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I'm a computer technician, so please avoid the obvious diagnostic suggestions - info provided.

I'm working on a Compaq Presario CQ40. Originally, the battery would not charge or run off ac adapter. Tested battery - good. Tested AC adapter - good. Tested dc jack with questionable results - decided to replace with no avail.

I went back and replaced the battery/ac adapter with known good. Tested new dc jack with multimeter - continuity confirmed, voltage confirmed. I even tested the motherboard solder joints for voltage - everything checked out fine!

What the heck could possibly be causing this? I've fixed thousands of laptops and never seen anything like this.

Please help! Thanks so much!

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The circuit itself could have been damaged. In most charging circuits there is a diode that will allow the excess voltage to go to ground or shunt when the charge is complete. This diode could be damaged. –  Pow-Ian Feb 22 '13 at 20:35
    
Interesting. If that was the case couldn't I test for voltage passed the suspected diode, which would yield no current.. right? –  Joe Feb 22 '13 at 21:06
    
I would assume yes you could however who knows where the diode is, or if it is even a diode. They could have used another switching mechanism all together. Are you able to get it to power on without the battery installed? –  Pow-Ian Feb 22 '13 at 21:17
    
No. Battery only, unfortunately. –  Joe Feb 22 '13 at 21:23
    
So the power is getting to the board from the battery and not from the adapter. This defiantly sounds like the charging circuit got yacked. Obviously there is a switching circuit that allows voltage from the battery or from the plug to run the machine. It would seem this circuit is damaged because it is not letting voltage from the adapter into the board. You may need a new mother board unless the charging board is not integrated, sometimes they are separate. –  Pow-Ian Feb 22 '13 at 21:26

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