I have a LED alarm clock that would randomly add or subtract an hour. It was frustrating because it would keep perfect time for weeks until it would change by an hour in the middle of the night, waking me up an hour early or an hour late.
I took it apart expecting to find bad decoupling caps or a bad solder joint, but everything looked good. Looking over the ribbon cable that ran from the control buttons to the clock IC, I noticed tiny pinholes in the insulation. It wasnt until I tried putting the clock back together that I noticed the cause. The cable ran between the PC board and the cover.
The LED leads protruded through the back of the PC and pushed against the ribbon cable when the cover was closed. The signals driving the LED would short against the button scanning controls, changing the hour. The odd thing is that the buttons only allow incrementing the hour, not decrementing it. Somehow the short could decrement the hour. So the malfunction surprisingly delivered new functionality.
Adjustment of the cable, trimming the LED leads, and liberal use of Kapton tape fixed the issue.
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