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Master Model H-1 Desktop Calculator

Updated 1/9/2017

This old calculator appears to be quite unique and unusual. It was found by my aunt at a Seattle-area flea market. She paid $0.50 for it! The machine is from the mid-1972 timeframe, though based on the technology in the machine, it was likely introduced sometime in the late 1970 to early 1971 timeframe. It was made by Master Calculator Co., a division of 6/C Inc, of Grand Prarie, TX. If anyone has information about Master Calculator Company, 6/C Inc., or the Master II calculator, please contact the museum by visiting the EMail page. It is hoped that someone associated with either Master Calculator Co. or 6/C Inc. will find this website and contact the museum and be able to fill in details about these mysterious companies.

The SCM F-16

This calculator was also marketed by Smith Corona Marchant(SCM) as the SCM F-16. It isn't clear what kind of business arrangement was made for SCM to market the F-16. It could be that an OEM agreement was formed between Master Calculator Co. and Smith Corona Marchant, whereby Master Calculator Co. built the machines and shipped completed calculators minus badging and serial number tags to SCM, for them to brand, market, sell, and support under the SCM name. Or, it could possibly have been the other way around, with SCM designing and manufacturing the machine, and Master Calculator Co. simply putting their branding on the machine. This relationship will likely remain a mystery unless someone out there can shed some light on the situation.

Master calculator company marketed a follow-on machine to the H-1, called the Master II (on the front panel) and Model AAA-1 on the serial number tag, utilizing newer IC technology to shrink the number of Large Scale Integration (LSI) chips from four to two. The Master II is has a twelve digit display, using a seven-segment gas-discharge display technology similar to that of the Master H-1. The Master II is a fixed-decimal point calculator, with a large decimal point selection dial on the front panel providing settings for 0, 1, 2, 3, 4, 5 and 6 digits behind the decimal. The Master II also has a single accumulating memory register, with a Light Emitting Diode (LED) situated to the right end of the display to indicate when the memory has non-zero content. The machine uses a two-chip Mostek-made chipset -- the PMOS-technology MK5013 and MK5014, which were part of a three-chip set known as Mostek's R1200 chipset. The MK5015 was the third chip in the set, which provided an interface to a drum-type impact printer for printing calculators. These chips were introduced sometime around mid-1971, which probably means that the Master II model was introduced sometime in the late 1971 to early 1972 timeframe. The Old Calculator museum has recently acquired a Master II calculator, and an exhibit should be prepared for it soon.

Inside the Master H-1

The Master H-1 is a rather unusual four-function desk calculator. It has a full 16-digit capacity, but only has an 8-digit display. A special key on the keyboard (a double-ended arrow, e.g. [<->]) toggles the display back and forth on each press between the most significant and least significant eight digits of the 16-digit number. It is possible to enter numbers larger than the eight digits on the display, however, when entering numbers from the keyboard, the [<->] key doesn't work...you end up entering numbers larger than 8 digits blindly. The [<->] key only works with results in the display.

The H-1 uses fixed decimal point logic, and it took a while to figure out how to set the location of the decimal point. There is no dial, switch, or other obvious selector to tell the machine where the decimal point should be located. When the machine is powered up, the machine is set to no digits behind the decimal point, making the machine effectively an integer-only calculator. After poking around for quite some time, I found that pressing and holding the [CE] key, while at the same time, pressing a digit from 0 to 7 on the keyboard, sets the fixed decimal point location. This is a very similar method of fixed decimal location to the Marchant Cogito 412 and Cogito 414 calculators.

Detailed view of Master H-1 Circuitry

The Master H-1 is based on a four-chip LSI chipset made by Electronic Arrays, Inc., of Mt. View, California. EA no longer exists, having been bought out by NEC after falling upon hard times in the late 1970's due to extreme competition in the memory chip marketplace. The chips are numbered "190B-7010", "280B-7008", "310B-7014" (Input Chip), and "150B-5005" (Output Chip). All of these IC's have date codes with the earliest being 7148, and the latest 7201. This chipset appears to be an updated version of Electronic Arrays' first calculator chipset, used in another machine in the museum, the ICM 816. The LSI's are all in 24-pin plastic packages. A National Semiconductor DM8880 chip performs display drive functions. The guts of the machine (except the power supply transformer) are mounted on a double-sided fiberglass circuit board, which uses plated-through holes for feedthroughs. The power supply is a simple transistor-regulated linear supply. The keyboard uses individual key modules which contain magnet-activated micro-switches. The keycaps are molded plastic with printed-on (rather than molded in) keycap legends. Painted legends on keycaps was less-expensive than the double-shot injection moulding required to embed the keycap legends in the key. Painted-on legends tend to wear off over time, while molded-in legends held up much better. Given the high quality of the keyswitch modules used, it is surprising that the manufacturer went cheap on the keycaps.

The Display Module

The H-1 uses a neon-gas discharge display, similar to a Burroughs Panaplex, but it is definitely not made by Burroughs. The Burroughs displays have transparent electrodes deposited on the glass plates, while the display module used in the H-1 uses a metal mesh screen for the digit grid, and metal electrodes for the segments. The display is arranged as seven-segment digits, with a decimal point to the right of each digit. The display is driven by a combination of the forementioned National DM8880 IC, and discrete transistor drivers. The calculator does not provide leading or trailing zero suppression. Negative results are indicated by a very early LED situated to the left of the display. The display digits glows orange, however, the case has a red filter which is situated in front of the display modules, making the digits look very much like red LED digits rather than gas-discharge digits.

Display showing most significant and least significant digits of result of 99999999 X 99999999

The Master H-1 is rather fast machine for its day. Considering that it calculates to a full 16-digits of capacity, it is actually very fast, especially given that it is based on a fairly early MOS LSI chipset, which typically were somewhat slower than calculators made with bipolar integrated circuits. Sixteen 9's divided by 1 takes less than 1/3 second to complete. Eight 9's times itself results in an almost instantaneous answer, with the most significant digits displayed by default (with no decimal point present), and the least significant digits being displayed by pressing the [<->] key, as shown in the images above.


This exhibit is dedicated to the memory of my Auntie Dyann, who found this calculator for me. She was an amazing woman, and a wonderful aunt who treated me so well. I miss her so much.

Thanks to LaFarr Stuart for information on Electronic Arrays

Text and images Copyright ©1997-2017, Rick Bensene.