Texas TI-58: Repairing the keyboard bounce and miss to register keystrokes

[jebem]

I have this Texas TI-58 calculator missing a few parts used to repair other Texas machines.

One of the missing parts was the keyboard, used to repair one of my TI-59.
The original TI-59 keyboard had a few troubles, including battery acid leaking light corrosion and it failed to register a few keys and sometimes several keys were registered from a single keystroke - a classic case of bouncing, I would say.

So as I had nothing to lose, I decided to disassemble the TI-59 keyboard piece by piece, fix the issues and put it back on operation on my TI-58.

Looking to the keyboard white plastic frame, we can see a number of small holes in front of the crimping/welding points. I think Texas made a less than perfect job when welding these joints, resulting in a lot of failing keyboards.

In my case, these mechanical/electrical joints were broken in several positions, and I believe this was causing both bouncing and missing keys as well.
Pressing a key can either cause multiple contacts spaced in time in a way that the debouncing software routine can't handle, or a total failure to register.

After reading so much reports from others here and there, I have noted that some people can fix some of these keyboard issues by just manipulating the original foam that sits between the plastic keys and the domes.
This makes sense to me, as the foam makes some pressure to the domes forcing them to make a better contact with the keyboard frame wires.
A thicker foam should give better results as well. I did it on one TI-57 some time ago with some success.

To test my hypothesis, I decided on soldering all the 9 dome's connection points where originally it were crimped/welded at factory.


Separating the 9 dome rows from the keyboard assembly.

Once the keyboard is desoldered from the main board, the starting point is to remove the plastic cover from the keyboard base, where the 9 dome rows comes attached to it.
Some of the joints of the 9 rows were lose, creating the issues. Other joints were good, so some force is required to pull all the set apart from the keyboard base.
The plastic cover and the metal domes are resistant, though, so no damage was caused by this procedure.

Next step involved the separation of each one of the 9 dome rows from the plastic cover.



Preparing the keyboard base 9 wire rows dome's joint points by tinning them.
This is a delicate operation. Needs to be done fast and precisely. The base plastic is very soft and melts easily if applying too much heat for to much time. One to two seconds maximum, using a small but powerful solder gun.



Prepare the 9 metal dome rows by tinning the joint points on the both sides.
Very delicate operation, as one needs to apply the least amount of solder as possible on the underneath side to tin just a small area. Otherwise it can short circuit the 5 columns wires as well.
Tinning the underneath side first:



Tinning the upper side of the dome row allows to do a fast and precise soldering operation when putting it back into the keyboard base. Before tinning it, it is necessary to remove a small portion of the strong glue layer that covers the entire dome row.
Tinning the upper side next: