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retroclouds

Logic in TI-99/4a keyboard?

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This week my daily driver TI-99/4a console (that's the one with an F18a) broke down.

I thought the easiest thing to do was to just swap the mainboard and keep on going for now (I have plenty of spare consoles).

 

Oh boy, was I wrong. The tricky part was to transplant the F18a. I took out the F18a and you guessed it, I managed to break a pin connecting to the VDP socket. Felt like an idiot there.

Luckily I was able to solder on another pin to the F18a board, so got the F18a working fine again (took me several hours and multiple attempts putting on that pin. I'm not a hardware guy so was pretty proud of myself doing the soldering without damaging the F18a board) .

 

Anyway, what I noticed after putting everything back together is that when I typed on the keyboard, there was a very fast repeating of the keys while typing. Tried to type "call clear" and got "caaaalllll cleeeaaar"

So I thought something was wrong with my replacement mainboard and swapped it again, this time being absolutely sure to be as careful as possible not to damage the F18a.

 

I put the keyboard back on, and got the same behaviour. So this time I swapped the keyboard as well and now everything works as expected.

 

Now here to my question, is there any real logic in the keyboard? I was under the assumption that it's just a matrix you poll.
What would explain the above behaviour? At first I thought there could be something wrong with the clock, voltage being slightly of or whatever on the mainboard. But it did not occur to me the keyboard itself could be the culprit.

 

The keyboard in error has the manufacturer "HI-TEK", the one that's working has no manufacturer at all, presume it's made by Texas Instruments (did they even do that?).

 

Anway, long story short. My TI-99/4a is working again and I'm a happy camper. Never thought I'd have such an issue because of malfunctioning keyboard.

 

 

retroclouds

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No logic just switches. Perhaps bad connections if they are actually switches. Membrane has a bad Tendicy to repeat due to poor connection in the membrane

 

Sent from my LG-H872 using Tapatalk

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Seconded... keyboard is a dumb array and repeating keys are usually caused by dirty contacts. Sometimes hammering on the bad key a few dozen times will clean off the gunk, but you can also try taking it apart and cleaning it. This helps the membrane types the most but the membranes are also prone to sticking and tearing, so results vary widely. If it's a switch-based keyboard, you might be able to get some electronic contact cleaner and try squirting that into the switches, then working it in with repeated taps.

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