Hello all,

I just picked up an HP-35 calculator from a guy that was emptying out his garage and preparing to leave the state. I got it for free (along with some other items). I thought it was interesting, but worthless. Before I threw it in the trash, I decided to check out e-bay and I saw that they are collectible. That's when I stumbled across this informative forum. So.......

I believe it is a "Version 2". It is ser. no. 1143a 3****. It has the raised dot on the 5. It appears to be in good working order and it pretty good condition. It has the original grey case, as well as the charger and the owners manual. Also the leather case. The owners manual also has the accessory order card and the service card, in the back.

Is there anything else that i should check for to see if this unit is special? (I know it's not a "red dot". And the serial number is inside the battery compartment instead of between the feet.) What is the approximate value? (Although I don't think I want to sell it) This is all so interesting.

Maybe I have found myself a new hobby.



David --

Glad to see another person who appreciates the technological history!

My HP-35 has a serial #1143A2nnnn, and is a 2nd version. It says only "HEWLETT-PACKARD" on the bottom edge, not "HEWLETT-PACKARD 35", because it was HP's only calculator model at the time. My unit has the serial number between the feet, however.

The only models that are really "distinctive" to collectors are the early "red dot" units, some of which have a computational bug, where eln 2.02 is calculated as 2.00.

I think that the 4th version is much better aesthetically -- previous versions look almost like prototypes.

Is the section about the HP-35 at this website's homepage helpful?

-- KS


Is there anything else that i should check for to see if this unit is special?

It is an HP-35, the world's first handheld calculator with transcendental functions. It is quite special! Granted, it might be a little more special if it were a red-dot, but it is a version 2, and has the earliest possible serial number prefix. Following-up on something Karl mentioned, early units had a few "bugs" where they did not provide absolutely correct answers to some very specific arguments in some of the functions. As Karl mentioned, taking the natural logarithm of 2.02 and then the antilog of the result returned 2., rather than 2.02. There was also a bug in the trigonometric functions for certain inputs. Some examples were:
Function                Buggy                   Correct        
ARC SIN 0.0002 5.729577893 x 10E-3 0.01145916
ARC COS 0.0002 89.99427042 89.98854
ARC TAN 0.0002 5.729577893 x 10E-3 0.01145916
ARC TAN 1.00020002 45 45.00573
So checking for the above anomalies will tell you if your unit was sent back to HP for repair. If it has the bugs, you have an original, pristine, untouched unit with its “quirks” still intact. If not, you have a unit that the owner went to the trouble to send back, and HP repaired a problem that in the present day would be ignored. Either way, it is special.

Approximate value based on eBay sales would be $40 to $100 (assuming it is not essentially "mint-in-box"). Unless you really need $40 to $100, you should by all means keep it.
Maybe I have found myself a new hobby.

If so, be prepared for quizzical looks from friends and family. Also, beware the slippery slope from hobby to, ...obsession. You will obviously need a version 2 with or without bugs as required to complement the unit you have. Versions 3 and 4 are also a necessity, and a version 1 would be really, really nice. One each of the 45, 55, 65, 70 and 80 to fill out the classic series would also be nice. But wait, the 45 comes in at least three versions, so you’ll need one of each of those.......

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