I fixed the display glitch in the healthier HP35. Turns out I was browning it out! Longer numbers on the LED display require a lot more current. It's happy at 135 mA. It's difficult to capture in photos just how good these old displays look. Super crisp. The digits get a little blurred in photos, likely due to the high frequency PWM.
This first HP calculator has no built in self test, but manual tests suggest it's fully working. 🎉
The water damaged one, not so much.
Reading about these displays, it's remarkable how much better LEDs have gotten since 1970. If I put 400 mW into modern LEDs of this size, it'd be blinding.
Whereas these little fellers are just doing their darnedest to be visible in normal indoor lighting.
They still look great though.
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