HP 35

HP 35

This HP 35 was the first scientific pocket calculator and was introduced in 1972.  It stayed in production until 1975 and retailed at a price of $395.00. The 35 was allegedly a hack by an HP engineer of the HP 9100 which was a large desktop calculator with a price of over $1000.00. The engineer shrunk the 9100 down and incorporated a number of trigonometric and exponential functions that no other “pocket” calculator. In fact, all pocket calculators at the time were just four function calculators.

Other engineers at HP liked the 12 prototypes that were built so HP decided to make it a full fledged product. It was named the HP-35 because it has 35 keys. Early versions of the 35 only had the Hewlett Packard name on the faceplate but later versions added “35” to the name. It took two years and about one million dollars to develop.

HP 35 Case

The HP-35 utilizes a 15 digit LED display that would illuminate each LED segment separately as opposed to the entire digit. It is 5.8 inches long and 3.2 inches wide which is just right to fit in a shirt pocket. HP didn’t originally feel that there was enough demand to produce the calculator at the nearly $400.00 price point. But, in just the first year they sold over 100,000 units and sold over 300,000 during its production life. HP even made the HP 35s in 2007 which is a retro model paying homage to the original 35.

HP 35 Battery Compartment

Obviously Bill Hewlett’s vision and ability to go against conventional marketing wisdom at the time led to a revolution in calculating that is still with us today.

One bug in early HP-35’s was 2.02 ln e^x which gave the result 2 as opposed to the correct 2.02. 25,000 35’s had been sold by that time and HP offered those customers replacements of which only about 25% returned theirs.

HP 35 Main Board

As I mentioned before there were basically four production differences over the life of the 35. The first production run had a small red dot to the right of the on/off switch that would show when the calculator was turned on and did not have the “35” designation on the label.

The second version got rid of the red dot.  The third version had the “35” designation. The fourth version had all the function notation molded into the keys as opposed to printed on the calculator body. The list below shows the 4 production variations as well as two prototype versions.

HP 35 Label
  • Prototype version 1: beige case, no label, yellow keys for basic functions.
  • Prototype version 2: same as first version except with a black case.
  • Production Version 1: no “35” designation, small red dot, letters on the face .
  • Production Version 2: no “35” designation, no red dot, letters on face.
  • Production Version 3: added “35” designation, no red dot, letters on face.
  • Production Version 4: “35” designation, no red dot, letters molded into keys.
HP Prototype
HP 35 Red Dot

Obviously the prototypes are nearly impossible to find.  The first version with the red dot is very difficult to find as well so if you see one scoop it up. The others aren’t too difficult to find since so many were made and built to last. The red dot version sells for upward of $1000.00 in excellent functioning condition while the rest sell for around $75.00 to $300.00 depending upon the accessories included.

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HP 65

HP 65

This fairly hard to find Hewlett Packard made HP 65 calculator was produced from 1974 up until 1977 and retailed for $795.00.  It was HP’s first programmable pocket calculator and had a built in magnetic card reader – writer.  It featured a single line red LED display and used RPN logic. It was also HP’s third scientific calculator. It is known for being used in 1975 by the astronauts of Apollo to make calculations that helped them dock with the Soviet space station Soyuz. The HP-65’s were used as backups in case the main computer system on the Apollo went down.

The HP65 was the first appearance of the “Tall” keys where each key had the capability of implementing 4 functions.  As you can see, each key has a function on its face, one listed above the key as well as one on the forward face of the key. By pressing the yellow second function key the user could access the inverse of the function listed above the key.

HP 65

HP made a subtle design change with the HP 65 as well. As you can see in the picture below the older HP-35 on the left has a silver trim line going under the display and above the on/off switch. This silver trim would wear quickly, as it has on this calculator, when the calculator was turned on and off over time. So, with the HP 65 the trim was repositioned around the top of the display and away from the on/off switch. This kept the calculator looking newer longer.


The HP-65 could read from and write to magnetic cards.  So, the user could input their own programs, of up to 100 lines, into the calculator and save them to the magnetic card. They could also use pre-programmed magnetic cards that came in what HP called “Pacs”.  The HP 65 came with a “Standard Pac” of 40 magnetic cards that contained 19 programs. There were also additional “Pacs” available including:

Aviation Pac 1                  Chemical Engineering Pac 1
EE Pac 1                          EE Pac 2
Finance Pac 1                  Machine Design Pac 1
Math Pac 1                      Math Pac 2
Medical Pac 1                   Navigation Pac 1
Stat Pac 1                       Stat Pac 2
Stress Analysis Pac 1       Surveying Pac 1

HP 65 Pac

The HP 65 came with a hard plastic case for the calculator and accessories as well as a soft leather case for the HP 65 itself.  Accessories included the HP-65 reference guide, HP-65 Owners Handbook, Power supply, and Standard Pac magnetic cards.

HP 65 Box

The fully functional HP-65 is a difficult calculator to find and prices reflect that. Many times the card reader does not work as the reading mechanism gets gummed up over the years. Still, a fully functional HP-65 will sell for around $400.00 alone. Throw in the original case and accessories and the price can reach $700.00

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HP 70

HP 70

This HP 70 is a fairly difficult calculator to find in good working condition. It is a business calculator that Hewlett Packard offered as a lower end choice to the HP 80.  As you can see it has a unique color scheme with the orange keys and green/gray case.  It also does not have a shift key and the “Save” button was relabeled to “Enter”.  The HP 80 has two memory positions entered with the STO key followed by either the K or M buttons.  The HP 70 lasted only a short time on the market, hence its relative scarcity, and was supplanted just a year later by the HP 22 which had more features and was cheaper.

HP 70

Introduced in 1974 the HP 70 utilizes a LED display and RPN logic.  The top row of buttons are time value of money functions and it has 2 memory registers.  There was memory storage addition to the M register but not the K register.

As mentioned above the HP 70 was not on the market for very long which translates into few units produced and even fewer remaining today. So, collectors will pay a high price for it.  Sales in the $300-$400 range are not uncommon and can go higher if the condition is excellent and all accessories are included.

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