28S Heat stake size? (Repairing a mostly working 28)

[wyatt8740]

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?