HP 55 and HP 45 restorations

Just finished buying an HP 45 for parts but ended up restoring it. I will post pictures probably tomorrow of the dis-assembly, restoration and assembly, but I do have a question.?

What was HP thinking when it produced the HP 45 with an extremely inaccurate timing function?

Mine is 10 seconds slow per minute. I know I could calculate a correction factor to multiply the resulting times to correct for the tardiness but just what was the timing intended for? Okay thats two questions related!

PS I will be converting it to a quartz crystal for the fun of it and for an accurate timing circuit.

Cheers, Geoff


The 'Timing Function' in the HP45 was sort of a 'Hardware Easter Egg'. It was not intended to be used; it is NOT documented in the official HP45 documentation. The function was experimental and not quite production ready, but the design engineers wanted to leave access to it in the final machine, planning on having it finalized for the next, newer model, with of course the proper crystal. This was then (obviously) released in the HP55.

They are fine machines. Good luck with your restoration! Looking forward to the pictures.


ps: See some details here:


Edited: 18 Jan 2008, 9:44 a.m.


My first HP calculator was the HP-55 which appealed to me as an engineering student. The timer, 20-memory register, and even the limited programming steps were all well used. The built-in unit conversion and regression stats were also a big plus. I chose this machine over the HP-65 because of all the above built-in features.

I remember using the timer (and storing time values) in studying the precipitation of chalk (I had a little lab at home).



Thanks for the response, check out the pictures at the HP 45 restoration post above

Cheers, geoff



I bought an HP-55 for exactly the same reason, but instead of the HP-65--which I couldn't afford--my other choice was the HP-25. Also, I was a senior in high school at the time and getting ready to move to Fresno State to major in civil engineering. In fact, I and a friend who piggybacked a purchase of an HP-25 on my order were the first two students at our high school to obtain programable calculators, thus starting the geek revolution at our alma mater. BTW, my friend is just finishing up a PhD in Computer Science.


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