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D3 looks like a Zener regulator very simple regulator and easily opens or shorts to cause problems. If opens then would feed the full supply voltage to the chip and BANG. I can't really see the joins underneath properly though.
Q1 is the high current switching which is fed from the main supply voltage and bias by the 555.
So need to make sure the right voltage zener is in place (If it is actually a zener).
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