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28S Heat stake size? (Repairing a mostly working 28)
05-15-2018, 08:27 PM (This post was last modified: 05-16-2018 12:12 AM by wyatt8740.)
Post: #1
28S Heat stake size? (Repairing a mostly working 28)
Hi. I bought myself a HP-28S as my first RPL calculator a while back (with an intact battery door!), and I love it. I'm planning on getting a 48SX or 48GX eventually, but I really, really like the full alphabetic character set on the clamshells, and as an occasional LISP/Forth programmer appreciate the rigorous and consistent RPL approach the 28 takes, spartan as it may be compared to the relative 'luxury' offered by a 48. So I'd like to keep my 28 around. Unfortunately in the last few weeks it's started acting up (arrow keys don't all work unless pressing down on the shell).

I see on the forum archives that at least two people have fixed their calcs with styrene rods and new foam, and I'd like to take that route, but I cannot currently take the stickers off the calculator to measure the rods. The HP journal article on the 28C, which describes the manufacturing process and includes internal photos, does not give measurements on the heat stakes either.

If anyone has one of those rare translucent ones, or a 19BII with the battery cover on the back (you can see the stake holes in the battery bay), or has done this and remembers/can measure, could you tell me approximately how thick the rods I should buy would be? My eyeball guess is around 2mm diameter, but I don't live in a metric system country and only have a vague idea of how big a millimeter actually is.

Also, is the foam replacement a vital part of making the keys work again? Or is it just something people like to get done while they have the calc apart?
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05-16-2018, 03:52 PM (This post was last modified: 05-17-2018 11:52 AM by jebem.)
Post: #2
RE: 28S Heat stake size? (Repairing a mostly working 28)
If you can't contact Geoff Quickfall, try reach this other member at his site as apparently there is also a project for creating a new case from scratch and probably you can get the correct rod sizing there.

That said, sooner or later you will have to open the calculator to fix it, and then you will will be able to take all the measures you need.

(edited to add information)
ok, I took some readings on my 19BII:
base, outer: 5mm or 0.197" or 13/64"
base, inner: 3mm or 0.118 or 1/8"
the rod: 2mm or 0.079" or 5/64"
Some more details here.


edited again for additional information:
Neil Hamilton (Ottawa) member have posted the information you are looking for.
According to him, he used 3/32" styrene tubing instead of rods, and then he realized it was a little bit thicker than required to install them without additional corrective actions, so he used a little bit larger diameter than my measurements above at 5/64.



Please share your findings here, once you have your answers.

Jose Mesquita
RadioMuseum.org member

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