TI99-4a strange keyboard issue

[deadmeat73]

Hi, I've acquired a TI99/4A that I've repaired (faulty RAM) and cleaned. It seems to be 100% functional with the exception of a strange defect: in basic when pushing and releasing the shift buttons (both) it display the letter g (lowercase). Shift button works fine when pressed together with another key it shows the uppercase (including the letter G) but when pushed alone this weird thing happens when released.

 

I'm not particularly familiar with the way TI99 reads keypresses, but my first guess would be something in the 74LS156 area but I would expect more than one problem if there's a faulty / marginal line in the decoder. 

 

Any leads before I disassemble everything? 

 

[+mizapf]

This is the keyboard matrix. The columns are selected one by another, and a keypress shorts the connection to the respective interrupt line.

What you can directly check from this is whether there are more relations between column 1 and column 5 (pins 12 and 9). When you press Shift, and the computer has selected the column 5, Shift should not be active, but it seems to have an effect, so it appears as g. What about the pairs Space/H, =/N, Fctn/5, Ctrl/T, and Enter/Y?

[Duewester]

Me personally, I'd examine the keyboard wiring plane with a microscope. If it's one of the membrane type, look for stray particles (I actually found the little punched out mylar discs floating around in mine). 

On one of mine with a very different kb from the other three, I used one of the Shift keys to replace a defective Space key. With key removed I still had to install a bodge wire across the NC contact solder holes. Perhaps the G key is actually the problem.

Again, I'd closely inspect the kb.

[HOME AUTOMATION]

How's the JOYSTICK port doin'?

 

We've seen this happen...

[deadmeat73]

Thanks for the answers, just a few comments I've seen that Shift and G are actually on the same row... that seems to indicate something wrong on line 9 or 3, but what's really weird is that every other key works fine space + h, shift + anything else, and even the G key works, shift + G works... it only happens when I release the shift key that's why I thought the decoder since it seems something stay "locked". Joystick port seems to be fine but at this point first thing is to carefully inspect the keyboard as suggested... too bad I've just finished reassembling it and it is such a pain to disassemble... 

[Mtlatc]

18 hours ago, HOME AUTOMATION said:

How's the JOYSTICK port doin'?

 

We've seen this happen...

Right.......Always check the JS Port.......

[johannesmutlu]

I personally wouldn’t be surprised if that keyboard problem has to do with the mailor membrane wich texas instruments used on their first models of their ti99 systems,those mailor membraines tend to stick on the motherboard overtime and then no longer respond to any key presses,i got the same problem with mine in wich many keys just don’t respond to any presses or they respond in overtour or they respond just normal,it differs per key,

it’s all randomly messed up,

now since later models of their ti99 do uses switc based keyboards ,that problem shouldn’t appear on those models,it’s possible to replace those mailor membrane keyboards with switch based ones but good luck finding one,

trough i still wish texas instruments did thinked twice before deciding to use cheap ass non everlasting mailor membranes into their ti99 keyboards,because from now on many ti99 systems do suffer from this flaw,all because of a determent decision from texas instruments 44 years ago,ouch🥲🥲

[+acadiel]

3 hours ago, johannesmutlu said:

I personally wouldn’t be surprised if that keyboard problem has to do with the mailor membrane wich texas instruments used on their first models of their ti99 systems,those mailor membraines tend to stick on the motherboard overtime and then no longer respond to any key presses,i got the same problem with mine in wich many keys just don’t respond to any presses or they respond in overtour or they respond just normal,it differs per key,

it’s all randomly messed up,

now since later models of their ti99 do uses switc based keyboards ,that problem shouldn’t appear on those models,it’s possible to replace those mailor membrane keyboards with switch based ones but good luck finding one,

trough i still wish texas instruments did thinked twice before deciding to use cheap ass non everlasting mailor membranes into their ti99 keyboards,because from now on many ti99 systems do suffer from this flaw,all because of a determent decision from texas instruments 44 years ago,ouch🥲🥲

IMO, I think it's about time we made a PCB for some current key switches for a drop in replacement for the TI Keyboard

 

I haven't measured one, but it's in my "one of these days" backlog that I can't ever seem get to.

 

[JB]

4 hours ago, johannesmutlu said:

I personally wouldn’t be surprised if that keyboard problem has to do with the mailor membrane wich texas instruments used on their first models of their ti99 systems,those mailor membraines tend to stick on the motherboard overtime and then no longer respond to any key presses,i got the same problem with mine in wich many keys just don’t respond to any presses or they respond in overtour or they respond just normal,it differs per key,

it’s all randomly messed up,

now since later models of their ti99 do uses switc based keyboards ,that problem shouldn’t appear on those models,it’s possible to replace those mailor membrane keyboards with switch based ones but good luck finding one,

trough i still wish texas instruments did thinked twice before deciding to use cheap ass non everlasting mailor membranes into their ti99 keyboards,because from now on many ti99 systems do suffer from this flaw,all because of a determent decision from texas instruments 44 years ago,ouch🥲🥲

Other way around. The mylar keyboard module was a later cost-reduction decision. 

[johannesmutlu]

10 hours ago, JB said:

Other way around. The mylar keyboard module was a later cost-reduction decision. 

Huh?

seriously? I tout that ti decided to replace that maylar keyboard with switch based keyboards after complainments of many users having problems with their keyboard on many ti99 systems,(according to some youtubers),now i don’t know long those maylor membranes could last before starting to wear out,maybe by two,three,or four years??

BUT however if it turns out that the maylar membrane could last a whopping 10 years,then they wouldn’t saw any reason to replace that maylar membrane with switches because by that point ti would,ve already out of the computer business,mmmm:

[+Ksarul]

12 hours ago, johannesmutlu said:

Huh?

seriously? I tout that ti decided to replace that maylar keyboard with switch based keyboards after complainments of many users having problems with their keyboard on many ti99 systems,(according to some youtubers),now i don’t know long those maylor membranes could last before starting to wear out,maybe by two,three,or four years??

BUT however if it turns out that the maylar membrane could last a whopping 10 years,then they wouldn’t saw any reason to replace that maylar membrane with switches because by that point ti would,ve already out of the computer business,mmmm:

Having owned my TI-99s since the original 99/4 with a chiclet keyboard was the only option, I can tell you definitively that the YouTubers are wrong. TI used the switch-based keyboards up until late 1983, and switched to the mylar keyboards because the reliability was BETTER at the time. Switch keyboards got dirt inside and started double-tripping or not tripping at all. A good cleaning resolved the problem, but not everyone was technically savvy enough to do that simple fix. The mylar keyboards didn't have that problem at all, and were seen as superior at the time. Spin forward 20 years or so, and the situation reverses, as now the mylar keyboards are long past their design life and work like crap. The mechanical switches, on the other hand, are still plugging along nicely with an occasional cleaning.

 

Yes, the mylar keyboards are NOT a good solution today, but in late 1983/early 1984, they were the best keyboards out there for regular use.

[JB]

On 9/25/2023 at 5:42 AM, johannesmutlu said:

Huh?

seriously? I tout that ti decided to replace that maylar keyboard with switch based keyboards after complainments of many users having problems with their keyboard on many ti99 systems,(according to some youtubers),now i don’t know long those maylor membranes could last before starting to wear out,maybe by two,three,or four years??

BUT however if it turns out that the maylar membrane could last a whopping 10 years,then they wouldn’t saw any reason to replace that maylar membrane with switches because by that point ti would,ve already out of the computer business,mmmm:

At the time, there was no indication the mylar keyboard modules would have long-term reliability issues. And TI was not planning on exiting the home computer market, they were still planning to dominate it. Their departure from the market was a sudden and abrupt decision due to the home computer division's absolutely CATASTROPHIC financial performance(the one division lost enough money in one quarter that the company as a whole posted a loss, for the first time in TI's history)

 

You can see the cost reduction in other ways besides "two sheets of mylar and a ribbon cable beats a few dozen switches and a solder bath". The mounting bracket on the mylar modules is molded plastic instead of stamped steel, and most of the various types of microswitched keyboard modules are black while most of the mylar modules are beige. The beige cases mark the beginning of earnest cost-reduction efforts.