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Nelno

Hi-Tek 373-70229A Keyboard Disassembly

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On 1/7/2021 at 8:56 PM, Ed in SoDak said:

Excellent work! It could give a second life to a lot of TI keyboards.

Thanks. I have a 4K resin (SLA) printer on the way that, supposedly, will print with an accuracy down to 0.01 mm. Even though I've got the PETG ones working well, they still need a bit of sanding. I am hoping with the SLA printer I will be able to simply print them and be done without any sanding.

I was able to also fabricate the copper contacts used for each key to replace the one that I broke.

 

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I purchased a 0.15 mm thick copper roll and basically cut these pieces out with a tin snipper and bent them into the shape of the one original that I removed without breaking it. I actually received 0.25 mm copper roll, but it worked perfectly well. The original contacts are 0.16mm thick and that is extremely hard to source. I only found it one place after hours of searching, but I went with 0.15 mm that had a somewhat sooner arrival time (still took about a month to get here).

 

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Attached are some pictures of the contacts. Probably the most useful is the profile pics that show how the contacts need to be bent to actually work correctly (touching when the key is depressed and separated when it is not).

You can see from the pictures that I didn't bother with the tabs on the outside, or the retaining clip that is punched in the middle of the original. I couldn't find any way to reproduce the clip with the tools I have, and I tried the tabs but accidentally cut them off. My tin snips are very unwieldy.

 

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I don't think this will be a problem, however, once I solder the copper strips in. The clip really only keeps the strips secure when the solder is removed, but it doesn't prevent them from wiggling around like the solder does. The tabs on the side just prevent the copper strip from going too deep, but the same can be achieved by careful sizing of the part and the notch that is cut on the bottom to make the solder pin.

To be really useful I need to figure out a way to reproduce these with more precision, though I would recommend not breaking them as the best solution. :) My current thinking is to make a 3D model of a form that can be printed, then the copper strips can be bent to conform to that. Another possibility would be to make a 3D model of the contacts, make a mold from that and then poor in molten copper to make the contacts.

 

Edited by Nelno

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Turns out that a Tech Tangents YouTube channel else has already gone through this process in 2018, made a YouTube video about it, and even released the model files on Thingiverse:
 

 

 

Comparing these to the ones I designed:

- The X/Y inner and outer dimensions are exactly the same.

- The nubs on the bottom are less pronounced, which would make them easier to pull out than mine.

- The inner spring rest is about 1.7 mm lower than mine. This may affect the rebound way the keys feel. I measured this again on the original plungers and it's 10.5 mm from the bottom, the same as my model.

- There's no beveling on the outer corners (I included beveled and unbeleved because it might make difference depending on how your printer slicing software works, and whether you use PLA, PETG or some other material)

- There's a lip on the top that doesn't exist on the original pieces.

- I maintained the split in the pieces, though for PLA parts this can be brittle depending on the quality of your filament. For PETG it helps with flexibility.

- I angled the inner spring rests at 45 degrees on one side so that they print without any supports on any printer than can handle 45 degree overhangs. The Thingiverse version has horizontal overhangs which are unlikely to work at all on my printer. You definitely don't want to have to add and clean up supports on a piece this small.

 

I haven't tried these in a TI, but the Thingiverse link (https://www.thingiverse.com/thing:2871775) does say they are also for TI.

I searched all over for something like this before starting, including on Thingiverse, and never saw this, but from what I have been able to tell, Thingiverse has a terrible search engine.

Anyway, if you're looking to replace your key plungers, you now have two options to try. One or the other may yield better results for your printer.

 

Edited by Nelno
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1 hour ago, Nelno said:

 

- There's a lip on the top that doesn't exist on the original pieces.

 

This 1 mm extra lip is probably an improvement over my design (and the original pieces). First, it makes the part of the plunger holding the key cap, which is a weak point in the original pieces, somewhat stronger. Second, it allows the hole at the top to be beveled, which will make it a bit easier to insert the key caps. Third, it will also act a bit like the optional brim I added to help the pieces stick to the printing surface. Finally, it may prevent the pieces from falling all the way into the waffle frame if you are testing for fit without the spring, though that shouldn't happen if the nubs are large enough, but did happen to me a few times during iteration of my design.

Edited by Nelno
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