Commodore 899A ghosting digit in the LED display - a repair
Effecting a repair to a ghosting digit in the LED display of this Commodore 899A calculator.
I am sharing the video in part as an aid to diagnosing the cause of the issue of ghosting LED digits (flickering or unintended LED illumination of a digit or 'junk' signals). I hope I explain how the LED display works, and what has caused the ghosting in this example unit. The repair is to add a pull-up resistor to the LED digit's common cathode to offset the issues that are lurking inside the IC chip.
Since these electronic calculators exist mainly as 'calc-on-a-chip' designs, when these old devices start to demonstrate unintended behaviours, the outcome is often irreparable, since the issues lie entirely within the chip itself (no external components to replace).
I am pleased to manage a fix, if not a cure, for the internal IC issues and hope this can help others trying to keep their calculators alive too!
Thanks for watching.