Caltronic Model 912

Harber Industries Limited in Hong Kong entered the calculator business in 1972 with the Caltronic 812 and soon offered its product line as white-label products to OEMs (Original Equipment Manufacturer) like Quelle and Neckermann in Germany and UK-based Plustronics Ltd. Although total sales approached one million calculators in 1973, the business was not sustainable for the contract manufacturer, Coltronics, a subsidiary of Cable & Wireless Ltd. in Hong Kong, which discontinued calculator production in 1974.

As implied by its model number, 812, the Caltronic 812 is built around a calculator chip with 12-digit internal precision but an 8-digit LED display. The discrepancy between internal precision and visible output results in several surprising operating characteristics (Examples shown with Decimal Point switch set to [0 2]:

Introduced in September 1972, at the dawn of the Calculator War, the Caltronic 812 came under intense cost-reduction pressure. As a result, Coltronics implemented numerous design and manufacturing changes throughout the calculator's production life:

In addition, Caltronic introduced with the Models 606 and 612 two variations using 6-digit display and replacing the Texas Instruments Klixon keyboards to further reduce manufacturing costs.

We acquired this Caltronic 912 calculator together with three other calculators on our quest to untangle the relationships between the MK6010L, MK5012, MK5010, and Cal-Tex CT5002 single-chip calculator circuits. Each calculator went through our "Teardown Treatment", Characterization of Single-Chip Calculator Circuits and De-capping and Chip-Photography:

In addition did we acquire multiple Caltronic calculators and their OEM products manufactured for Neckermann and Quelle (Privileg brand) to understand the various cost-reduction measures and hoping to discover the rumored Cal-Tex CT5012 chip.

Using the Mostek MK5012 calculator brain instead of the Cal-Tex CT5002 used in the original Caltronic 812 results in minor changes to the calculator's overflow indication:

Dismantling the featured Caltronic 912 calculator manufactured in February 1973 in Hong Kong reveals a completely revised internal construction compared with the original Caltronic 812, replacing the connector between its Main-PCB, the Display-PCB and the Keyboard Assembly with short wires:

To gain some knowledge about the differences between the MK5012 located in this Caltronic 912 and the Cal-Tex CT5002 used with the Caltronic 812, we decided here at the Datamath Calculator Museum to give it a "Teardown Treatment" and sharing our findings accordingly.

Calculating Unit: When Mostek introduced in November 1970 with the MK6010 the World’s first "single-chip calculator circuit", it was sold exclusively to Nippon Calculating Machine Corp of Japan for their Busicom Model 120-DM small desktop calculator, also known as Busicom Junior, and not available to other customers. Busicom recognized the potential of Mostek's ion-implantation process, which led to the introduction of the low-voltage, low-power MK6010L in February 1971. The first application of the MK6010L was a remarkably compact, battery-operated handheld calculator with LED displays - quite the opposite of the original MK6010, which had been designed for a large, AC-powered desktop calculator with a VFD. Although Busicom succeeded in squeezing the required DC/DC converter and LED drivers into an exceptionally compact housing, the manufacturing costs of the LE-120A and its successor, the LE-120S, remained prohibitively high and the company started early in 1972 to struggle financially. Busicom decided to forfeit its right of exclusivity, resulting in Mostek rebranding the MK6010 to MK5011 and its low power sibling MK6010L to MK5012.

Display: The featured Caltronic 912 calculator, manufactured in February 1973, utilizes a 9-digit LED display assembly composed of three Litronix DL-33 3-digit seven-segment LED displays housed in 12-pin packages and soldered onto the double-sided Display-PCB. In addition to the numeric displays, the Display-PCB incorporates an indicator LEDs for low-battery status. The Display-PCB is connected to the Main-PCB of the Caltronic 9812 with 21 hand-soldered short wires.

Display Driver: The MK5012 single-chip calculator circuit and its companion, the MK6010L, originated from the architecture of the Busicom Model 120-DM desktop calculator, better known as the Busicom Junior. This calculator was built from 22 JMOS integrated circuits and featured low-voltage vacuum fluorescent displays (VFDs). To facilitate display interfacing, the designers chose a negative-logic configuration for the digit and segment outputs, enabling straightforward drive circuitry based on discrete transistors. This design philosophy was carried over to the MK6010 and MK6010L. Consequently, both devices provide negative-logic digit and segment outputs, requiring PNP bipolar junction transistors (BJTs) to drive the anodes and cathodes of the 9-digit LED display. The necessary discrete transistors are arranged on the Main-PCB of the Caltronic 912 calculator.

Clock: The MK5012 single-chip calculator circuit used in the Caltronic 912 operates from a two-phase clock running at approximately 20 to 25 kHz. The clock signal is generated by an astable multivibrator consisting of four BJTs, which is located on the calculator's Main-PCB.

Power Supply: The Caltronic 912 calculator is powered by a disposable 9 V alkaline battery. A switched-capacitor DC/DC converters generates an additional supply voltage required by the MK5012 calculator chip:

We measured the operating current of the featured Caltronic 912 calculator for four different cases:

Calculating the power consumption at 9 Volts for the Caltronic 912 results in about 350 mW displaying a '0.' and about 500 mW with all segments illuminated, rather disappointing high numbers for an LED-based calculator designed in 1972. An earlier Caltronic 812, based on Cal-Tex' CT5002, clocked in at 120 mW and 250 mW, respectively - clearly demonstrating the optimization of the CT5002 for LED-based, battery-operated calculators.

Keyboard: The Caltronic 912 uses a Klixon™ 1KS116 keyboard assembly consisting of the "Basic 1KS" keyboard array with 17 snap-action Klixon discs mounted on a printed circuit board for the interconnections of the switches to the external calculator circuit and a stainless steel mounting frame and the molded keytops. In addition, the keyboard assembly integrates two two-position slide switches: one for selecting the decimal-point position and the other for switching the calculator power on and off. The keyboard assembly is connected to the Main-PCB of the Caltronic 912 with 22 hand-soldered short wires.

Here at the Datamath Calculator Museum we use the DCM-50A Platform to Characterize and Reverse-engineer Single-chip Calculator Circuits. Many designs of electronic calculators do not use all features of their calculator brains and it would be difficult to unleash the full potential of the calculator chips in these cases. Additionally are electronic calculators "closed systems" with limited flexibility to measure signals, change voltages or clock frequencies, provide additional input keys or even change the display technology or specifications additional digits. Core idea of the DCM-50A is providing a generic platform to access all features of a single-chip calculator circuit and with the DCM-50A (PLAYGROUND) we increased the scope from Texas Instruments products to offerings from their competitors in the 1970s, namely AMI, Cal-Tex, Commodore/MOS Technology, Electronic Arrays, General Instrument, Hitachi, Litronix, Matsushita, Mitsubishi, Mostek, National Semiconductor, NEC, Omron, RFT, Rockwell, Sharp, Toshiba, and Western Digital.

On our quest to document Mostek's MK6010 Chip and its many descendants like the MK5010, MK5011, MK5012 and Cal-Tex' CT5001, CT5002, and CT5012, we developed here at the Datamath Calculator Museum three additional tools for our DCM-50A (PLAYGROUND):

Comparing the Calculator Logic Implementation of the MK5012 chip used with the featured Caltronic 912 calculator with the Calculator Logic Implementation of the CT5002 retrieved from a Caltronic 812 calculator reveals only subtle differences: An Entry or Calculating Overflow is signaled with the MK5012 showing '0.0.0.0.0.0.0.0.0.0.0.0', while the CT5002 is showing '00000000.0.0.0.0.'. Pressing the [C] or [CE] key lit up the display with all 12 digits showing '888888888888' with the MK5012, while the CT5002 is showing '000000000000'.

In a next step, we decided to "decap" the CT5002 and MK5012 chips salvaged from the Caltonic 812 and Caltronic 912 calculators and asked Sean Riddle to provide us with high-resolution images of the silicon dies.

First surprise: The silicon die inside the MK5012 package is marked with MK6015 and bears the NCM logo.

Second surprise: The MK6015 (MK6010L/MK5012) chip looks almost identical to the CT5002 chip.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, July 4, 2026. No reprints without written permission.