Printing Calculator Teardown

A regular occurrence when I visit my parents’ house is being given one or more things to fix. It could be anything from an electronic key fob for a car to a broken dishwasher handle. Oftentimes the fix is relatively quick, as they don’t usually get too far before giving up – so it may be just a matter of opening up a housing, changing a battery or bending a spring connector or cleaning out some dirt or grease, and putting it back together.

At Christmas this year, my Dad handed me his Sharp EL-1750V printing calculator and told me that ever since he had dropped it, it was no longer loading paper. I was rather surprised that anyone still used a printing calculator, while he was surprised that anyone could do their taxes without one. He of course had a backup printing calculator, but didn’t like it as much as his trusty Sharp, so asked if I could fix it. I rolled my eyes a bit, but after looking at it a little more closely, saw a low-cost DC motor that reminded me of my days in the electronic toy industry, and an intriguing array of axles and gears that made me think I might learn something just from the teardown. The best thing about trying to fix something like this (that is otherwise too cheap to waste time trying to fix) is that there’s nothing to lose.

So, after opening it up, the core is a module made by Epson, the M-32TL. The best reference I found from Googling that was a series of blog posts by an engineer named Reece Pollack, who cannibalized the same module from a Canon P170-DH and, with some help from Arne Rossius’ page on the similar (but single color) Epson M-31A module, reverse engineered it sufficiently to build his own driver with a PIC microcontroller on a breadboard. The M-32TL appears to also be in the Canon P23-DH (based on this teardown), and I would guess most other 2-color printing calculators made today (also that work with the IR-40T ink roller or compatible variants).

As I removed this Epson module from the calculator housing, the position sensor assembly burst apart – not just the top and bottom housings, but all four wires/brushes popped off. What should look like this:

Instead, after a great deal of effort and hot glue to hold things in place looked like this:

While the brushes are somewhat bent, it will probably work okay, but instead of stopping there and putting everything back together, I decided I really wanted to understand the mechanism, including how the single solenoid is used to both press one character (of three) against the paper for printing, and advance the print head to another column. More on that to come.

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