Foam and foil conversion on TRS-80 clone

[mauriciomunuera]

Hi!
I'm fixing my Prologica CP-500 M80c (a brazilian clone for the TRS-80 model 3). The keyboard on this computer is a capacitive foam-and-foil (keytronic style), but it was never particularly good even when it was new.
Yes, I know I can replace the foam and foil discs, but that's not what I want. My goal is to make it actually good to use while keeping it fairly original.

The pcb is screwed onto the bottom of the switch units, which are mounted on a metal plate. The keycaps have two legs that snap into the plunger on each foam-and-foil unit.

My first thought was to install new switches on the plate, but then I would not be able to use the original keycaps. Thats's a deal-breaker because there are no modern keycaps sets with the same color scheme and legends or modern switches that can take the original keycaps.

So I came up with another idea: replace the foam and foil on the plungers with a small round magnet and install reed switches on the original pcb (it already has holes with traces that go to the capacitive pads, not sure why).
I've been looking around to see if anyone had tried this already but found nothing. I believe it would work, but thought it would be best to ask someone more experienced first.
What do you guys think?

[Sangdrax]

Magnets are a much bigger mess. Anything capacitive will do the trick. You just need something with a conductor and an insulator over it. I used to use punches from antistatic bags as they were an insulative sandwich of plastic over metalized material. Worked great. I also used harder closed cell foam like the super glue brand wall mount foam for a more satisfying thunk when depressed. It gave enough to connect but lessened the squish feel of original foam.

Also, first post in a long time!

[mmm]

Sounds like a great idea. No idea how it will work out or whether it is possible to reuse the PCB for this kind of sensing. Sounds nice circumventing the inherent mushiness of spring+foam&foil.

[mauriciomunuera]

Thanks for answering, guys! I've been reading stuff here for years but never joined. I'm glad I finally did it

So, I thought about replacing the foam and foil but I already replaced most of them about 10 years ago, and even though it worked well, it never worked well enough... and they are already failing again. I wanted a fix that would last me a bit longer.
I've seen people use stickers covered in graphite, Ruffles bags, safety blankets, rooftop thermal insulator, etc. Sure, they work, but I don't really like it.
Also, it seems like the previous owner already worked on the pcb, sanding and adding solder tin on some of the contacts. So I think I'd need to fix the board as well... Too much trouble for inconsistent key presses and unpleasant feel.

My biggest uncertainty about adding any kind of switches to this pcb is because I don't really know how the capacitive keyboard works. I mean, sure, I understand a little of the principle, but not enough to confidently mod it like that. Maybe some electronics wizard around here might be able to tell me "sure, go ahead. It might work" or "Do you want a house fire? Because that's how you get house fires" hahaha

I've checked the technical manual and it does explain the matrix and the way the cpu reads it, but I'm not sure if adding switches (reed, cherry or whatever) would send the wrong voltages somehow... Don't think so, but for once I thought it'd be better to ask someone more knowledgeable first. I usually just go for it and mess around until it works, only to find out later that someone already did the same thing and I reinvented the wheel using much worse techniques.

[Sangdrax]

Capacitive controllers can be tuned to work with regular switches. The PCB for a capacitive foam and foil usually has pads on two sides and has them lacquered over. These would be in the way of any switch leads. If it has a switchplate, I wouldn't bother with destroying the existing PCB and just handwire up the replacement with a regular controller.

[mauriciomunuera]

That makes a lot of sense, I agree with you. The only problem is the keycaps won't fit any switch I can think of. I thought maybe they'd fit the Alps mount, but no...
So to keep the keycaps I'll have to keep the housing and plunger. That's why I thought of using magnets and reed switches.
Well, I'll make some experiments and let you guys know how it went

[mmm]

That makes a lot of sense, I agree with you. The only problem is the keycaps won't fit any switch I can think of. I thought maybe they'd fit the Alps mount, but no...

Little known fact: They fit (some versions) of "Proto-Topre". Not ordinary Topre though.



I like your idea though, I hope you find a solution!

I have it on my todo somewhere in my future to replace the PCB with an assembly of either FLS "springs" or cheap microswitches, where the slider will touch those switches instead, thus eliminating the mushyness nature of foam and foil. Unsure whether I'd have to remove the external springs, depends on how it works out (if I ever get to it).

[snacksthecat]

I also used harder closed cell foam like the super glue brand wall mount foam for a more satisfying thunk when depressed.

Interesting. That type of material has never crossed my mind but it sounds like a great idea. Very clever!