Ensure that the keypad membrane is perfectly clean before trying to trace each corresponding keypad keys that is not responding. Majority of each group of connection has three keys are connected together, therefore if only one or two keys are not working, the problem may lies on open line tracks. If more than three or all of keys are not working the problem may lies on a damaged EMI-ESD filter chip.
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Nokia 2690 keypad IC jumper ways and line tracks, full solution