MBO Expert (7C-903)

This MBO Expert was one of the last "four-banger" calculators in decent manufacturing quality during the heights of the Calculator Wars in 1975, before products like the Far East Generic Design I took over.

MBO International Electronic GmbH, Jena was founded 1973 and started with the MBO Junior the production of the first portable calculator in Germany. Later MBO actually placed their nameplates on Far East OEM products.

Dismantling this MBO Expert calculator manufactured in October 1975 in Asia reveals a clean design based on a single-sided printed circuit board (PCB) for the main electronics, a double-sided PCB for the keyboard and powered by a disposable 9 Volts battery.

The Main-PCB is centered around a TMS0833 single-chip calculator circuit manufactured by Texas Instruments and a Bowmar BD5026 Digit Driver chip. The few remaining components on the PCB are mainly used to generate the clock signal for the internal timing of the TMS0833 and an additional transistor for the 8^th digit of the LED display. Well, what we wrote here in September 2022 is not exactly accurate. With the featured MBO Expert calculator being a loaner from our friend Ken, we didn't dig to deep into it. It is neither a transistor, nor has the display only 8 digits. Learn more in our most recent update of this page.

The TMS0833 is closely related to the TMS0800 Product Family and tracing back to the TMS1802NC, the first available standard calculator building block on a chip, later renamed into TMS0102. The TMS0800 kept the size of the Instruction ROM (Read-Only Memory), but decreased the Data Memory from 13 Digits Registers to 11 Digit Registers and added both integrated Segment Drivers for the LED display and a clock generator.

The unusual BD5026 display driver located with this MBO Expert manufactured in October 1975 is a relict of Bowmar's unsuccessful venture as manufacturer of Integrated Circuits before filing in 1976 for bankruptcy.

The display module of the MBO Expert uses nine small LED chips bonded on a substrate with an additional magnifying lens to enlarge the digits.

Update (August 2024): On our quest to Record the ROM Content of the TMS0833 single-chip calculator circuit, we acquired an MBO Expert in Germany (Thank you, Frank!) and studied its electronic circuitry completely. Disassembling the donor calculator with serial number #501250 and manufactured most likely in October 1975, resulted in three surprises:

Calculating Unit: The TMS0833 is a member of the TMS0830 Product Family, introduced in Winter 1974 and closely related to the TMS0800 Product Family, but adopting the low-voltage PMOS process developed for the TMS0950 used with the TI-1200. While the original TMS0800 design is using for V_DD and V_GG two different power supplies of -10.0 V and -15.8 V, respectively, can the TMS0830 be powered directly with a single 9 Volt battery. One key feature of a PMOS process is its threshold voltage for inputs signals, we measured around -2.6 V on a TMS0801 and less than -1.3 V on a TMS0833.

With low-cost battery operated LED calculators in mind, Texas Instruments added a so-called Timeout feature to the TMS0830 devices. When no key presses are detected for about 20 seconds, the display blanks out and shows only a '-' in the leftmost digit to reduce power consumption of the calculator. Looking closely at the printed circuit board (PCB) traces of the donor MBO Expert, you'll recognize that Pin 10 (WDK) and Pin 8 (KN) are connected to effectively disable the Timeout feature. The 2-pin "NPN bipolar junction transistor (BJT)" is acting as a diode to isolate the KN node of the keyboard matrix from the corresponding TMS0833 input pin. Difficult to understand with the mediocre battery life of the MBO Expert.

Display: The MBO Expert donor calculator manufactured around October 1975 makes use of a WB7-44 9-Digit Calculator Numeric 7-Segment LED display module from an unknown manufacturer. It uses nine small 7-Segment displays chips bonded onto a PCB and magnified with a clear plastic lens. The display module is connected with 17 pins to the Main-PCB and follows the pinout of similar displays.

Display Driver: The Main-PCB of the disassembled MBO Expert calculator makes use of a BD5026 9-channel digit driver manufactured by Bowmar for the LED display, while the TMS0833 chip drives the segments directly. Nine digits and a 16-pin package? Nine inputs, nine outputs and two power supply pins - that doesn't fit into a 16-pin package! Perhaps not in Dallas, TX, but obviously in Acton, MA. A closer look at the PCB traces shows only five connections between the TMS0833 single-chip calculator circuit and the BD5026 digit driver. To completely understand the magic of the BD5026, we used a simplified breadboard setup with the chips retrieved from the donor calculator and connected a Logic Analyzer to all input and output pins. Learn more about the innovative "interpolating" digit driver technology developed by Bowmar.

Clock: The MBO Expert makes use of the internal clock oscillator of the TMS0830 chip, we identified a resistor with 200k Ohm connected between Pin 14 (R_EXT/Clock Select) of the TMS0833 and the negative power supply line, resulting in a clock frequency of about of 82 kHz.

Power Supply: The MBO Expert calculator is powered with a disposable 9 Volt battery and three resistors limit the peak currents of the 9-digit LED display:

With all these efforts we expected a very low power consumption of the MBO Expert but measured:

Calculating the power consumption at 9 Volts for the MBO Expert results in about 120 mW displaying a '0.' and about 500 mW with all segments but the minus sign illuminated. A Canon LE-84 calculator using four disposable 1.5 Volt Alkaline batteries and a DC/DC converter for its TMS0801 chip clocks in at around 100 mW and 320 mW, respectively. A fresh 9 Volt Alkaline battery would last around 10 to 30 hours in the MBO Expert calculator, while 4 AA-sized Alkaline batteries would power the Canon LE-84 up to 100 hours. At first glance might these numbers be astonishing, but they are easy to explain with Kirchhoff's second rule (the loop rule). In a closed loop, whatever energy is supplied by a voltage source, the energy must be transferred into other forms by the devices in the loop. Kirchhoff's loop rule states that the algebraic sum of all potential differences, including the voltage sources, in any loop must be zero.

With the numbers from our donor MBO Expert, we calculate in a first step the average current per segment. The differences between the '0.' and '88888888.' displays are 50 segments and 43 mA, or 0.86 mA per segment. The TMS0833 chip is scanning the digits and segments with a 1:10 duty cycle, resulting in 8.6 mA average segment current per Digit Time. Each Digit Time has 11 State Times with S1 and S11 blanking the segments for a peak current of 10.5 mA per segment and around 80 mA per digit. The high-efficiency LED chips used with the calculator have a voltage drop of around 1.7 V at 10 mA segment current, and with a 9 Volt battery the sum of all other components in the loop will "burn" around 7.2 V:

The Canon LE-84 calculator on the other hand is using four 1.5 Volt batteries in series for the LED driver circuitry and consequently needs to "burn" just around 4 V between power supply and LED chips. And the true master is Texas Instruments with its TI-2500-II design, using only two 1.5 Volt batteries and "burning" just 1 V between power supply and LED chips.

Keyboard: The keyboard assembly of the MBO Expert uses spring-supported plastic keys pushing a small copper element against a pin soldered onto a double-sided phenolic PCB combining both long-travel keys with reasonable manufacturing costs while maintaining longevity of the calculator. The keyboard module is connected with 14 short wires to the Main-PCB.

With the DCM-50A Platform developed to Characterize and Reverse-engineer Single-chip Calculator Circuits we could proof that the TMS0833 uses the same Program Code as the TMS0803 known from the TI-1500 or TI-2550-II.

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

© Joerg Woerner, September 6, 2022. No reprints without written permission.