Model 1000

If we trace back the history of Melcor we locate the rare Mariner NRC 7200 as starting point of their calculator business. The NRC 7200 was introduced already in December 1971 and looks similar to the Bowmar 901B. Both designs used mainly technology and components from Texas Instruments, e.g. the calculator chip, keyboard, display drivers and even the display. In 1972 TI started their manufacturing of calculators and consequently changed their perspective for Bowmar and Melcor from component manufacturing to competitor. As a logical result a lot of "TI-free" calculators appeared. This Model 1000 uses:

Dismantling this Melcor Model 1000 calculator manufactured in October 1973 in the United States reveals a very innovative design using a single-sided printed circuit board (PCB) centered around an AMI S2144 single-chip calculator circuit connected to a 9-digit LED display, a keyboard assembly and powered by a 9V alkaline battery. A very prominent feature of the Melcor Model 1000 calculator are the two [ON] and [OFF] buttons instead the common sliding or rocker switches found in other calculators designed in 1973 to turn the calculator on and off.

The featured Model 1000 calculator uses a Nine-Digit Calculator Numeric Seven-Segment LED Displays manufactured by an unknown OEM and mounted directly on the Main-PCB and controlled with two integrated digit drivers and the 8 segments driven directly from the AMI S2144 calculator chip. The other components on the PCB are used for the power supply of the S2144, its clock generator, detection of a low-voltage situation of the 9V battery and the already mentioned electronic power on/off-circuitry. And in case you wonder about the unusual [PD] key, it is used to set the calculator chip either in fixed-point [DP][0...7] or floating-point [DP][.] mode for the number display.

On our quest to fully understand the features of the rare AMI S2144 chip used with only a few other known calculators like the RFT minirex 74, we decided to completely reverse-engineer the PCB of the Melcor Model 1000 on a component level. One of the interesting findings is the use of two different, yet pin-compatible "SN75492-style" 6-channel digit drivers for the LED display and we learned that the ITT 492 driver at position A2 controls the five right-most digits while the Fairchild SL23557 driver at position A3 controls the four left-most digits and has one of the remaining input pins connected to the center tap of a potentiometer and the corresponding output connected to an input pin of the AMI S2144. Obviously not the intended use of a "digit driver" for LED displays and probably the reason to use the SL23557 chip in position A3. Evaluating the functionality of the PCB schematics reveals that this part of the circuitry is used as a simple "under-voltage" detection of the 9V battery signaled by the S2144 on the left-most decimal point of the display.

Update (August 2024): We measured the Output Voltage vs Input Voltage of both the Fairchild SL23557 and ITT 492 devices to retrieve their "Signature" and noticed that the SL23557 has a lower threshold voltage and a higher gain than the ITT 492.

Understanding the schematic diagrams of both the Melcor Model 1000 and an RFT minirex 74 calculator demonstrated the compatibility of the AMI S2144 with our DCM-50A Platform and we were able to operate it in the right-most TMS1000 Textool Test Socket with some extra patches, hence allowing us to go through our process of Characterization of Single-Chip Calculator Circuits.

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

© Joerg Woerner, February 5, 2023. No reprints without written permission.