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After about one hour span time since surgery room preparation to the few minutes post-surgery set of severe tests at the Intense Care Unit, the HP25C Serial # 1511Axxxxx recovered its full operating capabilities. Unfortunately, donator HP25 (ID not revealed so far) is kept frozen for later replacement as it becomes available.
Thanks to specialist advisors Ellis Easley, David Smith amongst intl. cooperators around the world not mentioned here because of a small lack of information, the offended ROM was identified as being the one at the left side of the ACT chip. A brief report mentions that the removed ROM was also tested at the donator's "guts", which showed the same behavior observed in the HP25C before transplantation: a blank display followed by a complete set of "zeroes" after any key pressed except number keys, [STO], [RCL], [f] or [g] and full operating RAM, registers and program steps. This ROM (1818-0154) is definitely toasted.
Thank you all for your attention and patience... reading this silliness.
Luiz C. Vieira - Brazil.
(This story is based in actual facts; original identity of both calculators was not revealed to respect their privacy)