Repairing or replacing the Mitsumi keyboard?

[Calab]

I just acquired a TI-99/4a system and all seems well, except that the majority of the keys don't work, or don't work every time. Turns out that this beige TI-99 has a Mitsumi keyboard.

 

I'm going to try and repair the mylar. Any suggestions? It pretty much seems like a lost cause as the mylar is "stuck" to the pc board.

 

Are there replacement keyboards for these machines to be had anywhere?

[+Ksarul]

There are a couple of options for replacement keyboards. Sometimes NOS keyboards show up for sale on eBay, but a majority of those (in the Radio Shack packaging) are Mitsumi keyboards, which won't help much unless the keyboard is one of the other types (about 30-40 percent are other types). Some used keyboards also turn up (just watch out for the Mitsumi types). Any of the non-Mitsumi boards can be safely used in any /4A. There are also several PC-style keyboard adapters (for XT, AT, PS2 (IIRC), or USB keyboards). Recently, discussion has also focused on the possibility of building new keyboards with modern switches (but that option isn't cheap or even available yet).

 

One or two folks have successfully rejuvenated a Mitsumi keyboard. There is an existing thread on the process here (and the FAQ may even point to it). It isn't easy--and that mylar is really easy to tear. . .

 

I have a whole case of original keyboards that I bought about 20 years ago--but they are ALL Mitsumi boards, so I haven't been able to use them for their intended purpose.

[Calab]

I successfully restored my Mitsumi keyboard. It was not fun as the white layer under the mylar did not want to let go of the pcboard. A good scrub with DeOxit and some foam tape behind the row of contacts connecting the mylar to the pcboard seemed to do the trick. No idea for how long though.

[gferluga]

Lucky you. An hour ago the C key stop to work at all on mine ?

[+dhe]

Has anyone tried transplanting cherry keys switches on to the main board?

[emarquis1]

This thread is geting old but I just wanted to chime in on what I have done to repair a totally non-functioning Mitsumi keyboard.  I disassembled the keyboard and carefully peeled off the white mylar back. I purposefully DID NOT peel it all the way off because I did not want to separate the compressed connections at the side of the board closest to the space bar.  These are the connections from the mylar to the circuit board.  

 

Then I used this silver pen: Amazon.com : Circuit Scribe Non-Toxic Conductive Ink Pen for Kids Circuit Building Set (2-Pack) | Silver Ink Rollerball Point Pen Kit, Science Experiment, DIY Science Projects (New & Improved Formula) : Office Products to paint each of the black carbon dots on the mylar sheet.  I let them dry for about an hour before reassembling the keyboard.

 

It works perfectly now with no missing or multiple hit keystrokes.  You can also repair a torn carbon trace using the silver pen.  Hope this will help somebody revive a bad keyboard or two.

[Duewester]

8 minutes ago, emarquis1 said:

This thread is geting old but I just wanted to chime in on what I have done to repair a totally non-functioning Mitsumi keyboard.  I disassembled the keyboard and carefully peeled off the white mylar back. I purposefully DID NOT peel it all the way off because I did not want to separate the compressed connections at the side of the board closest to the space bar.  These are the connections from the mylar to the circuit board.  

 

Then I used this silver pen: Amazon.com : Circuit Scribe Non-Toxic Conductive Ink Pen for Kids Circuit Building Set (2-Pack) | Silver Ink Rollerball Point Pen Kit, Science Experiment, DIY Science Projects (New & Improved Formula) : Office Products to paint each of the black carbon dots on the mylar sheet.  I let them dry for about an hour before reassembling the keyboard.

 

It works perfectly now with no missing or multiple hit keystrokes.  You can also repair a torn carbon trace using the silver pen.  Hope this will help somebody revive a bad keyboard or two.

Hard to type with my fingers crossed for you. I tried an almost identical trick with my Frankennine keyboard. My problem is the mylar is too stiff and the fix didn't last. I ended up building my own keyboard with Kahli (I used blue)keys, switches and boards from Adafruit. I had to reuse the one latching key from the original keyboard. Getting a sturdy 3d printed frame was the biggest challenge. Soldering the matrix wasn't much fun but, not the worst solder job I have ever done 🤐 Somewhere in these forums are pictures ( I'm on my phone at the moment and multitasking isn't my favorite thing to do from here or I'd link).

[emarquis1]

The mylar on mine is quite flexible (maybe newer). Will be using it for my daily use machine for a while to see how it holds up. Will post an update at some point.  I had read your thread earlier on how you made your own keyboard.  Cool stuff!

[Duewester]

Thanks.

My first sign of returning failure was stuck keys and then hard press required on others. That's what motivated me to make my own.

Now I'm back to making my own numerical keypad for the Frankennine.