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Ty_Eeberfest wrote: Take a look at the schematic. If the 7805 was bad it's highly unlikely that LED1 would be lit. If 7805 failed open the LED would get no power. If 7805 failed shorted the LED would get the full VIN voltage so it would probably light up quite bright. This is all speculation of course, the real test is to read VCC, if it's 5 volts the 7805 is okay. As for why the clock would not work if VCC is 5V, there are a multitude of possibilities.