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how can i find a short in the motherboard when it cant power on ac adapter is okay geting 18-19 dc volts from the power jack

it is because i tried to run some checks and yes i checked all that you have mention includind leaving the motherboard whithout being connected to anything except having the memory and processor on it it did not power on but when i tried connecting the battrey it tried to come on for some minutes and went dead again