Prinztronic Asset

Dixons was founded 1937 by Charles Kalms in Southend, United Kingdom as photographic studio but started soon their own mail order division and sold in the 1970s a large variety of desktop and handheld calculators under their Prinztronic brand. This Prinztronic Asset caught our attention when started to research early single-chip calculator circuits manufactured by NEC (Nippon Electric Company) Corporation of Japan and we acquired the featured, non-working calculator in April 2025 from a seller in the United Kingdom.

To fully understand its design centered around a µPD274 chip, we decided here at the Datamath Calculator Museum to give the featured calculator a full "Teardown Treatment" and share the findings of our 4-step approach accordingly:

Step 1: Dismantling the featured Asset calculator manufactured in January 1975 by an unknown Original Equipment Manufacturer (OEM) in Taiwan reveals a cost effective design using a single-sided printed circuit board (PCB) for the main electronics and powered by four disposable 1.5 Volts batteries or a 6-Volt DC power adapter The Main-PCB is centered around a µPD274 single-chip calculator circuit manufactured by NEC and the few other remaining components on the PCB are mainly used to generate the different supply voltages for the µPD274 and Vacuum Fluorescent Display (VFD) and to bias the anodes and grids of the display with respect to its filament.

The µPD274 located in the featured calculator is the direct successor of the µPD273, considered NEC's first single-chip calculator circuit with 8-digit display capability and very basic functionality. The short-lived µPD273 was following the original µPD271 and its low-voltage sibling µPD272. Here at the Datamath Calculator Museum we don't qualify the µPD271 / µPD272 as a true single-chip calculator circuits, they are using with the µPD261 an external segment decoder and driver chip for the calculator display. The µPD273 added both an internal clock oscillator and segment decoder to the feature set of its predecessors, rendering it a "true" single-chip calculator circuit. It was complimented in December 1973 with the µPD277, a design with 8-digit display capability and integrated 2-key Memory.

The featured Prinztronic Asset calculator manufactured in January 1975 makes use of an 9-Digit low-voltage VFD manufactured by Itron and known as Type DP95A, soldered with its 19 wires directly to the Main-PCB.

The Prinztronic Asset makes use of a keyboard assembly with 19 individual spring loaded long-stroke plastic keys pushing small conductive carbon discs against against gold plated contacts etched on a single-sided phenolic printed circuit board (PCB). The sliding switch for Power On/Off is using a small stamped metal part pressing against two contacts on the PCB while the Constant function of the calculator is hard-wired on the PCB, using a diode placed on the Main-PCB of the calculator.

To understand the functionality of the rather uncommon µPD274 retrieved from the featured non-working Prinztronic Asset, we decided here at the Datamath Calculator Museum to give the featured calculator a full "Teardown Treatment". A first glance at the Main-PCB of the calculator after removing its main components reveals a marking "MK 800S", a reference to the identical looking Automath MK-800 calculator.

A closer inspection of the Main-PCB and the Keyboard-PCB and their PCB traces gives a few hints about the µPD274:

Comparing the pinout diagram of the µPD274 with the µPD273 doesn't reveal any differences - time to dig deeper. Enter Bread-bording.

Step 2: Based on the reverse-engineered schematics of the featured Prinztronic Asset and salvaged µPD274 chip, we bread-boarded the clock oscillator and measured some of its key parameters. To our surprise did we measure with the µPD274 a much higher clock frequency than with the µPD273 in the same setup (V_DD = -6.0 V, V_GG = -11.0 V, R_EXT1 = 150 kOhm, R_EXT2 = 300 kOhm, C_EXT1,2 = 100 pF):

While measuring the operating frequency of the internal clock oscillator over its supply voltages V_DD and V_GG, we noticed a second, major difference. While both chips have the same nominal operating conditions, does the µPD274 work with much lower supply voltages than the µPD273:

We conclude from these observations that the µPD274 is manufactured in a different PMOS process than the µPD273. Understanding the requirements to operate the salvaged µPD274 - time to operate it. Enter DCM-50A Platform.

Step 3: 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.

Inserting the µPD274 salvaged from the featured, non-working Prinztronic Asset into the 42-pin ZIF Socket of the DCM-50A Playground DIL42 Adapter mounted with the DCM-50A Playground Frame Carrier on our DCM-50A Platform and wiring all its pins accordingly, resulted in a working calculator and we could measure its timing of the Display and Keyboard Scanning Cycle. Again, no differences to the µPD273 and even the Segment Output SH to display the Fancy-Four is supported with the µPD274.

With most of the µPD273 and µPD274 functionality hidden in its software, we decided to look closer into it. Enter DCM-50A Platform, Part 2.

Step 4: Having with the DCM-50A (PLAYGROUND) a fully operational calculator with all Keymatrix Positions available, allows looking deeper into the Calculator Logic Implementation of the µPD274. In many cases a calculator does not use all the functionality available with its calculator circuit, the featured Prinztronic Asset as an example uses 19 keys while the keyboard matrix of the µPD274 would support up to 21 keys and switches (10 Digit Outputs, 2 Keyboard Matrix Inputs and separate [C] Input). Testing the missing key positions didn't reveal any additional functionality of the µPD274 compared to the µPD273.

The µPD273 uses for the Constant Function a very unusual - and buggy - approach for Multiplication (1^st number used as constant), Division (2^nd), Addition (1^st), and Subtraction (1^st) that we refer here in the Datamath Calculator Museum as (M-D-A-S) 1-2-1-1 implementation. The µPD277 dropped Addition and Subtraction from the Constant Function for an (M-D-A-S) 1-2-X-X implementation, while the later µPD276 expanded it again to a more common (M-D-A-S) 1-2-2-2 implementation. The µPD273 was plagued with another bug related to the Percent Function, is displaying after some calculations a negative zero and entering a ninth digit is resulting in an inconvenient overflow condition.

To our biggest surprise does the µPD274 exhibit exactly the same bugs - meaning its software is identical to the µPD273.

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

© Joerg Woerner, May 3, 2025. No reprints without written permission.