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Joined: Jan 1970
Some time ago (4 months?). A message was posted about taking apart and repairing an HP42 by using bits from an HP17Bii. The posting indicated that the internals are almost the same. You can pick these up second hand on ebay, if you can find one that has a bad keyboard so much the better.
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From the Museum's main page, there's a "Repairs and Batteries" link. From there, a "Pioneer Repairs" link. That leads to my article posted here at http://www.hpmuseum.org/guest/brogpion.htm .
The 42S should be repairable. Move slowly -- ESPECIALLY in removing a good LCD -- they're very touchy, but it can be done. (Practice on a less-valuable unit -- an HP-20S should be good for that.)
Good luck, and contact me with any questions. (I'm on vacation for a couple of weeks, but will answer.)