This is a short story about fixing a 1970 Toshiba electronic calculator. It started with some attempts to find small nixie tubes such as the NEC LD-955A. It appears they were used in early consumer electronic calculator in the late 60’s. I found this faulty calculator, and only just for the 16 tubes it was a reasonable price. Once I got it, I thought it would be more fun to try fixing it.
The Toshiba BC-1625 is a 16 digits electronic calculator, Nixie display, two memories, square root and percent functions. You can also choose the rounding method and how many digits you’d like after the coma.
If you type “Toshiba BC-1625” in your lovely search engine, you’ll get several online calculator museum website. Which I happily discovered the existence. Eventually, you’ll find this website giving plenty of hints about similar model, and most incredibly you will find the work of Brent Hilpert. Brent worked on fixing a Toshiba BC-1212, and it is amazing. He reversed engineered the calculator, from schematics up to the algorithm itself and wrote down everything. While it is not the same model – 12 digits instead of 16 – the electronic is quite similar. Brent’s work was immensely precious, I’m not sure I could fix mine without it.
Opening it shows that several things was done before. In particular a weird rewiring of the main transformer. I unplugged the circuit boards before testing anything. If I put the input voltage selector to ~240V, the transformer works, but get hot quickly. After some trial to find the right ratio for a ~240V mains, I hard cabled it and bypassed the input voltage switch.
The logic levels are completely new to me, 0v for ‘true’ and -24v as ‘false’. Most of the logic gates, OR and AND are made of discrete diodes and resistors. Dozens of them. The integrated circuits are mostly flip-flops, and a pair of shift register, all in TO-101 packages. Which appears to be MOS technology circuits seems only used in Japan.
The typical fault describe by Brent is the failure of DC blocking capacitors. Once failed, these capacitors short the 180V directly, destroying the upstream integrated circuits. Most of the PNP transistor and some diodes driving the Nixies died as well. I started by replacing the capacitor, transistors and diodes.
In the meantime, powering the logic boards alone shows that not all the signals are correct. The oscillator and timings work, but not the ring counter used to cycle through the digits. Brent was right, the flip-flops failed due to the short of the bad capacitors. Luckily I could find these rare TM4103M flip-flops on Ebay.
With everything in place, the calculator is back in operation ! See few tests in the video below. Thanks for reading !