Designing a Custom Replacement Board for a Vintage Casio CQ-1

The Casio CQ-1, released in 1976, was sort of a cross between the digital calculators of the era and the PDA-style “pocket computers” that would end up seeing some popularity in the ‘80s. When the CQ-1 hit the market, home computers were expensive and weren’t portable at all. Digital calculators, while pricey by modern standards, were a more affordable option. Models like the CQ-1 integrated additional functionality, like an alarm clock and the ability to perform time and date calculations. YouTuber A Menadue's Casio CQ-1 was destroyed by leaking batteries, and multiple replacement units ended up having their own issues. So, he designed and built a custom drop-in mainboard for the vintage Casio CQ-1.

The first replacement he purchased had some sort of loose connection, and would only function if he pushed down on one of the clock oscillator pins. The second replacement simply didn’t function at all. The third replacement worked initially, but the VFD tube power supply was malfunctioning and caused the AA battery to go flat very quickly. With four non-functional Casio CQ-1 units on hand, A Menadue decided to design a new and improved mainboard using modern components. This mainboard would address the weak points of the original design and also improve the specifications.

A Menadue started by removing the Casio CQ-1 components that would be reused, including the VFD tube and the keyboard PCB. He then prototyped a replacement board based on an STM32F103C8T6 “Blue Pill” development board. That has an Arm Cortex-M3 processor running at 72MHz and 20KB of RAM. More importantly, he replaced all of the original power supply components. The original design was odd and relied on two 1.5V button cell batteries to power the clock and calculator, with a separate AA battery to power the VFD tube. This new circuit powers everything from a single source (either USB or a single AA battery) via DC-to-DC converters.

After testing the circuit on the prototyping board, A Menadue designed a custom PCB that fits in the original mainboard’s location. There are a few small errors that need to be addressed and completely new firmware has to be programmed, but the new mainboard reportedly works mostly as intended.