I'm interested in old calculators, particularly scientific and programmable ones. I'm building a collection of various models and types.

If you have an old calculator made in the 1960's, 1970's or 1980's, please don't throw it out! I save, repair, and restore old calculators from this era. Please send me an e-mail if your old calculator needs a new home!


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I collect early nixie tube, electro-flourescent, LED and scientific calculators from nearly any manufacturer. I'm particularly interested in any calculator or computer made by Hewlett-Packard before 1985.

Some of those HP calculators were made well before 1985. For instance, HP's first programmable calculator, the HP-9100A (pictured at right), was introduced in 1968. I would very much like to find 9100A and 9100B calculators, peripherals, manuals and other documentation. You can read more about the 9100 models here.

To the left is an HP-46, the desktop version of the HP-45, HP's second handheld scientific calculator (the HP-35 was the first handheld). The LED display on the HP-46 was an optional feature.

The photo to the left is from a Remington 1259S, which is internally identical to the Casio FX-1. The display is made up of nixie tubes, where each digit is represented by a shaped wire inside a sealed glass tube. I have more information about nixie tubes here.

Even though I'm interested in almost any old calculator, there are some models that I'm specifically looking for. These include:

  • APF Mark 55

  • Busicom 141PF

  • Casio AL1000 (AL-1000)
  • Casio 121-A
  • Casio 121-K
  • Casio AS-C
  • Casio AS-L
  • Casio FX-1
  • Casio FX-3
  • Casio fx 201P
  • Casio fx 202P
  • Casio PRO fx-1

  • Commodore 207CB (electro-mechanical)
  • Commodore M55

  • General Electric CE93

  • Hewlett-Packard HP-01 (wrist watch)
  • Hewlett-Packard HP-10
  • Hewlett-Packard HP-19C
  • Hewlett-Packard HP-55
  • Hewlett-Packard HP-70
  • Hewlett-Packard HP-81
  • Hewlett-Packard HP-91
  • Hewlett-Packard HP-9100A and 9100B
  • Hewlett-Packard HP-9805
  • Hewlett-Packard HP-9830
  • Hewlett-Packard HP-9831

  • Melcor SC-535

  • Omrom 12SR (SR-12)

  • Tektronix 21
  • Tektronix 31

  • Texas Instruments SR-22
  • Texas Instruments SR-60
  • Texas Instruments TI-45
  • Texas Instruments TI-150
  • Texas Instruments TI-450

I'm also looking for maintenance and repair information, especially Service Manuals, for the following machines:

  • Friden 130
  • Rockwell 960

I'd also like to find some specific accessories for the HP-71B handheld calculator/computer:

  • 82441A, FORTH/Assembler
  • 82478A, FORTH/Assembler/Debugger
  • 82478-60001, HP-71 Debugger
  • 82490A, HP-41/HP-71 Translator

Although I'm mostly interested in electronic models, I would also like to find a Curta type 1 or type 2 mechanical calculator.

  Probably the most common question I'm asked is, 'I have this calculator, what is it worth?'

That's not always an easy question to answer. It depends on a number of factors, including:

  • Rarity (rare models are often worth more than common units)
  • Desirability (calculators made by Hewlett-Packard, for instance, are often more prized than from lesser-known companies)
  • Condition (calculators that look and operate 'like new' are worth more than if they are physically or electrically damaged)
  • Accessories (AC adapters, covers, peripherals, and other optional items increase the value of a calculator)
  • Documentation (manuals, schematics, and so on also increase the value)
  • Original box (some collectors want the original packaging as well as the calculator)

Here is some further information about calculators that are commonly requested:

HP-11C Scientific Calculator

I really like the Voyager series of calculators, of which the HP-11C is one. It's programmable, pocket-sized, well laid-out and runs nearly forever on a single set of three A76-sized watch batteries. The value of these models very much depends on their physical condition (most of these models have dings and scratches in the aluminum faceplate) and if they include a case and an instruction manual.

I have documented the disassembly of an 11C here.


Just a quick note on operating the HP-11C - I've gotten this question numerous times regarding the commas and periods in the display. From the manual:

A radix mark is the divider between the integer and fractional portions of a number. A digit separator distinguishes the groups of digits in a large number. In some countries the radix is a decimal point and the digit separator is a comma, while in other countries, the reverse is true. To change the radix/digit separator convention on your HP-11C, turn off the calculator, then hold down the [.] key, turn the calculator back on, and release the [.] key ([.]/[ON]).
So, to summarize:
  1. Turn the calculator off
  2. Hold down the [.] (decimal) key
  3. Turn the calculator on
  4. Release the [.] key
This will allow you to toggle between one setting and the other.


You can reset the HP-11C's memory by following this procedure:

  1. Turn the HP-11C off.
  2. Press and hold the [ON] key.
  3. Press and hold the [-] key.
  4. Release the [ON] key.
  5. Release the [-] key.

The calculator will display Pr Error after this procedure.


If all else fails, this reset procedure is given in the manual:

If it appears that the calculator will not turn on or otherwise is not operating properly, use the following procedures.

For a calculator that does not respond to keystrokes:

  1. Press the [yx] and [ON] keys simultaneously and release them. This will alter the contents of the X-register, so clear the X-register afterward.
  2. If the calculator still does not respond to keystrokes, remove and reinsert the batteries. Make sure that the batteries are properly positioned in the battery compartment. [Editors note: you may need to leave the batteries out of the calculator for a significant period of time, perhaps 24 hours, in order for any residual charge to dissipate.]

For a calculator that does not respond to keystrokes:

  1. With the calculator off, hold down the [ON] key and press [x].
  2. Release the [ON] key, then release the [x] key. This initiates a complete test of the calculator's electronic circuitry. If everything is working correctly, within about 25 seconds (during which the word running flashes) the display should show -8,8,8,8,8,8,8,8,8,8, and all of the status indicators (except the * low-power indicator) should turn on. If the display shows Error 9, goes blank, or otherwise does not show the proper result, the calculator requires service.

HP-15C Scientific Calculator

A more advanced version of the 11C, still in the Voyager series.

There is an electronic petition being "circulated" to gather signatures in an attempt to convince Hewlett-Packard to resume production of the 15C. The original calculators were produced from 1982 to 1989.

You can read about the petition effort at

I'll editorialize here for a moment. The HPQ of today is not the Hewlett-Packard that created these now-legendary calculators. Hewlett-Packard of old was a company of engineers that built products for other engineers with a committment to quality and innovation.

HPQ today has become a commodity, "me too" sales organization. In particular, the new calculator products available from HPQ today don't come near the level of ruggedness, originality, or ease-of-use that the 15C and its siblings represent. Even the classic designs have been debased. For instance, the 12C financial calculator, similar in form and design to the 15C, is still in production in China. The quality of construction is very poor these days -- nowhere near the standards maintained during production in the U.S., Singapore, or Malaysia (all HP-owned facilities). But they're inexpensive, so HPQ continues to buy them from the Chinese-owned factory and sells them based on the reputation gained over the past 20 years.

Hint: If you're looking for a 12C, buy a used one with a serial number that has an 'A' (USA-made), 'S' (Singapore) or 'M' (Malaysia) as the fifth character. These serial numbers will have four numbers, then this place-of-manufacture code, then five numbers.

HP has also lost their corporate memory about their early calculators and computers. Click here to read HPQ's response to my inquiry about the HP 9830 Desktop Computer.

Apart from HPQ, there are various efforts underway to produce scientific Reverse Polish Notation (RPN) calculators, with varying levels of success. One such group can be found at

HP-45 Scientific Calculator

The HP-45, an early scientific LED handheld calculator, has a "hidden" timer function that can be accessed via a specific sequence of keys. You can read more about the feature here.

Casio H-801, Biorhythm calculator

This calculator is still surprisingly popular, despite the fact that biorhythms seemed to be a fad of the 1970's. The calculator itself is the usual clever Casio design, except that the date functions are not Year 2000 compliant.

KOSMOS-1, Biorhythm calculator

Another attempt to cash in on the 1970's biorhythm fad, this calculator computes cycles, critical and mini-critical days, and even has a "compatibility" feature. For those who have asked, I've transcribed the part of the Owner's Manual that addresses the biorhythm features here.


The following table of calculators that use Reverse Polish Notation is taken from the February 1978 issue of Ham Radio magazine.

Manufacturer   Model   Stack Of   Scientific
  Number of
  Number of
  R↔P   D.MS
Hewlett-Packard HP-19C 4 yes 30 98 yes yes
Hewlett-Packard HP-21 4 yes 1 0 yes no
Hewlett-Packard HP-22 4 yes 10+5 0 no no
Hewlett-Packard HP-25 4 yes 8 49 yes yes
Hewlett-Packard HP-25C 4 yes 8 49 yes yes
Hewlett-Packard HP-27 4 yes 10+5 0 yes yes
Hewlett-Packard HP-29C 4 yes 30 98 yes yes
Hewlett-Packard HP-35 4 yes 1 0 no no
Hewlett-Packard HP-45 4 yes 9 0 yes yes
Hewlett-Packard HP-55 4 yes 20 49 yes yes
Hewlett-Packard HP-65 4 yes 9 100 yes yes
Hewlett-Packard HP-67 4 yes 26 224 yes yes
Hewlett-Packard HP-70 4 yes 2+5 0 no no
Hewlett-Packard HP-80 4 yes 1 0 no no
Hewlett-Packard HP-91 4 yes 16 0 yes yes
Hewlett-Packard HP-92 4 yes 30 0 no no
Hewlett-Packard HP-97 4 yes 26 224 yes yes
National Semiconductor Novus PR4515/4615 3 no 1 0 no no
National Semiconductor Novus 4520 4 yes 1 0 no no
National Semiconductor Novus PR4525 4 yes 1 100 no no
National Semiconductor Novus NS 4640 4 yes 3 0 yes yes
Corvus 500 4 yes 9 0 yes no
APF Mark 55 4 yes 9 0 yes no
Omron 12-SR 4 yes 9 0 yes no


I'm also building up a collection of books, manuals, and pamphlets related to calculators from the 1970's and 1980's.

As with calculator and computer hardware, I'm interested in operating and programming manuals for these early calculators and computers, as well as books, pamphlets and other related materials.

The image to the right is the cover of Electronic Calculators, a 1974 book written by the president of MITS -- the company that had a series of calculator kits and would later go on to offer the Altair computer kit.

You can see some information related to MITS calculators here.

Besides operating and service manuals, I'm also looking for the following books:

  • Advanced Programming Tips for the HP-41 by McCornack
  • Programming the HP-41C/CV/CX by Thomas Adams
  • The Basic HP-71 by Richard Harvey

I'd also like to find copies of various Hewlett-Packard publications, including any issues of Personal Calculator Digest and Key Notes.

Publications from calculator "clubs" are also of interest, including PPC Calculator Journal.


I came across a CT5002 calculator chip and have started to collect information about it, which you can see here. I've also managed to find a few other early calculator chips:

Qty Chip
1 CT5001
1 CT5002
2 CT5005
5 Mostek MK5012P

TI awarded Japanese patent for miniature calculator

In August 1978, over the objections of several Japanese calculator companies, the Japanese Patent Office issued a patent to Texas Instruments covering the manufacture of practically all minicalculators incorporating their circuitry in a single IC. The Japanese patent is based on U.S. patent 3,819,921 originally filed in 1967 and granted TI in 1974.

The calculator described in the 1967 patent was the first to provide the compu- tational capability of larger machines. The heart of the TI minicalculator was an integrated circuit that performed four basic arithmetic functions. It measured 4 1/4 X 6 1/2 X 1 3/4 inches, and featured a small 18-pushbutton keyboard and a thermal printer to display up to 12 digits. Nineteen other countries to date have Issued patents to TI for manufacturing minicalculators.

Thinking Computers? Think Small, Radio-Electronics magazine, May 1967

An Electronic Desk Calculator You Can Build, Popular Electronics magazine, November 1971

Calculator Developments at the end of 1974, IEEE Spectrum magazine, April 1975

RPN Calculators in 1975, IEEE Spectrum magazine, April 1975

It's interesting to see how prices have changed on these calculators. To the right is an advertisement that ran in the magazine Popular Science in late 1977 (click on the ad to see a full-size version).

HP-10 $139
HP-19C $275
HP-21 $64
HP-22 $100
HP-25 $100
HP-25C $128
HP-27 $140
HP-29C $156
HP-67 $360
HP-80 $235
HP-91 $260
HP-92 $500
HP-97 $599

Also interesting (to me, at least) are the calculators that "didn't make it." There were a lot of unusual designs from manufacturers that faded from view.


An advertisement for a Cogito that used a cathode ray tube (CRT) as a display. This ad is from the May 1966 issue of Scientific American magazine.


An advertisement for a Dietzgen. The company was better known for their mechanical slide rules than their electronic calculators. This ad is from the February 1973 issue of Scientific American magazine.


[From the July 1975 issue of Radio-Electronics magazine:]

Worldwide calculator production totaled about 34 million units in 1974 and it will rise to more than 92 million in 1978, according to forecasts by Coleman & Co., a New York broker. Coleman predicts that the average factory price of all calculators will decline by 1978 from today's $36.56 to $22.39. Hand-held consumer calculators now average $26.12 at the factory and will drop to $11.40 in the same period.

L. J. Sevin, president of Mostek, a calculator manufacturing firm, warned American manufacturers that the calculator could go the way of the transistor radio - to the Far East. Before the end of this year, he said, Oriental assemblers may be able to put together six-digit calculators for considerably under five dollars. He gave this breakdown of costs: Vacuum-flouorescent display, 90¢; 4-function chip, $1.30; keyboard, 60¢; battery, 8¢; case, 20¢; labor, 13¢; packaging and instructions, 16¢.

I have a number of calculators that need some type of repair to get them back into operating condition. Many handheld machines are in the repair pile because the former owner failed to remove the batteries, leading to corrosion on battery terminals.

A list of machines needing repair and a brief summary of the problem(s) is available here.

I have documented the disassembly of an HP-11C here.

Along the way I've managed to acquire duplicates of many models. In general I am open to trades and/or sales of these units.

Comments to Dan Veeneman

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Last updated May 25, 2020