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My approach is not 100% sure: Use a synthetic tone that leaves the beeper in "buz mode". Halfnuts tend to "buz" louder. But you know the old surgeon saying: "Open it if you want to see clear."
Ciao.....Mike
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Joined: Jan 1970
1) No HP41 of any sort actually made as early as 1980 was built as a halfnut. That didn't happen for several years afterward. The orignal HP41s from 1980 would all have been fullnuts. There's just no way around this. HP did not change the HP41 manufacturing process that early. Period. :-)
2) If you actually HAVE an HP41C with a 1980 S/N on the back and a halfnut front / display, then it must be the result of either a) some sort of repair by HP where they replaced the front half of the calculator and left the back side of the calculator alone, or b) some sort of swap by a user/owner trying to get a calculator working again.
3) One way to test would be to take the back of the calculator off and take digital pictures to post and let some of us look at it. That would help determine the true inside nature of the beast.
4) Another way to test would be to load up the Sandbox ROM onto a Clonix / No-VRAM module (what? you don't have one of those?) and try out the lowercase display characters.
My feeling is that it is probably a result of one of the things listed in #2 above.
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According to Richard Nelson's and Jeremy Smith's article entitled "HALFNUT-- AN INTERNALLY IMPROVED HP-41C,CV,CX" in the CHHU Chronicle V2N4 Jul/Aug '85, pages 9-11, HP started the Halfnut project in mid-83. At the time the article was written, it was expected that cv and cx halfnuts would start shipping to dealers in Sep '85, and that the 41c Halfnut would only be used for service of returned 41c's. So If you have a real 41c Halfnut, it would have to be from having been serviced after the older stock (Coconuts) had already been used up. BTW, if a 41 is not a Halfnut, it's a Coconut, not a Fullnut, according to HP's internal project names.