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May 6, 2024
The Unitrex 80SR is a mid-1970s Japanese made vintage calculator with a nice LED bubble display. Overall this unit was in pretty good shape though it was missing the case and also unfortunately the battery door. The magnifying bubble display is a neat feature, meant to offer a readable display from the rather tiny individual LED segments by magnifying each digit with a bubble shaped lens. I powered up the calculator and checked the segments in each digit by filling the display with 8's. All of the segments on all of the digits worked, except one. The one segment was on a higher order digit, and would have no impact on lower order calculations, but it was worth trying to fix it, it has such a cool red glow.
After tearing down the calculator and cleaning pieces in the process, I tested the components on the board to ensure that all of the electronics were in good shape. Diodes, transistors, capacitors all passed their respective testing. The brains of the calculator, the Rockwell A4001 15471PC microprocessor appeared to be in good condition as well.
Unitrex 80SR main PCB
Rockwell A4001 15471PC
Key contact springs
With everything looking to be in good shape after the cleaning I moved on to looking to repair the broken LED segment that had been identified earlier. The individual digits are Litronix R68 displays which have bond wires welded to individual segments. The displays are incredibly tiny which is where the genius of the bubble lens comes in, magnifying the digits to something readable.
Litronix R68 LED displays under bubble lens
Litronix R68 LED with lens removed
With the bubble lens removed I was able to see that one of the segment wires had separated from the display PCB. Luckily it was still bonded to the LED. Using a microscope, some unsteady hands, and some dental tools I was able to carefully resolder the tiny bond wire back to its rail. With the wire back in place all of the digits were once again working. Now it was time to move onto modelling a replacement battery cover.
I know just enough FreeCAD to get by on simple work, and through a little trial and error I was able to land on a battery cover model which had a good fit.
The textured plate in my 3D printer provided a finish which was similar to the original finish on the 80SR which was a nice bonus.
Everything seemed to come together. The display was repaired, battery door replaced and this nice vintage calculator was given a new life... until tragedy struck.
Unfortunately someone visiting my bench thought the calculator looked cool and moments after picking it up it dropped to the floor. And now multiple LEDs were missing multiple segments. An investigation under the microscope showed that this time multiple bond wires had separated from the LEDs rather than from the PCB side. Unfortunately hand soldering the bond wires back to the LED wasn't possible as more advanced techniques and equipment is needed.
So on to 'Plan B'. Given that the original LED display is essentially unusable at this point I will order some more modern LED segements and see if I can retrofit the calculator with them. A modern display won't look as cool as the original bubble display, but at least the rest of the calculator will remain original. I'll have to pick up this project in the future once I have the parts, so eventually a Part 2 to this story.
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