)
7 hours ago, Duewester said:

Well, so far...

I've been doing some dry fitting of the keycap adapter. I had to adjust my "Hole Horizontal Expansion" up to +0.1 from -0.1. I'm using Kaihl Black (and a couple of Blue) switch's. They have a dust collar that goes all the way around the post so the fit is a little tighter but doable. They are spares from another keyboard project where I used Hot Swappable switches and a mounting plate verses a PCB.

 

I use an Ender 3 Pro with direct extrusion, dual Z stepper drives, through an updated hot end on a glass bed. Does great for one adapter at a time but the occasional booger and string tends to mess up doing two or more. Still have to stay on top of it after Auto bed leveling or it will extrude a big blob that curls up to the side of the nozzle and comes off at the wrong time during print.

I've had no issues with the 7x7 array of adapters. I'm using an Ender-3 V3 KE with a textured PEI build plate. It could be that my printer (and hence my nozzle and build plate) is relatively new (5-6 weeks).

Edited by visrealm
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