The unit may have the problem that befalls the Palm Pilots where something gets under the escutcheon and presses on the touch screen at the edge which overrides any other inputs.
Another possibility is a stuck button. The buttons are usually scanned as a matrix and at startup if a button is stuck that can make unit unresponsive.
Without a logic analyzer and the firmware listing you likely would not get far if one of the logic chips has gone bad unless it can be found using freeze mist and heat gun..
Stuck buttons or touch screen is usually easily found without schematic.
While the setting is shown in the display, press the CALIB key to
set the calibration.
Each time you press the key you will cycle through the following
settings.
0: 440 Hz 2: 442 Hz 4: 444 Hz 8: 438 Hz
1: 441 Hz 3: 443 Hz 5: 445 Hz 9: 439 Hz
When you have made the desired setting, wait approximately two
seconds without pressing the key.
The indication in the note display will blink three times, and the
calibration setting will be completed. You will then return to tuning
mode.
Just do it and stop on the 440 one.
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