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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 11th Jun 2020, 3:53 pm   #1
ronbryan
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Default TI LCD Programmer calculator key bounce

My LCD Programmer calculator has a problem with the keyboard - either the keys bounce or the key presses can get ignored until you try again. It has had many years of inactivity, but basically works with a couple of new SR44 batteries inserted and some exercising of the keys.

Before I risk damaging it by dismantling, has anyone succeeded in cleaning the key contacts? A youtube video I have seen says the calculator interior is mostly flexible printed circuit and the key matrix of dome type keys is spot welded together and therefore inacessible.

I did wonder if anyone had tried, for example, introducing IPA or DeoxIT into the keypad assembly with success.

Ron
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Old 12th Jun 2020, 11:37 pm   #2
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Default Re: TI LCD Programmer calculator key bounce

Can't help with any tips I'm afraid, but I had a ti57 with the same clicky keyboard when I was at college in 1983. After about 6 months of use it became unusable with keyboard bounce. Returned it to the shop (Woolworths I think) and they replaced it. The replacement went the same way in about another 4 months.
Truely terrible keyboard design!
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Old 14th Jun 2020, 10:47 am   #3
ronbryan
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Default Re: TI LCD Programmer calculator key bounce

It's interesting that the same key type (and problem) was carried over to other models.

Repeated exercising of the keys yesterday evening has made an improvement, but the difficulties with key bounce and occasional lack of response when keys are pressed are still a problem.

I don't know the internal construction of dome type keys, but the problem must be due to tarnish on the contacts, as the keypad should not be worn as it has had very little use in the past.

Ron
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Old 14th Jun 2020, 12:58 pm   #4
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Default Re: TI LCD Programmer calculator key bounce

I have a TI-54 with the same problem... or it did used to behave like this - now I can't get any of the keys to respond !
So I would also like to know if a repair is possible.
Needless to say, that was the last TI calculator I ever bought...

Andy
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Old 14th Jun 2020, 1:08 pm   #5
SiriusHardware
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Default Re: TI LCD Programmer calculator key bounce

Ron didn't state the model number of his but here is a detailed case where someone has dismantled and repaired a ti-58 keypad so this may give some advance idea of what to expect if you attempt it.

https://www.hpmuseum.org/forum/archi...read-2571.html
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Old 14th Jun 2020, 4:04 pm   #6
ronbryan
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Default Re: TI LCD Programmer calculator key bounce

There is a video showing the interior of the TI LCD Programmer calculator here:

https://www.youtube.com/watch?v=mlZiE5aG9IA

The keyboard construction is shown if you speed to about 3 minutes 20 seconds in and it does seem to be similar to the TI-58 referenced by Sirius.

The flexible printed circuit is shown attached to the wires of the keyboard on fingers at the bottom end of the calculator. To repair the keyboard would require removal of the flexi-circuit from the keyboard connecting point. Is it likely that the flexi traces are welded to the keyboard fingers, as the dismantler suggests, or are there other attachment techniques, such as conductive glue for instance or low temperature soldering?

If the joints are welded, some means of re-attaching them will be needed.

Ron
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Old 14th Jun 2020, 4:37 pm   #7
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Default Re: TI LCD Programmer calculator key bounce

I'm not at all sure that you would need to disconnect the flexi connector from the keypad and in fact I'm afraid that any attempt to do so might end in tears, best to leave those connections well alone. The video demonstrates that you can get to both sides of the keypad without disconnecting the flexy PCB, but you need to be careful not to bend those connections back and forth too much.

It looks to me on that video as though the 'welds' he refers to are the flattened heads of soft metal rivets visible on the back of the keypad. If you can remove the flattened part on either the blank side or the keypad side the dome strips should come free, as was obviously achieved by the poster in the link in #5.

It also looks to me as though those rivets are involved in electrically connecting each keypad row to whatever they are connected to, as there are only one, at most two, rivets per key row. If they were there purely to hold the keypad layers together you would expect more of them, equally distributed across the keypad.

To put it back together you might be able to use tiny countersunk-head machine screws and nuts to replace the rivets if there is enough space between the keypad and the ribbon PCB when the unit is reassembled.
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Old 14th Jun 2020, 4:48 pm   #8
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Default Re: TI LCD Programmer calculator key bounce

Actually looking again it looks as though those 'welds' are actually solder joints - it seems there are vertical wires running through the keypad backing and each row of keypad domes is soldered to one of those as it passes by underneath the row. There is probably a top layer of wide transparent sticky plastic covering the whole of the front of the keypad - if you carefully peel that off then maybe you can unsolder the dome strips on the 'key' side of the keypad in order to release the dome strips.
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