🇸🇰 Tesla PMD 85: Part 7 (Keyboard Issues?) [TCE

Continuing on from yesterday's video [TCE #0446], I replaced the Tesla programmable peripheral interface (PPI) chip for the keyboard, MHB8225A, with a Western CMOS equivalent. It seems to have stopped the "key stuck on repeat" issue.
However I think I still have keyboard issues. Thanks again to Jakub‪@jjermar1‬ and Yura ‪@yurasergienko6890‬ for their advice.
Jura pointed to a Hungarian video by‪@ATLAB‬ where he details about replacing the rubber domes, and also identifies a replacement from Aliexpress:
• PMD 85-2 számítógép ja...
On Aliexpress they are described as ... "Φ12*Φ7*H6mm silicone conductive single keypads silicone buttons" ... so I'm thinking 12mm outer diameter, 7mm inner diameter and 6mm height ... ? This one page (that exists as of May 2024) they are "model DGQSDZ-KP58" ...
www.aliexpress...
archive.is/0Sk...
I'm really wondering if designing a replacement keyboard might be a better solution ... ?
Time to have a look at the Robotron Kleincomputer KC 85 again I think ...

[@josephlunderville3195]

The broken pin can probably be replaced with one from another right angle pin header if you desolder the whole thing and yank it out from the back with pliers. It won't look perfect, but if you have trouble finding an exact replacement..

[@twinshobbytwinshobby3863]

Awesome Video !

[@charlesdorval394]

The aliexpress thing as been working quite well for me as well, kinda blows my mind tbh

[@tomanpcm]

:-)

[@VilleRaulos]

Why not use m2 or some other size screws to reattach the key switches?

[@JVHShack]

If worse comes to worse, there's always the "clone that thing" approach and using modern parts in the new design. I've done a few retro keyboards already myself.

[@Brfff]

Yep - I've been thinking about it for a while for this machine. I think the current key switches are 14mm square, so if there is an equivalent replacement ... ?

[@zxcygnus]

I just saw, you had in socket chip 8255 from Intersil. I have no idea how will it work in PMD 85, but in UR-4 paralel interface for ZX Spectrum it did not work well. There are slight differences. If you can, use NMOS version, maybe from Tesla, but NEC and other clones are good too.

[@Brfff]

Well I do now have a socketed Tesla 8255 in the PP01 that I could also try out!

[@MrWaalkman]

I found a 1/2 ton arbor press for ~$33 bucks, outdoorly.store/products/hhip-8600-0031-heavy-duty-arbor-press/ Which you could fit a die on the top with two pins to match the spacing and size of the pins on the back side of the keyboard, and a cup on the key side to support the board. Then hopefully cold press them out without damaging the board.

[@Brfff]

Ach, I finally get around to checking out this arbor press for only 33 Freedom Bucks ... but CostCo says no ... that links re-directs to: cosctodeal.online/forbidden

[@MrWaalkman]

@@Brfff It's still a good link for me. Must be a location thing. But if you have a friend with a drill press you could use it as a makeshift arbor press. Just don't turn it on. And Thingiverse has a few arbor press models that you can download. No idea how durable they would be. Probably good enough for the purpose.